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If you are standing somewhere
on Earth... this is the Earth,

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the mass of the Earth,
radius of the Earth,

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and you're here.

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And let's assume for simplicity

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that there's no atmosphere
that could interfere with us,

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and I want to give you one
huge kick, an enormous speed,

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so that you never, ever come
back to Earth, that you escape

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the gravitational attraction
of the Earth.

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What should that speed be?

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Well, when you're standing here
and you have that speed,

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your mechanical energy--

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which we often simply call E,
the total energy--

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is the sum
of your kinetic energy--

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this is your mass;

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this is your
escape velocity squared--

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plus the potential energy,
and the potential energy equals

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minus m Mg divided
by the radius of the Earth.

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So this is your kinetic energy

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and this is your
potential energy--

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always negative,
as we discussed before.

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Mechanical energy is conserved,

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because gravity is
a conservative force.

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So no matter where you are
on your way to infinity,

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if you are at some distance r,

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that mechanical energy
is the same.

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And so this should also be

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one-half m v at a particular
location r squared

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minus m M earth G
divided by that little r.

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And so at infinity,
when you get there--

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little r is infinity,
this is zero,

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potential energy at infinity
is zero--

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and if I get U at infinity
with zero kinetic energy,

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then this term is also zero.

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And that's the minimum amount
of energy that I would require

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to get you to infinity

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and to have you escape the
gravitational pull of the Earth.

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If I give you a higher speed,

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well, then, you end up
at infinity

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with a little bit
net kinetic energy,

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so the most efficient way
that I can do that is

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to make this also zero, so you
reach infinity at zero speed.

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So this is
for r goes to infinity.

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And so this E equals zero then.

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And so this term is
the same as this term

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for your escape velocity.

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And so we find that one-half m
v escape squared equals

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m M earth G divided
by the radius of the Earth.

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I lose my little m, and I find

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that the escape velocity
that I have to give you

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is the square root
of two M earth G

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divided by the
radius of the Earth.

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And this is enough,
is sufficient to get you

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all the way to infinity
with zero kinetic energy.

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If you substitute in here

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the mass of the Earth
and the radius of the Earth,

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then you will find

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that this is about 11.2
kilometers per second.

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That is the escape velocity
that you need.

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It's about
25,000 miles per hour.

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Again, we assume

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that there is no air
that could interfere with you.

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If the total energy

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when you leave the Earth
with that velocity--

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if the total energy is
larger than zero,

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you do better than that.

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You reach infinity

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with kinetic energy which is
a little larger than zero.

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We call this unbound orbit--
larger or equal.

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If E is smaller than zero,

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that the total energy
that you have is negative,

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then you will never escape the
gravitational pull of the Earth,

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and you will be
one way or another

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in what we call a bound orbit.

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Let's pursue the idea
of circular orbits.

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Later in the course we will
cover elliptical orbits,

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but now let's exclusively talk
about circular orbits.

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Now, this is
the mass of the Earth,

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and in a circular orbit is an
object with mass m, a satellite,

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and m is way, way, way smaller
than the mass of the Earth.

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And the radius
of the orbit is R,

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and this object has a certain
velocity v, tangential speed.

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The speed doesn't change,
but the direction changes,

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and there has to be
a gravitational force

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to hold it in orbit,
and the gravitational force

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is exactly the same
as the centripetal force--

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we've discussed that
many times before.

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And so the gravitational force

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which is necessary
to make it go around--

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I could also say the
centripetal force is necessary

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to make it go around
in a circle--

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that gravitational force equals

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m M earth G
divided by R squared.

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This is now the distance
from the Earth to the satellite,

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and that must be equal
to m v squared divided by R,

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and that is that tangential
speed that you see here,

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which a little later in time,
of course, would be here.

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I lose my m, and so you see now
that the orbital speed--

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not to be mistaken
for escape velocity--

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the orbital speed is exactly
the same what we have there

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except the square root of two
divided by R.

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This is now R.

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And there it was R earth.

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If you know R, then you can
calculate the speed in orbit.

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If you know the speed in orbit,
you can calculate R.

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And so the period
of going around in the orbit,

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T equals two pi R
divided by the orbital speed,

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and when you do that,
you get two pi.

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You get an R
to the power three-halfs,

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and you have downstairs
the square root of G M earth.

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Let me move this in a little.

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Two pi R
to the power three-halfs.

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So again,
if you know the radius,

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if you know how far you are
away from the Earth,

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the period follows uniquely.

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If you know the period,

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then the distance to the
satellite follows uniquely.

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If we take the shuttle as an
example of a near-Earth orbit,

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so we have the shuttle.

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The shuttle may be

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400 kilometers
above the Earth's surface.

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So we have to add to the radius
of the Earth 400 kilometers,

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so you end up with
about 6,800 kilometers

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for the radius
of the orbit of the shuttle,

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and you substitute that in here,

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the mass of the Earth
and the gravitational constant,

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you'll find that
T is about 90 minutes.

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It's about 1½ hours.

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The shuttle takes
about 1½ hours to go around,

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and the speed,
that tangential speed,

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is very close to eight
kilometers per second.

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And that holds for all
near-Earth-orbit satellites.

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Whether they are
400 or 500 or 600 kilometers,

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that doesn't change very much.

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If you take the moon--

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the moon is much further away
than the shuttle,

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and you take the distance
to the moon--

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which is some
385,000 kilometers--

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you substitute that in this
equation, you will find

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that the period for the moon
to go around the Earth

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is about 27½ days.

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And its speed is only
one kilometer per second.

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It's much further out.

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If it's much further out,
R is much larger,

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and so you see
the speed will be much lower.

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If you take the Earth itself
around the sun--

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because we can use
all these equations--

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replace the mass of the Earth
by the mass of the sun,

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and then we can do this
for planets.

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So if we take the Earth
around the sun,

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then we have to put in
the mass of the sun,

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which is about two times
ten to the 30 kilograms.

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And the distance
from the Earth to the sun,

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we have seen that before--

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I call that the distance
from the sun to the Earth--

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is about 150 million kilometers.

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Forgive me for mixing up
meters with kilometers,

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but you have to convert that,
of course, to meters.

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And when you calculate

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how long it takes the Earth
to go around the sun,

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no surprise--
you will find 365½ days.

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So that's simply substitution
of these two quantities

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in the equation that I have here
and that I have here.

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The velocity of the Earth
in orbit

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is about 30 kilometers
per second.

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That's a substantial speed,
by the way,

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that the Earth is going
around the sun--

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30 kilometers per second--
way higher than the speed

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that the shuttle is in around
the orbit... around the Earth,

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which is only
eight kilometers per second.

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Jupiter is five times further
away than the Earth,

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and so the time for Jupiter
to go around

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goes with five to the power 1½.

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That's about 12,

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so it takes Jupiter about
12 years to go around the sun.

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Notice that this period
is independent

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of the mass
of the little satellite,

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and that was very unfortunate
for the Americans

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when on October 4, 1957,
Sputnik was launched.

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They could find the radius
very easily, because they knew

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the period that it took Sputnik
to go around the Earth.

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That was about 96 minutes.

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They could calculate
the velocity,

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they could calculate the radius,

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but they had no clue
about the mass, and that was

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a key piece of ingredient
that the Americans wanted,

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because if the mass was
very large of Sputnik,

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that would indicate, of course,

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that the Russians had
very powerful rockets.

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You cannot tell the mass
from the orbital parameters--

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it's independent of mass.

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Whether you have
a very light object

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or a very heavy satellite, they
have the same velocity in orbit

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if they are
at the same distance,

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and they have
the same orbital period.

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I mentioned earlier,

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notice that the orbital period
and the escape velocity vary

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by a square root of two if you
are at a particular position.

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For instance, you're
at a particular position

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around the Earth,
here at a satellite.

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If you want to escape from this,
you will need a speed

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which is the square root
of two times larger

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than that orbital velocity.

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And so if you wanted
to escape from the Earth,

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then you need your 11.2
kilometers-- we have it there.

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If you are near Earth in orbit,

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you are eight kilometers
per second,

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and eight times the square root
of two is exactly that 11.2.

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So you see

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the connection is always
through this square root of two.

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There is something remarkable
about these numbers.

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The total mechanical energy--

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and I will write that
once more here--

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which is one-half m v squared
at a given radius

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minus little m capital M G
over r;

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whether M is the mass
of the sun or the Earth

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is of no concern to me now.

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This is the kinetic energy

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for something in orbit
at this radius,

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and this is
the potential energy.

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But now notice, I can substitute
now for this v squared,

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I can substitute
the square of this--

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that is the orbital speed--
and then I get M G over R.

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And so this one equals
one-half m MG over R.

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And now compare the two.

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They almost look like
carbon copies of each other,

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except that there is a minus
sign here, which is crucial,

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and there is a half here,
which is missing here.

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And so the total energy E--

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which I've called
the mechanical energy--

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always for a circular orbit
is one-half U
 

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and is the same
as minus the kinetic energy.

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A remarkable coincidence,
you would think.

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It is not as much a coincidence
as you think, of course.

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But if something is in orbit,

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this is the orbital speed
at radius r,

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then always
is its total energy...

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is half the potential energy.

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It's always negative.

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Okay, later in the course
we will cover elliptical orbits.

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I will not do that today,

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and so I will march on
to a completely different topic,

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and that is the topic of power.

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So I will abandon for now
the orbits entirely.

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What is power?

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Power is work that is done
in a certain amount of time.

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dw/dt, if w is the work, is the
instantaneous power at time t.

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00:13:59 --> 00:14:05

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We also know power
in terms of political power.

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That's very different.

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00:14:07 --> 00:14:13
Political power-- you can do
no work at all in a lot of time,

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and you have a lot of power.

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Here in physics,
life is not that easy.

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The units of power are
the units of work,

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which is joules per second,
for which we often write W,

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which is named after
the physicist Watt.

257
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Don't confuse this w for work
with the W for watt,

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which is one joule per second.

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Now, the work that I do,
that the force is doing,

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is the dot product

261
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between the force and a certain
displacement of that force.

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We have dealt with that before.

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That is a little bit of work
that I'm doing, right?

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I have a force which is constant

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during a short,
small displacement.

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And so I can substitute that
in there, and so I get

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that the power is the derivative
of this versus time.

268
00:15:04 --> 00:15:10
And that is...

269
00:15:06 --> 00:15:12
if I keep this force constant
for that short amount of time

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is the dot product times
the velocity,

271
00:15:12 --> 00:15:18
because dr/dt is simply
the velocity of that object.

272
00:15:15 --> 00:15:21

273
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So power is also force dotted
with the velocity.

274
00:15:22 --> 00:15:28
If the force were perpendicular
at all times

275
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to the velocity vector,
then the power is zero.

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00:15:30 --> 00:15:36

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00:15:33 --> 00:15:39
Let's take an example.

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00:15:34 --> 00:15:40
I am on a bicycle--
here is my bicycle--

279
00:15:40 --> 00:15:46
and I'm sitting
on a bicycle here

280
00:15:42 --> 00:15:48
and I'm trying to get going.

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And I have a certain velocity,

282
00:15:50 --> 00:15:56
and I keep
that velocity constant.

283
00:15:53 --> 00:15:59
That's the way most people
would ride their bikes.

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Now, there is air drag,
which is unavoidable.

285
00:16:00 --> 00:16:06
We discussed that.

286
00:16:02 --> 00:16:08
And the air drag acts
like a force on me, F drag,

287
00:16:09 --> 00:16:15
and so somehow, I, Walter Lewin,
will have to come up

288
00:16:12 --> 00:16:18
with the force in this direction
to overcome this drag

289
00:16:17 --> 00:16:23
so that the speed
can be constant,

290
00:16:19 --> 00:16:25
because if the net force
is zero on me,

291
00:16:22 --> 00:16:28
then of course I will have
a constant velocity--

292
00:16:24 --> 00:16:30
no acceleration,
no change in the velocity.

293
00:16:27 --> 00:16:33
How do I do that?

294
00:16:29 --> 00:16:35
Well, I push on the pedals.

295
00:16:32 --> 00:16:38
But the pedals push back on me.

296
00:16:35 --> 00:16:41
Action equals minus reaction.

297
00:16:38 --> 00:16:44
So that causes no net force
on the bike at all.

298
00:16:42 --> 00:16:48
I push on the pedal; 
the pedal pushes back on me;

299
00:16:44 --> 00:16:50
those two forces cancel.

300
00:16:47 --> 00:16:53
We call them internal forces.

301
00:16:50 --> 00:16:56
Now, the pedals push
on the chain,

302
00:16:53 --> 00:16:59
and the chain pushes
on the wheel,

303
00:16:55 --> 00:17:01
and ultimately this wheel wants
to start rotating

304
00:16:58 --> 00:17:04
in this direction
because of my pedaling.

305
00:17:02 --> 00:17:08
And now here with the...
with the floor, with the road,

306
00:17:05 --> 00:17:11
there is friction, and so now
the wheel pushes onto the road,

307
00:17:10 --> 00:17:16
and the road pushes back.

308
00:17:11 --> 00:17:17
Action equal minus reaction.

309
00:17:13 --> 00:17:19
And it is that force,
which is really the friction--

310
00:17:17 --> 00:17:23
that is the force that Walter
Lewin has to come up with

311
00:17:21 --> 00:17:27
in order to make sure that
he can go with a constant speed.

312
00:17:25 --> 00:17:31
It's the friction that does it.

313
00:17:27 --> 00:17:33
You have to really think
about it.

314
00:17:28 --> 00:17:34
It's remarkable.

315
00:17:30 --> 00:17:36
If there were no friction of
that road, you couldn't cycle.

316
00:17:32 --> 00:17:38
I could do this, and I would
just stay still, right?

317
00:17:34 --> 00:17:40
There would never be
any force here

318
00:17:36 --> 00:17:42
that would drive me in this
direction so you can go.

319
00:17:40 --> 00:17:46
Of course, if you had a speed
that you were sliding,

320
00:17:43 --> 00:17:49
then, of course, you would
always maintain that speed.

321
00:17:45 --> 00:17:51

322
00:17:47 --> 00:17:53
I want you to appreciate that
the power that I have to deliver

323
00:17:52 --> 00:17:58
is an extremely strong
function of the speed.

324
00:17:56 --> 00:18:02
If we are here

325
00:17:57 --> 00:18:03
in the domain of what I called
earlier regime two,

326
00:18:01 --> 00:18:07
which is the
pressure-dominated regime,

327
00:18:05 --> 00:18:11
then the drag force is
proportional to v squared.

328
00:18:10 --> 00:18:16
Let's say it is
a constant times v squared.

329
00:18:14 --> 00:18:20
We spent a whole lecture
on this.

330
00:18:15 --> 00:18:21
That's regime two.

331
00:18:17 --> 00:18:23
Let's assume it's there.

332
00:18:18 --> 00:18:24
Then if I have
ten miles per hour here,

333
00:18:22 --> 00:18:28
I drive ten miles per hour,
and I tell you that the power...

334
00:18:26 --> 00:18:32
this is a given, this is not
something that I show you,

335
00:18:29 --> 00:18:35
that is just a given,

336
00:18:31 --> 00:18:37
that that is about a power
of 0.02, 1/50, of a horsepower,

337
00:18:37 --> 00:18:43
and one horsepower is some
crazy unit-- 400... 746 watts.

338
00:18:44 --> 00:18:50
So this is about 15 watts.

339
00:18:48 --> 00:18:54
So I'm pedaling, and I keep
my speed ten miles per hour,

340
00:18:52 --> 00:18:58
and I have to generate
15 joules per second on average.

341
00:18:58 --> 00:19:04
But now I want to go
to 25 miles per hour.

342
00:19:01 --> 00:19:07
So here we get 25.

343
00:19:04 --> 00:19:10
That is 2½ times higher.

344
00:19:06 --> 00:19:12
But now the power
that I have to generate

345
00:19:11 --> 00:19:17
is the dot product between
the force and the velocity.

346
00:19:15 --> 00:19:21
Now, the force and the velocity
are in the same direction,

347
00:19:17 --> 00:19:23
so the dot can disappear,

348
00:19:19 --> 00:19:25
so I get that the power is
k times v to the third,

349
00:19:24 --> 00:19:30
and so now if I want the speed
to go up by a factor of 2½,

350
00:19:28 --> 00:19:34
the power that
I have to generate

351
00:19:29 --> 00:19:35
is 2½ to the power three
times higher,

352
00:19:33 --> 00:19:39
and that is
about 15 times higher,

353
00:19:35 --> 00:19:41
so now you're talking
about 0.3 horsepowers,

354
00:19:39 --> 00:19:45
and you're talking about
something like 230 watts.

355
00:19:43 --> 00:19:49
And that is quite a power,
let me tell you.

356
00:19:48 --> 00:19:54
I wonder whether there are many
here in the audience

357
00:19:50 --> 00:19:56
who could generate this
for more than even half hour.

358
00:19:55 --> 00:20:01
Most of us could probably do it
for a few minutes,

359
00:19:57 --> 00:20:03
but not for hours.

360
00:19:58 --> 00:20:04
It depends entirely, of course,
on your condition.

361
00:20:02 --> 00:20:08

362
00:20:05 --> 00:20:11
There is also heat energy,

363
00:20:08 --> 00:20:14
and heat energy is expressed
in a very different way.

364
00:20:12 --> 00:20:18
We express that
in terms of calories.

365
00:20:17 --> 00:20:23
And a calorie is defined
in a very special way

366
00:20:22 --> 00:20:28
as the energy which is needed

367
00:20:24 --> 00:20:30
to increase one gram of water
by one degree centigrade.

368
00:20:29 --> 00:20:35
And so in general we can write

369
00:20:31 --> 00:20:37
that Q, which is heat energy,
which is in calories,

370
00:20:38 --> 00:20:44
is the mass of the object
times the specific heat,

371
00:20:43 --> 00:20:49
which for water would be

372
00:20:46 --> 00:20:52
one calorie per gram
per degree centigrade

373
00:20:52 --> 00:20:58
times the temperature increase
that we apply.

374
00:20:57 --> 00:21:03
So we increase
the temperature of an object.

375
00:20:59 --> 00:21:05
The object has a mass m.

376
00:21:00 --> 00:21:06
We increase the temperature
by this much,

377
00:21:02 --> 00:21:08
so many degrees Kelvin
or degrees centigrade--

378
00:21:05 --> 00:21:11
that's the same--
and then this is

379
00:21:07 --> 00:21:13
the number of calories
that you have to put in there.

380
00:21:11 --> 00:21:17
Um... I gave you
the specific heat for water

381
00:21:16 --> 00:21:22
in calories per gram,
not per kilogram.

382
00:21:18 --> 00:21:24
If I gave it per kilogram, which
may be nicer for this course,

383
00:21:21 --> 00:21:27
then of course it would be
thousand instead of one.

384
00:21:25 --> 00:21:31
Aluminum has
a specific heat of 0.2.

385
00:21:30 --> 00:21:36
Lead is unusually low--
it's only .03.

386
00:21:34 --> 00:21:40
It's very, very low.

387
00:21:35 --> 00:21:41
Ice is only half
the specific heat of water.

388
00:21:39 --> 00:21:45
Ice is only...

389
00:21:40 --> 00:21:46
ice is only one-half calorie
per gram per degree centigrade.

390
00:21:47 --> 00:21:53
The physicist James Joules,
after we...

391
00:21:52 --> 00:21:58
we call after him
the unit of work--

392
00:21:56 --> 00:22:02
was the first to demonstrate

393
00:21:58 --> 00:22:04
that heat energy and mechanical
energy are really equivalent.

394
00:22:01 --> 00:22:07
He did an ingenious experiment.

395
00:22:04 --> 00:22:10
Of course once you hear it,

396
00:22:05 --> 00:22:11
you said, "Well, I could have
thought of it myself."

397
00:22:08 --> 00:22:14
He takes objects with masses
which hang from strings

398
00:22:11 --> 00:22:17
and he lowers them
in a gravitational field

399
00:22:14 --> 00:22:20
over a certain distance.

400
00:22:16 --> 00:22:22
So he knows what mgh is.

401
00:22:18 --> 00:22:24
And he uses this rope to rotate
scoops which are in water.

402
00:22:23 --> 00:22:29
And these scoops are driven.

403
00:22:25 --> 00:22:31
There is mechanical energy,
mgh comes out in the scoops,

404
00:22:28 --> 00:22:34
and what does he notice?

405
00:22:29 --> 00:22:35
That the temperature
of the water goes up.

406
00:22:31 --> 00:22:37
And he measures
the increase in temperature,

407
00:22:33 --> 00:22:39
and he knows
how the calorie was defined,

408
00:22:36 --> 00:22:42
and so he found that one calorie
is approximately 4.2...

409
00:22:42 --> 00:22:48
now it's called joules.

410
00:22:44 --> 00:22:50
At that time
it wasn't called joules yet.

411
00:22:46 --> 00:22:52
So there is a direct connection
between the two.

412
00:22:51 --> 00:22:57
I would like to... I'm going
to throw several numbers at you

413
00:22:56 --> 00:23:02
during this lecture,
and I prepared a view graph.

414
00:23:02 --> 00:23:08
Don't copy the numbers,
because it's all on the Web.

415
00:23:05 --> 00:23:11
But some of these numbers
I will return to,

416
00:23:08 --> 00:23:14
and therefore I thought I might
as well compile them in one.

417
00:23:11 --> 00:23:17
You see there on the top there
that one calorie is 4.2 joules.

418
00:23:16 --> 00:23:22
You also see the horsepower

419
00:23:18 --> 00:23:24
and other units that will come
up very shortly

420
00:23:21 --> 00:23:27
are all defined there.

421
00:23:24 --> 00:23:30
When we burn something,
there is a chemical reaction

422
00:23:29 --> 00:23:35
which produces heat,
in many cases.

423
00:23:32 --> 00:23:38
Gasoline produces per gallon

424
00:23:35 --> 00:23:41
something like close
to a hundred million joules.

425
00:23:38 --> 00:23:44
Your body produces heat.

426
00:23:42 --> 00:23:48
Your body is roughly
at a temperature

427
00:23:44 --> 00:23:50
of 98 degrees Fahrenheit,

428
00:23:46 --> 00:23:52
unless you happen to run
a high fever today.

429
00:23:49 --> 00:23:55
And your body is radiating
electromagnetic radiation.

430
00:23:52 --> 00:23:58
You can't see it with your eyes,
because it's infrared.

431
00:23:55 --> 00:24:01
But when it's dark and
you hold someone in your arms,

432
00:23:59 --> 00:24:05
you can feel that heat.

433
00:24:01 --> 00:24:07
That heat is a fantastic amount.

434
00:24:04 --> 00:24:10
That is about 100 joules
per second that you radiate--

435
00:24:08 --> 00:24:14
100 watts.

436
00:24:09 --> 00:24:15
You radiate at the same level
as a 100-watt light bulb,

437
00:24:13 --> 00:24:19
but it's, of course, distributed
over a much larger area,

438
00:24:16 --> 00:24:22
so you're not that hot
as a 100-watt light bulb.

439
00:24:20 --> 00:24:26
But it's a fantastic amount--

440
00:24:22 --> 00:24:28
a hundred watts that you radiate
for the simple fact

441
00:24:25 --> 00:24:31
that your body has to be kept
at that temperature.

442
00:24:30 --> 00:24:36
It means that in one day

443
00:24:32 --> 00:24:38
about ten to the seven joules
that you generate.

444
00:24:37 --> 00:24:43
Ten to the seven joules--

445
00:24:40 --> 00:24:46
that is what you generate
in terms of heat,

446
00:24:44 --> 00:24:50
ten to the seven joules per day,

447
00:24:47 --> 00:24:53
and that is about
two million calories per day.

448
00:24:53 --> 00:24:59
Where does the body get it from?

449
00:24:57 --> 00:25:03
Food.

450
00:24:58 --> 00:25:04
You better eat two million
calories per day.

451
00:25:02 --> 00:25:08
Now, I can see some of you turn
pale and green and purple,

452
00:25:06 --> 00:25:12
and say, "Over my dead body!

453
00:25:08 --> 00:25:14
"Two million calories per day?!

454
00:25:10 --> 00:25:16
You must be out of your mind!"

455
00:25:12 --> 00:25:18
Well, not quite.

456
00:25:14 --> 00:25:20
You see, when you read
on the packages "calories,"

457
00:25:17 --> 00:25:23
then it is called
a capital C-a-l

458
00:25:21 --> 00:25:27
and that is really
a kilocalorie.

459
00:25:25 --> 00:25:31
So you have to divide this
by a thousand

460
00:25:27 --> 00:25:33
to compare it
with the packages that you buy,

461
00:25:30 --> 00:25:36
how many calories
there is in the food.

462
00:25:32 --> 00:25:38
So you have to eat roughly daily

463
00:25:34 --> 00:25:40
about 2,000 kilocalories'
equivalent of food.

464
00:25:39 --> 00:25:45
And if you eat
a lot more than that,

465
00:25:41 --> 00:25:47
well, you pay a price for that
sooner or later.

466
00:25:46 --> 00:25:52
How about mechanical work?

467
00:25:48 --> 00:25:54
Don't we have to eat also

468
00:25:50 --> 00:25:56
for all the mechanical work
that we do?

469
00:25:52 --> 00:25:58
We work so hard, and I'm sure

470
00:25:53 --> 00:25:59
there must be a lot of energy
going into that work.

471
00:25:55 --> 00:26:01
Well, I have a surprise for you.

472
00:25:57 --> 00:26:03
It's very disappointing.

473
00:25:59 --> 00:26:05
The kind of work that
you and I do in one day

474
00:26:02 --> 00:26:08
is so embarrassingly little
in terms of mechanical work

475
00:26:06 --> 00:26:12
that you can
completely neglect it.

476
00:26:09 --> 00:26:15
Suppose we go up three floors.

477
00:26:13 --> 00:26:19
We walk up three floors,
which is about ten meters high.

478
00:26:16 --> 00:26:22
And let's say we do that
three times per day.

479
00:26:20 --> 00:26:26
And let's give you a mass
of about 70 kilograms.

480
00:26:24 --> 00:26:30
It's about my mass.

481
00:26:26 --> 00:26:32
How much work do I do
when I do that three times...?

482
00:26:31 --> 00:26:37
Oh, let me do it
five times per day.

483
00:26:33 --> 00:26:39
Boy, I really go out of my way.

484
00:26:36 --> 00:26:42
Five times per day
I go three floors up.

485
00:26:39 --> 00:26:45
Well, the amount of work
that I do is mgh.

486
00:26:42 --> 00:26:48
mgh.

487
00:26:44 --> 00:26:50
The ten meters have to be
multiplied by five,

488
00:26:46 --> 00:26:52
because I do it five times,

489
00:26:48 --> 00:26:54
and so I get 35,000 joules
of work that I do.

490
00:26:54 --> 00:27:00
35,000 joules.

491
00:26:58 --> 00:27:04
Compare that with the
ten to the seven joules per day

492
00:27:01 --> 00:27:07
that your body generates
in terms of heat.

493
00:27:05 --> 00:27:11
You think you have to eat
a little bit more

494
00:27:07 --> 00:27:13
for these lousy 35,000 joules?

495
00:27:09 --> 00:27:15
Forget it-- it's nothing.

496
00:27:11 --> 00:27:17
In fact, your average power
if you did...

497
00:27:14 --> 00:27:20
if you walked up these stairs
and you spread it out

498
00:27:16 --> 00:27:22
over a day, and say you...
it took you ten hours.

499
00:27:22 --> 00:27:28
You go once up in the morning
and then sometime the afternoon,

500
00:27:25 --> 00:27:31
and you go up in the evening
and maybe twice in the evening.

501
00:27:28 --> 00:27:34
It takes you ten hours

502
00:27:29 --> 00:27:35
to go five times
up these three floors.

503
00:27:32 --> 00:27:38
Then the average power
that you have done,

504
00:27:34 --> 00:27:40
that you have generated,

505
00:27:36 --> 00:27:42
is 35,000 joules
divided by 36,000 seconds.

506
00:27:42 --> 00:27:48
That is embarrassingly little.

507
00:27:44 --> 00:27:50
That's about one watt.

508
00:27:47 --> 00:27:53
Compare that with your body,
which generates

509
00:27:50 --> 00:27:56
a hundred joules per second
every second-- 100 watts.

510
00:27:53 --> 00:27:59
So it is completely negligible.

511
00:27:56 --> 00:28:02
However, if you climb
a mountain-- 5,000 feet--

512
00:28:02 --> 00:28:08
and you do that,

513
00:28:04 --> 00:28:10
then the work you have to do
is a million joules.

514
00:28:08 --> 00:28:14
Now, a million is
no longer negligible

515
00:28:10 --> 00:28:16
compared to the ten
to the seventh.

516
00:28:12 --> 00:28:18
And so now you feel hungry,

517
00:28:14 --> 00:28:20
and now you really need
more food.

518
00:28:16 --> 00:28:22
And if you do that in two hours,

519
00:28:19 --> 00:28:25
the power that you have
generated is substantial.

520
00:28:23 --> 00:28:29
You will have generated
an average power of 160 watts--

521
00:28:27 --> 00:28:33
more than the body heat--

522
00:28:29 --> 00:28:35
during those two hours,
of course.

523
00:28:32 --> 00:28:38
And so now the body says,
"I want to eat more.

524
00:28:35 --> 00:28:41
I want to be compensated for the
work if I climb this mountain."

525
00:28:40 --> 00:28:46
If I climb 5,000 feet
and I have to do an extra work,

526
00:28:46 --> 00:28:52
which is ten to the six joules,
you got to eat more.

527
00:28:50 --> 00:28:56
Now, you would think
that you have to eat

528
00:28:53 --> 00:28:59
only ten percent more
than you normally eat,

529
00:28:55 --> 00:29:01
because you say,

530
00:28:57 --> 00:29:03
"Ten to the six is only ten
percent of ten to the seven."

531
00:28:59 --> 00:29:05
But that's not true;
you have to eat a lot more,

532
00:29:02 --> 00:29:08
because the conversion
from food to mechanical work

533
00:29:04 --> 00:29:10
is very poor--
something like 20%.

534
00:29:07 --> 00:29:13
So you may have to eat

535
00:29:08 --> 00:29:14
40% or 50% more than
you normally do in one day.

536
00:29:12 --> 00:29:18

537
00:29:16 --> 00:29:22
Suppose I wanted to take a bath,

538
00:29:18 --> 00:29:24
and I want to calculate
how much energy it takes

539
00:29:21 --> 00:29:27
to heat the bath--
a wonderful thing to have.

540
00:29:25 --> 00:29:31
Well, we now know
how to do that.

541
00:29:27 --> 00:29:33
Q is the number of calories,

542
00:29:29 --> 00:29:35
m times C times delta T--
that's the equation.

543
00:29:35 --> 00:29:41
A bath would contain
about 100 kilograms of water.

544
00:29:40 --> 00:29:46
That is about 28 gallons.

545
00:29:43 --> 00:29:49
And let us assume

546
00:29:44 --> 00:29:50
that the temperature increase is
about 50 degrees centigrade,

547
00:29:49 --> 00:29:55
which is the same
as 50 degrees Kelvin.

548
00:29:52 --> 00:29:58
We have water,
and so you'll find

549
00:29:55 --> 00:30:01
that Q then becomes
roughly 5,000 kilocalories--

550
00:30:01 --> 00:30:07
that's how much heat energy
it takes--

551
00:30:03 --> 00:30:09
which is two times
ten to the seven joules.

552
00:30:08 --> 00:30:14
So that's the energy that
is needed to heat up a bath

553
00:30:13 --> 00:30:19
and enjoy that pleasure.

554
00:30:16 --> 00:30:22
I'll get back to this bath
very shortly.

555
00:30:19 --> 00:30:25
There are many forms of energy.

556
00:30:21 --> 00:30:27
As we're all familiar with,

557
00:30:23 --> 00:30:29
there is electric energy,
there is chemical energy--

558
00:30:26 --> 00:30:32
I mentioned that already,
gasoline burning--

559
00:30:28 --> 00:30:34
there is mechanical energy,

560
00:30:29 --> 00:30:35
when we move things
in a gravitational field,

561
00:30:33 --> 00:30:39
and there is nuclear energy.

562
00:30:36 --> 00:30:42
A waterfall is
mechanical energy-- mgh.

563
00:30:41 --> 00:30:47
You can convert that
to electricity.

564
00:30:44 --> 00:30:50
You can convert it to heat.

565
00:30:47 --> 00:30:53
Electricity will power
your coffee machine.

566
00:30:50 --> 00:30:56
It will power your TV,
your radio, your VCR,

567
00:30:53 --> 00:30:59
your electric toothbrush--
everything.

568
00:30:56 --> 00:31:02
It may power your electric
blanket, if you have one.

569
00:30:59 --> 00:31:05
Electric blanket is
only 50 watts.

570
00:31:02 --> 00:31:08
Compare that with
a human being-- 100 watts.

571
00:31:06 --> 00:31:12
Much nicer to have a human being
with you in bed

572
00:31:09 --> 00:31:15
than one electric blanket--
believe me.

573
00:31:12 --> 00:31:18
(students chuckle )

574
00:31:15 --> 00:31:21
LEWIN:
Nuclear energy can be
converted into heat,

575
00:31:19 --> 00:31:25
and that can be converted
into mechanical energy

576
00:31:22 --> 00:31:28
and again into electricity.

577
00:31:24 --> 00:31:30
Chemical energy--

578
00:31:27 --> 00:31:33
gasoline, fossil fuel can be
burned, converted to heat,

579
00:31:32 --> 00:31:38
converted to electricity.

580
00:31:34 --> 00:31:40

581
00:31:36 --> 00:31:42
I have here a device
that allows me

582
00:31:40 --> 00:31:46
to convert mechanical energy
to electric energy,

583
00:31:45 --> 00:31:51
and I would like to invite
a student to come up here,

584
00:31:49 --> 00:31:55
a volunteer, a he or a she,
who is going to show

585
00:31:53 --> 00:31:59
how he or she can convert
mechanical energy

586
00:31:57 --> 00:32:03
into electric energy.

587
00:32:00 --> 00:32:06
We'll have the
special light conditions

588
00:32:03 --> 00:32:09
so that we can see it well.

589
00:32:05 --> 00:32:11
So, who wants to do that?

590
00:32:07 --> 00:32:13
Yeah, please come.

591
00:32:09 --> 00:32:15

592
00:32:11 --> 00:32:17
There is a 20-watt
light bulb here.

593
00:32:13 --> 00:32:19
You will see it very shortly.

594
00:32:16 --> 00:32:22
And this man has a lot
of power, I can tell.

595
00:32:19 --> 00:32:25
More than 100 watts.

596
00:32:22 --> 00:32:28
Go ahead.

597
00:32:23 --> 00:32:29
Power that 20-watt light bulb.

598
00:32:25 --> 00:32:31
Put your foot on here.

599
00:32:28 --> 00:32:34
Take it easy.

600
00:32:30 --> 00:32:36
(apparatus trundling )

601
00:32:33 --> 00:32:39
Quite impressive, eh?

602
00:32:35 --> 00:32:41
Okay, now we'll tighten
the nuts a little on you.

603
00:32:38 --> 00:32:44
Here we have six of them.

604
00:32:40 --> 00:32:46
So now go ahead,

605
00:32:42 --> 00:32:48
and now you are trying to
generate 120 watts of power.

606
00:32:47 --> 00:32:53
You think you can do it?

607
00:32:48 --> 00:32:54
STUDENT:
I'll try.

608
00:32:49 --> 00:32:55
LEWIN:
Try it.

609
00:32:51 --> 00:32:57
(apparatus trundling )

610
00:32:54 --> 00:33:00
They look pretty dim to me.

611
00:32:59 --> 00:33:05
Nowhere near.

612
00:33:01 --> 00:33:07
(apparatus trundling )

613
00:33:04 --> 00:33:10
Nowhere near--
keep going, man, keep going!

614
00:33:06 --> 00:33:12
(students laugh )

615
00:33:08 --> 00:33:14

616
00:33:11 --> 00:33:17
LEWIN:
You're not even
at the level of 120 watts.

617
00:33:16 --> 00:33:22
It's hopeless. It's hopeless.

618
00:33:18 --> 00:33:24
(more laughter )

619
00:33:19 --> 00:33:25
LEWIN:
You can't do it.

620
00:33:21 --> 00:33:27
And even if you could do it,
you would have to do this

621
00:33:24 --> 00:33:30
for 48 hours in a row
to heat up my bathtub.

622
00:33:26 --> 00:33:32
Think about that.

623
00:33:27 --> 00:33:33
For one bath, 48 hours.

624
00:33:30 --> 00:33:36
But you can't even do it.

625
00:33:31 --> 00:33:37
120 watts is too much.

626
00:33:33 --> 00:33:39
I don't blame you--
I can't do it either.

627
00:33:35 --> 00:33:41
(students applaud )

628
00:33:37 --> 00:33:43

629
00:33:46 --> 00:33:52
There are batteries.

630
00:33:47 --> 00:33:53
Batteries convert chemical
energy to electricity directly.

631
00:33:53 --> 00:33:59
We are all used
to these fancy dry cells,

632
00:33:57 --> 00:34:03
but in the old days,
and still nowadays in your car,

633
00:34:00 --> 00:34:06
there are acid batteries.

634
00:34:02 --> 00:34:08
If I have here
a beaker with acid,

635
00:34:04 --> 00:34:10
for which most commonly is used
sulfuric acid,

636
00:34:08 --> 00:34:14
and I put here in a zinc wire
and here in a copper wire,

637
00:34:14 --> 00:34:20
then this is a battery.

638
00:34:16 --> 00:34:22
I believe this side
of the battery is positive

639
00:34:18 --> 00:34:24
and this is negative.

640
00:34:20 --> 00:34:26
Now, we have them here.

641
00:34:22 --> 00:34:28
We have this sulfuric acid and
we have zinc and we have copper.

642
00:34:26 --> 00:34:32
But if we use only one cell,

643
00:34:28 --> 00:34:34
then I won't be able to light
a small light bulb.

644
00:34:32 --> 00:34:38
Just like with your flashlight
that you have at home,

645
00:34:35 --> 00:34:41
you sometimes have to put in
several cells in series

646
00:34:38 --> 00:34:44
to get a higher voltage

647
00:34:40 --> 00:34:46
so that you can power
a small light bulb.

648
00:34:43 --> 00:34:49
The light bulb that we have here
is only a few watts.

649
00:34:47 --> 00:34:53
It's almost nothing, and
I will still try to get it lit,

650
00:34:51 --> 00:34:57
which is not so easy,

651
00:34:53 --> 00:34:59
because this battery has
a self-destruct in it.

652
00:34:56 --> 00:35:02
The moment that I put
this zinc in there,

653
00:34:59 --> 00:35:05
I get very violent
chemical reactions.

654
00:35:02 --> 00:35:08
The fumes are awful--

655
00:35:03 --> 00:35:09
you may actually smell that
in the first row;

656
00:35:05 --> 00:35:11
it's very awful--

657
00:35:06 --> 00:35:12
and the battery works
only maybe for a few minutes.

658
00:35:10 --> 00:35:16
So I have to do this very fast

659
00:35:12 --> 00:35:18
since it has
a self-destruct built in,

660
00:35:15 --> 00:35:21
and when I do it, I will make it
at the very last minute,

661
00:35:18 --> 00:35:24
I will make it completely dark.

662
00:35:21 --> 00:35:27
So the way I will do that is,

663
00:35:23 --> 00:35:29
why don't we turn
everything off?

664
00:35:27 --> 00:35:33
And now I leave
a few things on first.

665
00:35:29 --> 00:35:35
I can put the copper in.

666
00:35:30 --> 00:35:36
The copper is not the worst.

667
00:35:32 --> 00:35:38
Let me first put the copper in.

668
00:35:34 --> 00:35:40
That's pretty innocent.

669
00:35:37 --> 00:35:43
So I'm going to build four cells
and put them in series,

670
00:35:40 --> 00:35:46
and I have
the copper now in place.

671
00:35:42 --> 00:35:48
So that's not the worst.

672
00:35:44 --> 00:35:50
The moment I put the zinc in,

673
00:35:46 --> 00:35:52
then things begin
to be very unpleasant,

674
00:35:50 --> 00:35:56
but when I make it very dark,
I close the circuit,

675
00:35:55 --> 00:36:01
and I hope you will be able to
see the light-- no pun implied.

676
00:36:02 --> 00:36:08
So let's leave something on
and turn all the rest off.

677
00:36:06 --> 00:36:12
I'm going to make it
very dark very shortly.

678
00:36:09 --> 00:36:15

679
00:36:12 --> 00:36:18
First you still have dim light.

680
00:36:14 --> 00:36:20

681
00:36:17 --> 00:36:23
Aah-- one thing goes in.

682
00:36:19 --> 00:36:25
Ugh! I already smell it.

683
00:36:21 --> 00:36:27
Two things go in.

684
00:36:24 --> 00:36:30
And the third goes in,

685
00:36:25 --> 00:36:31
and now I'm going to make it
completely dark.

686
00:36:30 --> 00:36:36
And now I have to close the loop
with the last piece of zinc.

687
00:36:33 --> 00:36:39
Look at that little light bulb
that is right there.

688
00:36:35 --> 00:36:41
There it goes!

689
00:36:36 --> 00:36:42
There! I see the light!

690
00:36:38 --> 00:36:44
Did you see it?

691
00:36:39 --> 00:36:45
It doesn't last very long,
but it's there.

692
00:36:42 --> 00:36:48
Boy, it was very bright,
wasn't it?

693
00:36:46 --> 00:36:52
You saw it, right?

694
00:36:48 --> 00:36:54
Unmistakable.

695
00:36:50 --> 00:36:56
I have to get this out,

696
00:36:51 --> 00:36:57
because otherwise we will all be
dead by the end of the lecture.

697
00:36:55 --> 00:37:01
(students laugh )

698
00:36:57 --> 00:37:03

699
00:37:02 --> 00:37:08
Okay.

700
00:37:03 --> 00:37:09

701
00:37:05 --> 00:37:11
And let's cover these also up,

702
00:37:06 --> 00:37:12
because this sulfuric acid--
ugh!

703
00:37:08 --> 00:37:14

704
00:37:10 --> 00:37:16
So your lead battery in your car
works with the same idea,

705
00:37:15 --> 00:37:21
except this is lead oxide
and this is lead.

706
00:37:18 --> 00:37:24
So it works
with lead oxide and lead,

707
00:37:20 --> 00:37:26
and it's a very, very
powerful battery.

708
00:37:23 --> 00:37:29

709
00:37:28 --> 00:37:34
There are batteries which are
very fancy which can be charged.

710
00:37:31 --> 00:37:37
Nickel-cadmium is a battery
that can be charged.

711
00:37:34 --> 00:37:40
My electric shaver works
on these batteries.

712
00:37:38 --> 00:37:44
It's wonderful.

713
00:37:39 --> 00:37:45
If I forget to shave
in the morning,

714
00:37:40 --> 00:37:46
I can still do it
before you come in here.

715
00:37:43 --> 00:37:49
That's the great thing
about batteries.

716
00:37:46 --> 00:37:52
This is probably...
this probably consumes 30 watts,

717
00:37:50 --> 00:37:56
30 joules per second
is my rough guess.

718
00:37:52 --> 00:37:58
And I can probably get
one hour of shaving out of that.

719
00:37:55 --> 00:38:01
Probably shave six, seven times,

720
00:37:58 --> 00:38:04
so that's a total
of 100,000 joules--

721
00:38:00 --> 00:38:06
that's not bad--
out of a battery.

722
00:38:02 --> 00:38:08
And you can even recharge it.

723
00:38:05 --> 00:38:11
Now, we all know when you really
need batteries, they're dead.

724
00:38:08 --> 00:38:14
When you're in the mountains
and you need your flashlight

725
00:38:11 --> 00:38:17
because it's really
an emergency,

726
00:38:13 --> 00:38:19
it just so happens
that the batteries are dead.

727
00:38:16 --> 00:38:22
And therefore

728
00:38:17 --> 00:38:23
almost every mountaineer has
with him or her a device

729
00:38:21 --> 00:38:27
which converts mechanical energy
into electricity.

730
00:38:27 --> 00:38:33
(device ratcheting )

731
00:38:29 --> 00:38:35
And this is it.

732
00:38:30 --> 00:38:36
Hey, you see nothing.

733
00:38:32 --> 00:38:38
Oh, my light bulb broke.

734
00:38:34 --> 00:38:40
(students laugh )

735
00:38:36 --> 00:38:42
LEWIN:
Oh, how sad.

736
00:38:38 --> 00:38:44
(more laughter )

737
00:38:39 --> 00:38:45
LEWIN:
When you're in the mountains,
you see, things never work.

738
00:38:44 --> 00:38:50
Let me see whether the bulb
may not have been tight.

739
00:38:48 --> 00:38:54
(Lewin chuckles )

740
00:38:49 --> 00:38:55
What a tragedy.

741
00:38:55 --> 00:39:01
(device ratcheting )

742
00:38:56 --> 00:39:02
No.

743
00:38:57 --> 00:39:03
The light bulb gave up.

744
00:38:59 --> 00:39:05
I can't show you the light.

745
00:39:00 --> 00:39:06
But you've seen it there.

746
00:39:01 --> 00:39:07
I'm sorry--
that's the way it works.

747
00:39:04 --> 00:39:10
I have to put
a new light bulb in.

748
00:39:05 --> 00:39:11

749
00:39:08 --> 00:39:14
I'll give you back
your view graph,

750
00:39:10 --> 00:39:16
because I'm going to talk
about a few more numbers,

751
00:39:13 --> 00:39:19
and they're all here, so you
don't have to copy anything.

752
00:39:18 --> 00:39:24
It'll all be on the Web.

753
00:39:20 --> 00:39:26
The world energy consumption

754
00:39:22 --> 00:39:28
of the entire world
of six billion people--

755
00:39:25 --> 00:39:31
by the way, the six billionth
was born two days ago.

756
00:39:27 --> 00:39:33
Have you heard about that
on the radio?

757
00:39:29 --> 00:39:35
6.00000 billion people
now on Earth--

758
00:39:33 --> 00:39:39
is about four times
ten to the 20 joules per year.

759
00:39:37 --> 00:39:43
That is the entire consumption.

760
00:39:39 --> 00:39:45
The United States has only
1/30 of the world population

761
00:39:43 --> 00:39:49
and consumes one-fifth of that.

762
00:39:45 --> 00:39:51
We are really energy spoilers,
big energy spoilers.

763
00:39:49 --> 00:39:55
The sun is a wonderful source
of energy.

764
00:39:54 --> 00:40:00
The sun has a power of four
times ten to the 26 watts--

765
00:40:03 --> 00:40:09
four times ten to the 26
joules per second--

766
00:40:06 --> 00:40:12
mostly in the visible light
and some in the infrared.

767
00:40:11 --> 00:40:17
If the sun is here
and the Earth is here,

768
00:40:15 --> 00:40:21
and you can calculate how much
of that energy reaches the Earth

769
00:40:19 --> 00:40:25
at the distance of the Earth--

770
00:40:21 --> 00:40:27
so you have to know the
distance, but we know that;

771
00:40:23 --> 00:40:29
that is 150 million kilometers.

772
00:40:25 --> 00:40:31
And so that energy goes out

773
00:40:27 --> 00:40:33
radially, symmetrically,
isotropically in all directions,

774
00:40:31 --> 00:40:37
and so it's very easy.

775
00:40:32 --> 00:40:38
You know that the surface area
of this sphere

776
00:40:34 --> 00:40:40
is four pi r squared,
and so you can calculate

777
00:40:37 --> 00:40:43
how much for every square meter
reaches the Earth.

778
00:40:42 --> 00:40:48
And that is a classic number
that almost everyone knows,

779
00:40:47 --> 00:40:53
certainly people
who are in solar energy.

780
00:40:49 --> 00:40:55
That is 1,400 watts
per square meter.

781
00:40:53 --> 00:40:59
That is what reaches the Earth.

782
00:40:56 --> 00:41:02
That is about 100 million joules
per square meter every day.

783
00:41:03 --> 00:41:09
It would be nice
if we could harvest that,

784
00:41:07 --> 00:41:13
and it would be nice

785
00:41:08 --> 00:41:14
if we could use that 100 million
joules per square meter per day

786
00:41:13 --> 00:41:19
to provide the world

787
00:41:15 --> 00:41:21
with this four times
ten to the 20 joules per year.

788
00:41:19 --> 00:41:25
To do that, you would need
ten to the ten square meters

789
00:41:26 --> 00:41:32
to absorb that solar energy.

790
00:41:28 --> 00:41:34
That's trivial.

791
00:41:29 --> 00:41:35
That's only the size of Holland.

792
00:41:31 --> 00:41:37
No big deal.

793
00:41:32 --> 00:41:38
If we lose Holland,
that's no big deal, so...

794
00:41:34 --> 00:41:40
(students laugh )

795
00:41:36 --> 00:41:42
However, there is a catch.

796
00:41:38 --> 00:41:44
There is day and night, which
we haven't allowed for yet.

797
00:41:41 --> 00:41:47
We just assumed that
the sun was always there.

798
00:41:44 --> 00:41:50
There are clouds.

799
00:41:45 --> 00:41:51
And then the sun rises
and the sun sets,

800
00:41:48 --> 00:41:54
and of course
if the sun is at the horizon

801
00:41:50 --> 00:41:56
and here is your plane
where you try to absorb the sun,

802
00:41:53 --> 00:41:59
you get nothing, so you have

803
00:41:54 --> 00:42:00
the cosine of the angle has
to be taken into account.

804
00:41:58 --> 00:42:04
And then the efficiency
of the units that you're using,

805
00:42:01 --> 00:42:07
with which you capture
the solar energy

806
00:42:03 --> 00:42:09
could be solar cells.

807
00:42:05 --> 00:42:11
It's a very low efficiency.

808
00:42:07 --> 00:42:13
And if you take all that
into account,

809
00:42:09 --> 00:42:15
you would need an area
more like 400 by 400 miles.

810
00:42:13 --> 00:42:19
Now you're really talking.

811
00:42:14 --> 00:42:20
That's something like
the whole of England

812
00:42:16 --> 00:42:22
and the whole of France.
 

813
00:42:17 --> 00:42:23
And so not only are the costs
staggering, but it is

814
00:42:21 --> 00:42:27
simply beyond our present
technological capabilities.

815
00:42:26 --> 00:42:32
So solar energy plays a very
small role in our world economy.

816
00:42:31 --> 00:42:37
Nuclear energy, which is the
fission of uranium or plutonium,

817
00:42:36 --> 00:42:42
was very popular in the '70s,

818
00:42:38 --> 00:42:44
but it has become a little bit
less popular lately.

819
00:42:40 --> 00:42:46
We had the Three Mile Island
accident in our own country,

820
00:42:43 --> 00:42:49
and you've heard
just a few weeks ago

821
00:42:45 --> 00:42:51
about the nasty accident
that there was in Japan.

822
00:42:49 --> 00:42:55
So people are,
understandably so,

823
00:42:51 --> 00:42:57
emotionally strongly biased

824
00:42:53 --> 00:42:59
against the use
of nuclear energy.

825
00:42:57 --> 00:43:03
But nuclear energy is all
around me, at least every day.

826
00:43:01 --> 00:43:07
I have a very special collection
of Fiestaware,

827
00:43:05 --> 00:43:11
which is American tableware

828
00:43:07 --> 00:43:13
which was designed and built
in the '30s, in 1937,

829
00:43:13 --> 00:43:19
and it went on until the '50s.

830
00:43:15 --> 00:43:21
And here I brought you
some of this.

831
00:43:17 --> 00:43:23
This is a ten-inch plate,
and this is called "Fiesta red."

832
00:43:22 --> 00:43:28
Even though it's orange,
we still call it Fiesta red.

833
00:43:24 --> 00:43:30
It has uranium oxide in it.

834
00:43:26 --> 00:43:32
That red is uranium oxide.

835
00:43:28 --> 00:43:34
That is the same uranium
that powers nuclear reactors.

836
00:43:32 --> 00:43:38
This is cobalt; 
it has no uranium in it.

837
00:43:35 --> 00:43:41
And this, again, it's
my cup of tea-- radioactive.

838
00:43:40 --> 00:43:46
Uranium oxide.

839
00:43:41 --> 00:43:47

840
00:43:44 --> 00:43:50
Okay.

841
00:43:45 --> 00:43:51
You ready for this?

842
00:43:47 --> 00:43:53
You hear this?

843
00:43:49 --> 00:43:55
This is a Geiger tube.

844
00:43:51 --> 00:43:57
It can measure the gamma rays
that the uranium emits

845
00:43:55 --> 00:44:01
when it spontaneously
breaks up in pieces

846
00:43:57 --> 00:44:03
and energy is released--
we call that fission.

847
00:44:00 --> 00:44:06
You'll hear a little beep.

848
00:44:02 --> 00:44:08
I'll hold it close
to my microphone.

849
00:44:04 --> 00:44:10
(rapid high-pitched beeping )

850
00:44:07 --> 00:44:13
That's the plate
from which I eat.

851
00:44:11 --> 00:44:17
(much slower beeping )

852
00:44:13 --> 00:44:19
This cup has no uranium oxide.

853
00:44:15 --> 00:44:21
But my cup of tea...

854
00:44:17 --> 00:44:23
(rapid high-pitched beeping )

855
00:44:21 --> 00:44:27
Radioactive.

856
00:44:23 --> 00:44:29
So if you want
to come for dinner,

857
00:44:25 --> 00:44:31
you're more than welcome
to do so...

858
00:44:27 --> 00:44:33
(students laugh )

859
00:44:28 --> 00:44:34
LEWIN:
But you know what you're in for.

860
00:44:30 --> 00:44:36

861
00:44:34 --> 00:44:40
We have fossil fuel on Earth.

862
00:44:37 --> 00:44:43
We are consuming at this moment
the fossil fuel

863
00:44:41 --> 00:44:47
at a rate which is
a million times faster

864
00:44:45 --> 00:44:51
than nature could create it--
one million times faster.

865
00:44:49 --> 00:44:55
And if we consume it
at the present rate,

866
00:44:52 --> 00:44:58
or increase maybe
by only three percent per year,

867
00:44:55 --> 00:45:01
then we won't have any left
in less than 100 years.

868
00:45:01 --> 00:45:07
So we have an energy crisis--
a real energy crisis.

869
00:45:05 --> 00:45:11
And we have
an environmental problem,

870
00:45:08 --> 00:45:14
because all these power plants
and all the industries

871
00:45:11 --> 00:45:17
cause pollution.

872
00:45:14 --> 00:45:20
And so what are we going
to do about it?

873
00:45:16 --> 00:45:22
My own energy consumption
is quite modest, I think,

874
00:45:19 --> 00:45:25
although I am also
in your country,

875
00:45:21 --> 00:45:27
so I'm sure I also consume

876
00:45:23 --> 00:45:29
six times more than the
average person in the world.

877
00:45:26 --> 00:45:32
I use electricity,
for which I get a bill.

878
00:45:29 --> 00:45:35
I have gas heat; 
I heat with gas.

879
00:45:32 --> 00:45:38
And I have also
cooking with gas.

880
00:45:35 --> 00:45:41
I use my car-- gasoline.

881
00:45:37 --> 00:45:43
And when I add that all up,

882
00:45:39 --> 00:45:45
I think I consume roughly
400 million joules per day.

883
00:45:44 --> 00:45:50
That 400 million joules per day
is the equivalent

884
00:45:48 --> 00:45:54
of having 100 slaves working
for me like dogs 12 hours a day.

885
00:45:57 --> 00:46:03
Think about that.

886
00:45:59 --> 00:46:05
What a luxury, what
an incredible time we live in.

887
00:46:03 --> 00:46:09
One hundred slaves are working

888
00:46:05 --> 00:46:11
for every single person here
in my audience 12 hours a day,

889
00:46:10 --> 00:46:16
working like dogs
to make you live comfortably.

890
00:46:16 --> 00:46:22
For one kilowatt-hour
of electricity,

891
00:46:18 --> 00:46:24
which is four million joules,
I pay only a lousy ten cents.

892
00:46:22 --> 00:46:28
My entire energy bill
for those 100 slaves

893
00:46:26 --> 00:46:32
is no more than $150 a month.

894
00:46:29 --> 00:46:35
What a bargain

895
00:46:30 --> 00:46:36
to have 100 slaves working
for you for $150 a month.

896
00:46:35 --> 00:46:41
But now comes
the $64 million question:

897
00:46:38 --> 00:46:44
How are we going
to continue this?

898
00:46:41 --> 00:46:47
Because we are running out
of fossil fuel,

899
00:46:44 --> 00:46:50
and nuclear energy has
its problems.

900
00:46:47 --> 00:46:53
Well, the only way
that we might survive--

901
00:46:52 --> 00:46:58
the quality of life is at stake
here-- is nuclear fusion.

902
00:46:56 --> 00:47:02
Not fission, whereby uranium and
plutonium breaks up in pieces,

903
00:46:59 --> 00:47:05
but fusion.

904
00:47:01 --> 00:47:07
If you could merge deuterium
with deuterium, you gain energy.

905
00:47:06 --> 00:47:12
Now, we have

906
00:47:07 --> 00:47:13
one out of every 6,000 hydrogen
atoms on Earth is deuterium,

907
00:47:12 --> 00:47:18
and we have a billion
cubic kilometers of water.

908
00:47:15 --> 00:47:21
Now, it is unclear
whether we will ever succeed

909
00:47:18 --> 00:47:24
in making a fusion reactor
working.

910
00:47:21 --> 00:47:27
That is still
completely unclear.

911
00:47:23 --> 00:47:29
People work hard on it.

912
00:47:25 --> 00:47:31
But if we succeeded,

913
00:47:26 --> 00:47:32
then simply the oceans
would provide the world,

914
00:47:31 --> 00:47:37
if we consume it

915
00:47:32 --> 00:47:38
at that same rate that
we are consuming today--

916
00:47:34 --> 00:47:40
four times ten to the 20
joules per year--

917
00:47:36 --> 00:47:42
we would have enough energy
for 25 billion years.

918
00:47:40 --> 00:47:46
All the worries are over,

919
00:47:42 --> 00:47:48
because the Earth is
not going to survive

920
00:47:44 --> 00:47:50
for any more
than five billion years.

921
00:47:45 --> 00:47:51
Five billion years from now,

922
00:47:47 --> 00:47:53
the sun will become a hundred
times bigger than it is now,

923
00:47:50 --> 00:47:56
and it will just swallow up
the world,

924
00:47:53 --> 00:47:59
and it will be the end of MIT,
of everything.

925
00:47:55 --> 00:48:01
(students chuckle )

926
00:47:56 --> 00:48:02
LEWIN:
So all we have to think of

927
00:47:58 --> 00:48:04
is in terms of energy
for about five billion years.

928
00:48:05 --> 00:48:11
I want to leave you with
what I call a brain teaser.

929
00:48:09 --> 00:48:15
I have here a very special ball.

930
00:48:13 --> 00:48:19
And I'm going to bounce
this ball,

931
00:48:17 --> 00:48:23
and I want you to look at it

932
00:48:20 --> 00:48:26
and tell me what you think is
the source of that energy.

933
00:48:25 --> 00:48:31

934
00:48:28 --> 00:48:34
It's important that
we have little light,

935
00:48:32 --> 00:48:38
because if there's
too much light,

936
00:48:34 --> 00:48:40
then you won't see it well.

937
00:48:37 --> 00:48:43
So, this is a ball.

938
00:48:41 --> 00:48:47
See, I have another one here.

939
00:48:43 --> 00:48:49
And I will bounce it here,
and then notice what you see.

940
00:48:47 --> 00:48:53

941
00:48:53 --> 00:48:59
Just keep looking.

942
00:48:55 --> 00:49:01

943
00:48:58 --> 00:49:04
It stops.

944
00:48:59 --> 00:49:05
The other one.

945
00:49:00 --> 00:49:06

946
00:49:07 --> 00:49:13
And the other one.

947
00:49:08 --> 00:49:14

948
00:49:11 --> 00:49:17
Now, I want you
to think about...

949
00:49:13 --> 00:49:19
you've seen now what happens.

950
00:49:15 --> 00:49:21
I bounce it, it starts blinking.

951
00:49:17 --> 00:49:23

952
00:49:19 --> 00:49:25
Clearly, there's mgh available
when I bounce it.

953
00:49:24 --> 00:49:30
Where does the energy come from
of the blinking light?

954
00:49:28 --> 00:49:34
Think carefully
before you give an answer.

955
00:49:31 --> 00:49:37
It took my graduate students
and me, embarrassingly,

956
00:49:34 --> 00:49:40
at least ten minutes
before we had the answer.

957
00:49:38 --> 00:49:44
Think about the fact
that they continue to blink

958
00:49:43 --> 00:49:49
and then stop.

959
00:49:44 --> 00:49:50
Talk about it among yourselves.

960
00:49:46 --> 00:49:52
Think about it
when you have dinner, breakfast,

961
00:49:48 --> 00:49:54
when you take your shower.

962
00:49:49 --> 00:49:55
And discuss it on PIVoT.

963
00:49:51 --> 00:49:57
See you next Friday.

964
00:49:54 --> 00:50:00

965
00:50:03 --> 00:50:09.000