A hot object contains energy in the form of the random microscopic motions. It would be great if we could capture all this energy and use it for something -- polishing the floors, running the lawnmower, pumping water out of a well. But the Second Law has an important consequence here, because the energy in the hot object is shared among all of its parts, and to concentrate it and organize it into a visible motion is going the wrong way, thermodynamically.

We have compared temperature to the level of a lake several times. Trying to turn a temperature into usable energy is like trying to run a water wheel using a lake: it only works if there is a way to let water run out of the lake, to a lower level. Similarly, if we have two objects that are at different temperatures, we can produce some usable energy while letting heat flow from one to the other. The usable energy is more ordered, but this is offset by the gain in disorder (energy is shared between the two objects). We can't turn all of the energy of a hot object into useful energy, but we can turn some of it into useful energy.

One way to turn heat into motion -- the "Drinking Bird"

Here is a toy that you have probably met already. This may not look like a device to produce useful energy, but you could imagine harnessing a few thousand of them to make a baby-rocking machine. The Drinking Bird keeps itself in motion, despite friction and all the other ways of sharing energy with the rest of the universe -- how does it do this? What is the fuel, and how is it converted into motion?

Let's study the drinking bird for a few minutes. You have one in your kit.

• Holding the bird upright, notice what happens when you are touching the bottom. The liquid inside is being forced up the neck.
  Is the bird a kind of thermometer?
• While continuing to hold the bird's bottom, touch his or her head with an ice cube. What happens?
• Which heat transport mechanism does this resemble? What is going on?
• Why does this cause the liquid to move up the tube?
• When the liquid gets too high, the bird becomes unstable and falls over.
  How does it get back up?
• In normal use, the bird doesn't have you there to warm its bottom. Yet the bird keeps on bobbing up and down.
  Why is the bird's head always cooler than the bird's bottom?
• According to the Second Law of Thermodynamics, the bird's behavior must be causing an increase in the disorder of the universe, and yet what we see is that it is managing to produce motion (a very ordered kind of energy) from heat.
  Where is this increase in disorder?

• Be patient. If the liquid is only slowly rising up the tube, you just have to wait.
• If the bird doesn't ever tip over, even though the liquid gets very high in the tube, his collar is too high. Push on the bottom while holding the wings of his collar, perhaps twisting slightly to get him to move.
• If the bird tips over and stays that way, try a taller glass. Raising the collar slightly might also fix this, but this is a tricky adjustment.
• The bird's head should be damp, and his bottom dry.

Refrigerators and heat pumps

The Second Law implies that energy can only move from hot to cold by itself. However, sometimes we want to make energy go the wrong way.

Refrigerators don't "make cold." They move energy from inside to the outside, making the ice cream colder while warming up the kitchen. Of course this can't happen by itself, but we can make it happen by expending some energy: you have to plug the refrigerator in.

The amount of energy that it takes to keep a refrigerator running is small compared to the amount of energy that is moved. This is the basic idea behind the heat pump, which is a kind of home heating system. It is exactly a refrigerator, but instead of removing energy from the freezer compartment, it removes it from your back yard; the energy ends up in your living room. Using electrical energy to pump heat into your house is a better idea than turning it directly into heat using a space heater or running the electric stove, because you get more heat for your electrical dollar. This is especially true when it isn't very cold outside, because pumping heat up a small temperature difference is like pumping water up a small height -- it doesn't require much energy.
Check the box when you are done:  

What have we learned about irreversibility?