This weekend David (age 11) and I spent a lot of time talking about perpetual motion machines. He has been convinced for about a year that he can create a perpetual motion machine from magnets. So we actually got out a bunch of magnets this weekend and tried many different ideas.
Why does David think that there is extra energy in magnets? The logic might go something like this: "When a magnet is repelling another magnet and holding it suspended in the air, do you know how much energy that takes? If we were to suspend something in the air with a rocket engine or a helicopter, it takes a huge amount of energy." My question back would be, "If you put a mailbox on a post, the post suspends the mailbox in the air. Why isn't the post 'creating energy'?"
"There is no air gap."
"What if we suspend the mailbox in the air with a rope. Now there is an air gap. Why isn't the rope 'creating energy'?"
"Because the rope has to attach to something."
That is the correct answer. And it is the same thing that is happening with the magnets. Because the bottom magnet is sitting on a table, the two magnets are acting like a post, not like a helicopter. The air gap is deceptive.
David is not the only person to have these ideas. There are plenty of adults dreaming about perpetual motion machines on the Internet. Here is a typical video:
In the video, all he has to do is raise up the magnet to get past the "hump" created by his magnet arrangement (if you ever play with strong magnets you can feel the hump he is talking about). The thing to understand is that the process of raising and lowering the magnet is what provides the energy that spins the wheel. If you were to raise and lower the magnet with a cam and lever attached to the wheel, it would not work. His fingers are pushing down with a little extra force, and that extra force pumps in enough energy to spin the wheel. He has no way to sense or measure the extra force, so it is invisible to him.
Here is another typical magnet arrangement for these types of machines:
So we talk all through this, we try out different arrangements of magnets, we look at examples on the Internet, and I think he is finally starting to understand. When it comes to energy in a system, you can't get something for nothing.
But then he switches over to another medium. What about the sun? It's creating energy out of nothing, right? Well, no, it is creating energy out of matter. How does it do that? This leads to a long chain of discussions on nuclear fission and fusion, nuclear decay, etc.
And somehow we end up at breeder reactors. His question, ultimately: "Is a breeder reactor a perpetual motion machine?" For example, the Fast breeder reactor page on Wikipedia states:
The fast breeder or fast breeder reactor (FBR) is a fast neutron reactor designed to breed fuel by producing more fissile material than it consumes. The FBR is one possible type of breeder reactor.
This sounds promising - a machine that makes its own fuel. But a little research shows it is not a perpetual motion machine either. You have to feed in something (like Uranium-238 or Thorium-232) in order to breed the new fuel. However, on a typical human time scale, it is close to perpetual. If we were using breeder reactors, our supplies of U-238 and Thorium-232 would look nearly infinite, without adding any CO2 to the atmosphere.