Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: perpetual motion?

Learn more physics!

Most recent answer: 10/22/2007
Q:
I don't understand the first law of thermogenics. Why can't a perpetual motion holder be made?
- Joseph McNeary (age 33)
Maryland Heights, MO usa
A:
I guess you mean the combination of the first and second laws of thermodynamics. The first law says that energy is conserved.  The second says that total entropy never decreases. It turns out that means that in order to pull thermal energy out of the vast sea of thermal energy around us, something has to get closer to equilibrium. Either two regions at different temperatures get closer to the same temperature, or some fuel gets used up, or something. After a while, when things get close to equilibrium, there's no more free energy left to power the machine, so it won't run forever.

I haven't really explained WHY entropy always increases. It's maybe the most universal law of physics, but we don't have a solid underlying explanation for why it has to be true.

In other words,  "There's no such thing as a free lunch"

Mike W.
Lee H

(published on 10/22/2007)

Follow-up on this answer.