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Q & A: hydrogen pertual motion engine?

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Most recent answer: 06/12/2009
Q:
Is it not possible to create a perpetual hydrogen engine? This would work the same way as a regular hydrogen engine, but the water vapor would come back through a separate valve and undergo electrolysis so that you would have the same amount of hydrogen and oxygen still in the vehicle constantly undergoing a perpetual energy source. Would this work?
- GAA (age 15)
Flower Mound, TX, US
A:
No, it won't. The laws of thermodynamics say that it always requires more energy input to separate the water into hydrogen and oxygen than you can get back out as work when they recombine. The engine wouldn't be strong enough to supply the power for the generator to run the electrolysis.

Mike W.

(published on 06/05/2009)

Follow-Up #1: energy sources

Q:
What if the energy needed to separate and combine the two was generated by wind turbines or solar panels?
- GAA (age 15)
Flower Mound, Texas
A:
Sure, you could use electricity generated by wind or solar to electrolyze water, then use the hydrogen for fuel. You don't need extra energy to combine it with oxygen to get water back. That's the step that releases energy to run something. The problem is that it always releases less usable energy than it took to generate in the first place.

You could use the electricity directly, which is more efficient. When a portable power source is needed, right now it's more efficient to charge batteries than to generate hydrogen, but it's conceivable that could change with new technology.


In any case you don't have a perpetual motion machine. The solar power (which also drives wind) will run out when the Sun runs out of fuel.

Mike W.

(published on 06/12/2009)

Follow-up on this answer.