Jeff_42 said:
“I think the only 100% effeciency of (something) to energy would be anti-matter/matter collisions. I’m talking from brain freeze here but I think it would only take about an ounce of antimatter to put the spcae shuttle in orbit. Nuclear weapons, as powerful as they are, only manage around a 3-5% mass-to-energy conversion…maybe less. Fuel combustion (i.e. burning gasoline) is on the order of .001% efficient in converting itself to energy.”
You’re talking about mass-to-energy conversion efficiency here, which is a rather different topic altogether!
The energy released in a chemical reaction or stored in a battery can be measured fairly precisely from its heating effect. For example, the energy released when burning 1 kg of gasoline is about 47 000 000 Joules. That is enough energy to raise the temperature of a tonne of water by 11 degrees celcius.
When we talk about “energy efficiency”, we generally mean “what proportion of the energy released can be employed to do something besides heating stuff up?” Usually for scientific purposes, we are talking about raising a weight against gravity. This is called “doing work”. In theory, those 47 000 000 joules of energy released by burning a kg of gasoline could raise a 4.8 tonne weight a height of 1000 metres.
However, if you actually got yourself a 4.8 tonne weight, and a gasoline engine and a winch and tried to haul the weight up a 1000 metre cliff, the best you could do would be about 250 metres. This is because a gasoline engine running under optimum conditions is only about 25% efficient at doing work with the energy released. The rest comes out as heat, mostly through the exhaust.
(automobiles do a LOT worse than this because the engines are throttled down almost all of the time.)
Batteries actually do quite well. If you take a fully charged lead-acid battery and discharge it through a heating element, measuring all the heat released (including the battery warming up) you find they store about 200 000 joules per kilogram.
Use the same battery and an electric motor to raise a weight, and you can get efficiencies of 75% or more if you’re careful.
As for the OP, I just don’t know. What you’d have to do is get a human to do a certain amount of work, say on a treadmill, and measure the amount of “fuel” they burnt by measuring the CO2 they breathed out while doing it. That’s one for the sports scientists. My hunch is that a battery-motor combination is more efficient, but they work in completely different ways.
Phase transition point - put a pan of water on the stove with a thermometer in it. When it reaches 100 celcius, the water doesn’t all flash to steam in an instant, right? Instead it sits there, boiling away. You have to put extra energy in it to actually turn it into steam, typically 4-5 times as much as it took to raise it to 100 celcius in the first place.
As for the AAA battery thing, the signal from your brain isn’t doing the actual work! It’s just switching your muscles on, and THEY do the work.