How Everything Works
How Everything Works How Everything Works

Question 708

Why is it impossible to make a wheel that turns forever, all by itself? — AWG, Karachi, Pakistan
While people are always trying to build perpetual motion machines, they will never succeed. All of these devices are intended to obtain useful energy—what physicists call "work"—from either nowhere or from less useful energy—what physicists often call "heat" or "thermal energy". Obtaining work from nowhere is really impossible; energy is a conserved quantity, meaning that it simply cannot be created or destroyed. For a machine to do work, it must obtain energy from somewhere or something else. So if anyone tries to sell you a car engine that doesn't take any fuel at all—thus creating work out of nowhere—don't buy it! It's a fraud.

As for machines that try to convert thermal energy completely into work, they are also impossible, but for a different reason. While they don't violate the conservation of energy, they do violate the laws of thermodynamics. Thermal energy is disordered energy—it is energy that has been distributed randomly among the individual atoms and molecules in an object so that it cannot be easily reassembled to do useful work. When you burn a candle, all of the energy the candle once had is still in the room, but it's much harder to use. Just as a coffee cup is much more useful before you drop it than after you drop it, so energy is much more useful before you disorder it than after you disorder it. The difficulty with reassembling thermal energy to do useful work is a statistical one: it's unlikely that this energy will spontaneously reassemble itself in a useful manner, just as its unlikely that a dropped coffee cup will spontaneously reassemble itself in a useful manner. The laws of mechanics don't prevent either of those reassemblies from occurring, but both reassemblies are statistically very unlikely to occur. How often have you dropped a broken cup and had it fall together rather than apart? So if someone tries to sell you a car engine that uses the thermal energy in the surrounding air as "fuel"—thus turning thermal energy completely into work—don't buy it! It's also a fraud.


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