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Q & A: Converting between kinetic and thermal energy

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Most recent answer: 01/06/2014
Kinetic energy can be turned into thermal energy, so why can't thermal energy be be turned into kinetic energy?
- Grace (age 11)

Hi Grace,

Actually, thermal energy is partly a form of kinetic energy; some of it refers to the motion of tiny particles inside a substance. This motion has kinetic energy, but we don't see it because the particle directions are random and change very rapidly. This is most easy to picture in a box of gas, where particles bounce randomly off each other at high velocity. Even in liquids and solids the particles move around with this thermal energy, but there's also potential energy from the springy contacts between the particles. So, the main difference between thermal and kinetic energy is that the first one is random motions of particles that you mostly can't see by eye, while the second is a unified motion of these same particles, which you can then see.

With a microscope, however, you can see particles just small enough to see that random thermal motion. It's called Brownian motion. (See )

As you say, kinetic energy of large objects can be converted into this thermal energy. For example, if you drop a water balloon onto the ground, its kinetic energy is converted mostly to thermal energy. If the balloon weighs 1 kilogram and you drop it from about 2 meters, it will heat up by less than .005 degrees Celsius. 

At first glance it is amazing that this visible kinetic energy from falling is so easily absorbed into a tiny change in microscopic motions. This makes sense, however, when you realize that the particles in most objects are moving much more rapidly than the balloon is falling, so the new kinetic energy is just a tiny addition of energy compared to what was already there. And how exactly is the kinetic energy turned to thermal energy? Simple, the unified downward motion of all the particles is turned to random motions by colliding with the earth.

So why can't thermal energy turn to kinetic energy? Actually, it can. In principle, if all the moving particles in a water balloon had a slight net upwards velocity, the balloon could lift off the ground in the reverse process as the falling balloon. This of course never happens, because it's just waaaay too unlikely; getting so many particles to randomly align would be like rolling a dice and getting the number 1 billions and billions of times in a row. However, we have found more clever ways to extract kinetic energy from thermal energy. These devices are called heat engines.

For example, in a steam engine, pistons remove some thermal energy from hot gases. The steam engine then uses these moving pistons to power whatever it is hooked up to, thus converting thermal energy into kinetic energy. These heat engines are always limited by the same laws of chance that keep the water balloon from just jumping up. They can't work if the temperature of all the parts is the same. They only convert a fraction of the thermal energy to big-scale energy, and that fraction gets small as the temperature differences of the parts gets small.

Hope that makes sense,

David Schmid

(published on 01/06/2014)

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