# Q & A: kinetic and potential

Q:
Hi! I am a student at the university of malta and I am studying education. I will soon to teaching in a school as part of my training. now energy is classified into two: Kinetic and Potential...but which types of energies are kinetic and which are potential?? I found different versions in different books and I am confused....the way I see it is, heat, movement, and sound are kinetic while all the others are potential. But is light actually potential energy??
- Amanda (age 20)
university of malta, europe
A:
Amanda- that's a nice question, but our answer will be a little indirect.

For some forms of energy, it's very convenient to classify them as kinetic or potential. If you're watching large objects move around, there's obvious kinetic energy associated with the motion. That kinetic energy is reduced if, for example, a ball happens to fly upward. We say that when it's up it has gravitational potential energy. That gets converted back to kinetic on the way down. Likewise, if you compress a spring, we say it has potential energy. Similarly, pulling two oppositely charged particles apart results in potential energy, which you can say is stored in the electric field.

Taking these categories which are useful for large-scale mechanical properties and imposing them on all phenomena can be misleading. Take light, for example. You can think of it as consisting of photons whizzing around, and call its energy entirely kinetic. Or you can say that it is made up of electrical and magnetic fields, and we just said that we usually call the field energy potential. Which is right? There is no answer, because we're just talking about name choices, not about how things actually behave.

Now on to some of your other specific energy forms, trying to give conventional answers:

Heat: The thermal energy in a material partly consists of particles jiggling around, so that's kinetic. In a gas, that's the main piece. In a solid, as they jiggle the particles squeeze and pull on each other, like little springs being compressed and stretched. So that part is potential. In a typical solid, the thermal energy is half kinetic and half potential.

Sound: In a solid, this is half kinetic and half potential, like the thermal energy. In fact, the thermal energy is just a bunch of sound waves moving around randomly. Something similar holds for a liquid. In a gas, the sound energy is kinetic, if you look at it on the microscopic scale of moving molecules.

So these classifications are useful but not to be taken too seriously because nature doesn't care what names we call it. What does count is which equations it obeys.

Mike W.

(published on 10/22/2007)