The Many Forms of Energy

I said last time that energy is nothing mysterious, only matter in motion. I also said there is a wide variety of ways that different kinds of matter can be in different kinds of motion. Here's some of them.

A single particle of mass moving in a straight line. Kinetic energy.

A big solid bunch of mass moving in a straight line. Also kinetic energy.

A solid bunch of mass rotating. Rotational kinetic energy.

A block of mass swinging from a rope. In this case kinetic energy is alternately taken up and released by gravity as the mass swings back and forth. As a system, the pendulum can be regarded as having vibrational energy.

A block of mass bouncing on a spring. Also regarded as a system. Kinetic energy is alternately taken up and released by the electrostatic forces between the atoms in the spring.

A block of mass ringing like a bell. This vibrational energy is very minute, but is basically the same as a bunch of little masses on little springs. A guitar string is a simple example of this.

A block of mass with all its atoms jiggling randomly. This is always happening anyway, but the hotter something is, the more vigorously it happens. Temperature is a direct measure of the amount of energy in the form of heat that something has.

In the real world, a block of matter could have all of these at the same time.

If we roll a ball along a level field, it doesn't keep going. Friction, we call it. But what's really going on?

The one thing to know about energy is that, while it can never be created from nothing and it can never disappear, it likes to spread out. It is always trying to disperse itself. Imagine you had a box of atoms that were all perfectly still, and you threw another atom in very hard. Now you have a box of perfectly still atoms with one very energetic, speedy atom. Will things stay that way for long? Of course not. As time goes on the speedy atom will share its kinetic energy with all the others through numerous collisions. Eventually, the energy will be evenly and randomly shared among all the atoms in the box.

In a similar manner, kinetic energy of large masses gets spread out by being shared with smaller objects, air molecules, the atoms of things it rubs against, or its own constituent atoms. For example, drop a bag of sand on the ground. Does it bounce? Where did the kinetic energy go that it had just prior to hitting? All the grains of sand inside it rubbed against each other and each of them became slightly hotter. Given half a chance, kinetic energy eventually turns into heat energy, because heat energy is more spread out.

Heat energy is still a form of energy, and as such tries to spread out even further. It's the one thing to know about energy: it is always trying to spread itself out. The result is that heat always flows in the direction of hot to cool.

Heat is one of the most fascinating forms of energy and one of the trickiest to understand. If you keep in mind the one thing to know about energy, you won't go wrong. Heat is made from more concentrated forms of energy and is always spreading itself out.

Next time, more about heat and how it relates to temperature.