What Exactly is Heat?

Energy is nothing more mysterious than motion. Things that are moving have energy, which is another way of saying that they are moving.

Suppose the countless atoms that make up an object are all moving with tiny, random motions in all directions at once. It's matter, and it's moving. So it's also energy. When it's in little pieces comprising a large object, we call it Heat.

Scientists have technical words they need to use such as "internal thermal energy," but we know that it's really just lots of little motions of lots of little objects in many directions at once. The atoms may be vibrating, spinning, or actually wandering about (if it's a liquid or a gas). It's not really different to the energy of a car whizzing down the street, just a lot smaller and a little trickier to keep track of. Remember, physics is mostly about being a good energy accountant.

Rather than a speedometer, we use a thermometer to keep track of heat energy. The temperature tells us the average speed of the many moving bits. The higher the temperature, the higher the average speed and therefore the more energy is in there. Trust me, it's easier that trying to put a little speedometer on each individual atom.

The temperature is higher when there's more energy. What if all the atoms were to stop at the same time? What would the temperature be then? Answer: -273 C. That's Zero on the absolute temperature scale, or 0 Kelvin.

I have a 1 kg block of ice in front of me that is at a temperature of negative 10 C, or ten below zero. If I add some heat to it, the temperature will go up. If I add 2 kiloJoules of energy (or heat) to it, the temperature will go up by 1 degree. If I add 20 kJ, the temperature goes up by 10 degrees. It's easy to do, but hard to keep track of. How do I add heat? By doing nothing.

Heat always spreads out. It does whatever it can to get away from high temperatures and move to lower temperatures. Just by leaving the block of ice sitting out, heat from the surrounding 20 C room moves towards the -10 C ice in whatever way it can. In this case, mostly through air currents.

Air next to the ice block gets cold. Cold air is heavier, and starts to sink down. Warm air then takes its place next to the ice, and the whole process repeats automatically. As the ice absorbs heat from the air in the room, the ice warms up. One result of that is that the rate of warming slows down. The other result is that the ice eventually starts to melt.

Ice melts at a temperature of 0 C. As we add more heat, more ice melts. But the temperature does not increase until all the ice is melted! Why not?

Water molecules in ice are stuck together and cannot move around. Water molecules automatically stick together when they are not moving very fast, in other words, when the temperature of the water is low (below 0 C, to be exact). If we want them to be unstuck and form a flowing liquid, then we have to give all the water molecules enough motion (meaning energy or heat) to move clear of each other. As we add heat, the temperature stays at 0 C until all the ice is melted. When we have added 335 kJ of heat, then the entire 1 kg block of ice will have melted. Scientists call that "The Latent Heat of Fusion," but we know it's just giving sufficient motion to the molecules to remain free. It's the same energy: motion.

I now no longer have a block of ice on my desk. I now have a litre of water in a pan which I had wisely placed under the ice. I knew what was going to happen, see. The water's temperature is still 0 C, but heat continues to flow towards it from the warmer surroundings. Every time the temperature increases by 1 degree, I know that another 4.2 kJ of heat has gone into the liquid water. Well, that's interesting! It takes more than twice as much energy to warm water as it does to warm ice! Why?

There are more ways for free water molecules to move. Up, down, left, right, forwards, backwards, and spinning in all directions. Previously, they could only wiggle a bit this way and that within the ice crystal structure. Each kind of motion takes a bit of energy to produce. Water molecules have to be doing all of them in order to make the temperature what it is. Therefore, raising the temperature of water requires more heat than it does for ice. Is that why the ice cubes in your drink never cause the entire drink to freeze? It's always the ice that turns to water, and not the other way 'round. How much ice would you have to put in a glass of water to make the entire glass freeze? Come on, accountants, get out your pencil and a calculator. It's not hard.

Eventually, when roughly 84 kJ of heat has entered the water, its temperature will be the same as the surroundings, and the flow of heat will trickle away to a stop. This happens increasingly gradually. Where the trip from 0 to 5 C may take only a few minutes, moving from 15 to 20 C may take hours.

One thing to remember about heat: our hands are not very good thermometers. They tell us when heat is moving into our hand (when we pick up something hot) or out of our hands (when we plunge them into ice water), but try picking up a handful of snow with a gloved hand. The insulation slows the heat flow out of our hand and we do not perceive the temperature as it actually is.

You already know that walking on carpet with bare feet does not feel as "cold" as walking on cement or tiles. But the carpet is exactly the same temperature as the tiles! Just something to think about.

Heat may be endlessly interesting, but never a mystery.

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.

Why do I say Science is Easy?

Science is not a collection of facts, equations and definitions. It is not. It may seem that way based on your (and my) experience in the classroom.

Science actually is a method for finding things out. It's as simple as making a statement of presumed fact, then testing that statement until it is proven false. Statements that are not disproven are added to, built upon, and expanded. In practical terms, such statements that are verified by experiment and observation become "facts." Extremely reliable facts.

Gaining an understanding of the physical world means discovering these facts for ourselves and convincing ourselves of their validity. As professionals, we know that there is no way to actually teach somebody something. The best we can hope for is to lead someone to a place where they can discover it for themselves. Only then does the change in thinking and the change in behavior occur which is known as "learning."

Here's the bit I'm prepared now to prove to you: Physics is no more difficult than accounting or bookkeeping. In bookkeeping one must be aware of the various ways money can come in and go out of an organization. Also, there are various ways money can become stuck or stored within an organization, such as in bank accounts, assets and so on. There are also various ways for negative amounts to become stored, such as liabilities, debts, entitlements, and so on. Once the bookkeeper is aware of these, it's a simple matter of adding and subtracting the various amounts to find out where we're at.

Physics is primarily the study of energy, and is no more complicated than bookkeeping. There are various ways for energy to enter and leave a system, or become trapped and stored within a system. Once we can picture this in our minds, it's a simple matter of adding and subtracting the debits, credits, the assets and the liabilities to see where we're at.

The universe has proven to us time after time that energy is never created or destroyed; only transformed from one form to another. But what exactly is this mysterious substance? Not mysterious at all, actually.

Energy is simply this: matter in motion. Anytime there is matter and it is moving, that's energy. As many different forms of matter and as many different kinds of motion there are, that's how many different forms energy can take. But it is nearly always convertible from one form to any other form, following this one rule. The transformation must occur through an actual, testable and observable "mechanism" or logical process.

Energy also responds to the three basic forces of nature: Electromagnetism, Gravity, and Nucleic "sticky" force. I'll explain that last one in a subsequent post.

How this works is fairly simple, too. Suppose an object moves away from the earth in spite of Gravity encouraging it otherwise. When that happens, energy is claimed and becomes locked up or stored. Throw something into the air and it slows until it stops completely, even if only briefly. Then when something moves where Gravity is encouraging it to, energy is liberated. A falling object picks up speed as it falls. The influence of electrical and magnetic forces operates in a similar manner.

All we have to know about energy is that it only transforms, it never vanishes.

Next: An overview of some of the kinds of energy we encounter.