By Don Lincoln, Ph.D., Fermi National Accelerator Laboratory (Fermilab)
There are a lot of misconceptions and misunderstandings about thermodynamics. One of the most common one is the notion that heat rises. It’s said so often that it’s not even questioned by people. Though there is a kernel of truth in it, it’s just oversimplified to the extreme. Let’s find out what’s right and what isn’t.
What Is Heat?
Everyone knows what heat means, at least sort of. Heat is what warms you up. You feel heat coming from a fire. You feel heat when you wrap your hands around a steaming cup of coffee on a cold day. However, there is the simpler and experiential idea of what heat is and the more technically correct one.
In physics, heat is a form of energy. Even more technically, heat is a kind of energy that can be transferred from a hot object to a colder one.
Trying to tie the technical and vernacular together, a hot thing is something that can transfer heat energy. If you take water straight from the tap, odds are that it’s pretty cool. Suppose you took two identical pans and filled each of them with an identical amount of water. Then you put one of the pans to the side and put the other on your stove to heat it up.
How are those two pans now different? Well, one is obviously cold and one is warm. So, how did that happen? Well, using the stove, you added energy—that is to say heat—to one pan. That energy is now stored in the water.
Where does that energy really go? Water is H20, which means that you can think of the pan of water as holding a whole bunch of tiny water molecules, basically like microscopic marbles.
When you add energy to water, the marbles start moving more quickly. They zoom around faster, and they also rotate. In the case of these water molecules, they can also vibrate. Water molecules look a little like a V, and the molecules can flex and shake and do all sorts of things.
At an atomic level, that’s what really is going on when you heat up a pan of water.
When people say that heat rises, what they mean is that one place gets warmed up and one place cools down. Taken literally, the phrase ‘heat rises’ presumably means that the place above a warm object will warm up, while the object itself would cool down. So, what we’re talking about is how heat energy is transferred.
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There are three ways to transfer heat. There is conduction, which involves directly touching the hot object. An example would be putting your hand in warm water. The vibrations of the molecules of hot water would vibrate the molecules of your hand, thereby speeding them up. And if molecules are moving, bending, and shaking, that’s what heat energy is. This means that your hand would heat up.
Now, you could put your hand at the top of the water, the bottom of the water, or anywhere in the water, and your hand would heat up. And in a pan, the water at the bottom might well be hotter. That pretty much shows that heat conduction doesn’t participate in heat rising.
In conduction, it’s just a matter of quickly vibrating molecules coming into contact with slowly vibrating ones, and the slower ones speeding up. The only direction that matters here is the interface between the vibrating and non-vibrating molecules.
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A second way that heat can be transferred is called radiation. That happens when an object is heated enough for it to give off electromagnetic radiation. If you start a fire, you see the glowing light. That’s visible light. However, you can’t see all kinds of electromagnetic radiation. Some types are invisible to the eye.
One kind is called infrared radiation. Infrared radiation causes objects to start moving—say rotating, vibrating, and so forth—which means they heat things up. Around a campfire, you don’t have to be above the fire to feel heat coming to you. You can sit beside it and warm your hands or feet. Obviously, that’s heat energy moving to the side and not up. So the transfer of heat energy due to radiation doesn’t follow the heat rises rule.
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However, if you put your hand above the fire and not just to the side, the result is much hotter. It’s actually pretty easy to burn yourself if you do that. If the temperature above a fire is hotter than the side of a fire, does that mean that heat rises? Well, sort of. It turns out that there’s a third kind of heat loss that does follow the heat rises rule, and that’s called convection.
Convection occurs when you take a heat source and put it in a movable medium, like air or water.
For example, if you put air near a hot object, the moving molecules of the hot object will pass heat energy to the air via conduction. But if that movable medium starts to absorb heat energy and its molecules start to move around, they will take up a little more space. That’s because the molecules move and bounce into other molecules and push them away. The effect ripples across the medium—in this example, air—and the result is that the air becomes less dense.
If a medium like air or water becomes less dense, then it rises through the medium. This is basically why air-filled objects like pool toys float on water. They’re less dense.
So in the case of, say, a room with a wood stove in it, what happens is that the air near the surface of the wood stove absorbs heat through conduction. The air then becomes less dense and rises through the remaining air and heads towards the ceiling. The colder ceiling encounters the warmer air and some of the moving molecules of the air cause the ceiling molecules to move, and thus the ceiling warms.
So, though colloquially it can be said that in the case of convection, heat rises, what really happens is that the air is heated by conduction.
The exchange of heat energy causes the air molecules to move faster, which causes them to move apart from one another and reduce the density. Colder and denser air falls to the bottom, which has the effect of causing the hotter and less dense air to rise, and the heat is carried away. But the initial actual transfer of heat to the air was caused by conduction.
Convection is a secondary process, which does, of course, carry heat from one place in a room to another.
Common Questions about Heat and the Transfer of Heat
Heat is a form of energy, and there are three ways that heat energy is transferred: conduction, radiation, and convection. A simple study of what happens during conduction of heat energy and infrared radiation of heat energy shows that heat doesn’t rise. In case of convection, colloquially one can say that heat rises, but what really happens is that the initial actual transfer of heat is caused by conduction. This is because convection is a secondary process.
In conduction, quickly vibrating molecules come into contact with slowly vibrating ones, and the slower ones speed up. That’s how heat is transferred. So, if you put your hand in hot water, the vibrations of the molecules of hot water would vibrate the molecules of your hand, thereby speeding them up and heating up your hand.
Infrared radiation is a form of electromagnetic radiation that’s caused by heating an object. Infrared radiation causes objects to start moving—say rotating, vibrating, and so forth—which means they heat things up.
Convection occurs when you take a heat source and put it in a movable medium, like air or water. So, for example, when air starts to absorb heat energy and its molecules start to move around, they take up a little more space. This results in air becoming less dense. However, because convection is a secondary process, the initial actual transfer of heat to the air is caused by conduction.