5.5 Heat and Temperature

Critical Questions:

  • What is the difference between heat and temperature?
  • What happens when water freezes or boils?

It may seem out of place to start talking about heat and temperature in the middle of a chapter about energy, but in fact, there are many connections between these ideas. And in order to understand the ways in which we can and can’t use energy, we have to know something about heat.

Heat Movie Poster
1995, starring Al Pacino.

The first thing science teachers do when teaching this topic to young people is try desperately to communicate the idea that heat and temperature are, in fact, two very different things. I never really understood why this seemed so important to them, but nevertheless, they’re right.

Heat, in physics, is a quantity of energy that is transferred from one object to another. When you hold your hand over a fire and your hands get hotter, heat has been transferred from the air into your skin.

Temperature, on the other hand, is a measurable property of any object. Specifically, it is a measure of how quickly the particles in that object are vibrating.

You see, it’s fine to picture solid matter as being made up of all of these atoms and molecules and whatnot, but these particles do not sit around waiting for stuff to happen. Even in a seemingly still, solid object, the particles that make up matter are constantly vibrating and bumping into each other.[1. This is called Brownian motion, named after the Scottish botanist Robert Brown, who noticed this behaviour when looking through a microscope at grains of pollen.] And when heat is transferred into an object, its particles vibrate more than before.

In case you didn’t notice it, this fits in just fine with what we now know about energy. Heat is transferred between objects, but that energy has to go somewhere; in this case, it turns into the kinetic energy of the vibrating particles.

This simple model is actually quite powerful – we can use it to explain many everyday phenomena. For example, imagine a block of ice. Its particles are all tightly packed together into rigid crystals, without much room for vibration. So what happens when head is added? Those particles are going to start vibrating faster and faster, and at some point they’ll have so much energy that they’ll break apart from the structure around them. In other words, the ice melts into water.

Melting ice cubes

The particles in liquids such as water aren’t as tightly packed as in a solid. They’re still loosely connected, but they’re also free to move around and slip past each other. This explains why water will change shape to fit into whatever container it’s put in, but also retains some surface tension, causing it to spill out of the container in drops rather than scattered molecules.

If we add more heat to the water, we’ll increase the particle vibration more and more. At some point, individual molecules will have so much energy that they’ll break those loose connections and fly off on their own. They’re now gas molecules – the hippies of phase states, flying around unfettered by the square-minded constraints of solids or liquids. That’s right, the water has started to boil.

It’s also important to note that different materials can absorb the same amount of heat but have different temperature changes. It takes a lot of heat to increase the temperature of water, for example – much more so than other materials like air. This is because in water, there is a large number of closely-packed particles. And any heat that gets added to it has to be shared among all of them, meaning that the kinetic energy of each individual particle won’t go up very much.

The amount of heat needed to raise the temperature of a substance depends on a few different factors – how densely-packed its particles are, the arrangements of those particles, and so on. This helps to explain why jumping into a lake often feels much colder than the air around it – the lake might actually be colder, of course, but the water molecules will absorb much more of the heat put out by your body than the air does. You’ll thus lose heat quicker, and feel colder.

Polar Bear Swim
Unless you’re wearing a protective Santa hat.

Another thing we can explain is why two objects left in contact with each other will eventually reach the same temperature – like a bucket of warm water left in a cool room overnight. The cool air particles are moving slower than the warm water particles. So whenever an air particle happens to collide with a water particle at the surface, the water particle will lose some kinetic energy and the air particle will gain some. If you can’t picture that, think of a fast-moving pool ball (A) running into a stationary one (B): ball A will slow down, and ball B will go rolling away.

It’s important to mention that we’re talking here about what usually happens, on average. In fact, temperature is a measure of the average kinetic energies of all particles in an object, but because of the chaotic nature of these particles, thermal energy is never distributed perfectly evenly. So there will be times when a particularly fast-moving air particle might run into a particularly slow-moving water particle, and the energy transfer will go in the other direction. But because there are such enormous numbers of particles involved, over time the energy is statistically guaranteed to flow from warm to cold.

Even if particles aren’t colliding with each other as we’ve been imagining so far, it’s still possible for them to exchange heat by radiation. Even in a perfect vacuum, with no other particles around to run into, all objects radiate heat energy out in the form of electromagnetic waves.[2. See section 7.2, eventually.] The amount of heat energy an object radiates is related to its temperature.

With these ideas in mind, I’d like to ask a quick question before we get back to discussing energy and machines: why is it useless to try to cool down your house by leaving the refrigerator door open? A refrigerator takes heat from its interior and moves it outside by pumping a special kind of substance through some pipes. The substance, as a gas, absorbs heat inside the fridge and then gets pumped into metal coils behind the fridge. Here, the motor presses the substance until its molecules get so close together that they become a liquid. The particles can no longer vibrate like they used to, so they’re forced to radiate all their extra energy in the form of heat. But the key point is that this heat is released at the back of the fridge, which is still inside the house. So if you leave the fridge door open, its motor works very hard but only moves the heat energy a couple feet backwards. This heat gets redistributed through the room and eventually makes its way back into the fridge, and the whole pointless cycle continues.

The main difference between a fridge and an air conditioner (beside the actual cooling mechanism) is that the AC sends the heat outside the house, leaving the indoors cooler.

But let’s leave the sad story of the open refrigerator behind for now. Heat may seem quite nice to you when your toes are cold and you don’t have a big enough blanket, but in many ways, heat is the most useless kind of energy of all of the types we’ve discussed so far, and it causes great problems for us when we want to use energy to do work. To see why, we’ll have to know a little more about an area of physics called thermodynamics, which details the relationship between heat and other types of energy.

Big Ideas:

  • Heat is a transfer of energy between two objects.
  • Temperature is a measure of the random kinetic energy of the particles in an object.
  • Phase changes occur because of changes in particle vibration.
  • Heat can be transferred by contact or by radiation.

Next: 5.6 – Entropy, Part 1: What Is It?

Previous: 5.4 – Perpetual Motion Machines