Storm in a Teacup: The Physics of Everyday Life(58)

As all that frantic paddling was going on, warm blood was flowing down the arteries of each duck’s legs. But those arteries were right next to the veins carrying blood back from the feet. The blood in the veins was cool. So the molecules in the warm blood jostled the blood vessel walls, which then jostled the cooler blood. The warm blood going to the feet got a bit cooler, and the blood going back into the body was warmed up a bit. Slightly farther down the duck’s leg, the arteries and the veins are both cooler overall, but the arteries are still warmer. So heat flows across from the arteries to the veins. All the way down the duck’s legs, heat that came from the duck’s body is being transferred to the blood that’s going back the other way, without going near the duck’s feet. But the blood itself goes all the way around. By the time the duck’s blood reaches its webbed feet, it’s pretty much the same temperature as the water. Because its feet aren’t much hotter than the water, they lose very little heat. And then as the blood travels back up toward the middle of the duck, it gets heated up by the blood coming down. This is called a countercurrent heat exchanger, and it’s a fantastically ingenious way of avoiding heat loss. If the duck can make sure that the heat doesn’t get to its feet, it has almost eliminated the possibility of losing energy that way. So ducks can happily stand on the ice precisely because their feet are cold. And they don’t care.

This strategy has evolved many times separately in the animal kingdom. Dolphins and turtles have a similar layout of blood vessels in their tails and flippers so that when they swim into colder water, they can maintain their internal temperature. It’s also seen in Arctic foxes—their paws have to be in direct contact with ice and snow, but they can still keep their vital organs warm. It’s very simple, but very effective.

Since my friend and I didn’t have any way of playing the same trick, we lasted a limited amount of time out in the snow. After watching a few more high-speed duck squabbles, and expressing suitable admiration for what must be the fittest ducks anywhere in the world, we went in search of giant scones.


AFTER MANY THOUSANDS of experiments carried out by generations of scientists, we have concluded that the fixed direction of heat flow seems to be a pretty fundamental law of physics. Heat will always flow from the hotter to the cooler object, and that’s just the way it is. However, that fundamental law says nothing about how quickly the transfer has to happen. When you pour boiling water into a ceramic mug, you can carry that mug around by the handle until the water has completely cooled down, and you won’t get burned because the handle doesn’t heat up very much. But if you put a metal spoon into the boiling water and keep hold of the end, you’ll be squeaking with discomfort after just a few seconds. Metal conducts heat very quickly, and ceramics conduct heat very slowly. That must mean that metals are better at passing on the vibrations from the most energetic molecules. But both metals and ceramics are just made of atoms that are anchored in place and can only vibrate about a fixed location. Why would there be a difference in conductivity?

The ceramic mug shows what happens if you rely on entire atoms passing on their vibrations. As we’ve said, each atom nudges the one next to it, which nudges the next, and eventually the energy gets passed along the chain. This is why you can hold the mug handle without getting burned—that method of passing the energy along is slow, and lots of it will be lost to the air before it ever reaches your hand. Ceramics, just like wood and plastics, are considered poor conductors of heat.

But the metal spoon has a shortcut. In a metal, most of the atom is locked in place, just like the ceramic. The difference is that each metal atom has a few electrons around its edge that aren’t very tightly bound to it. We’ll get on to electrons properly later. What matters here is that electrons are tiny negatively charged particles that sit in a swarm in the outer zone of each atom. In the ceramic they’re locked in place, but in the metal they can easily be swapped between adjacent atoms. So while the metal atoms themselves must sit in their lattice positions, those free electrons can wander through the whole structure. They form a cloud of electrons that are shared between all the metal atoms, and they’re extremely mobile. It’s these electrons that are the key to heat conduction in metals. As soon as you pour boiling water into the mug, the water molecules pass some thermal energy to the ceramic walls, and that gets slowly passed through the mug as whole atoms bump into each other. But as soon as the hot water touches the spoon, it passes its vibrations on to both the fixed metal atoms and their electron cloud. Electrons are tiny, capable of vibrating and of zooming through a structure very quickly. So while you’re holding onto the spoon, the minuscule electrons are shunting themselves about inside the metal, passing on thermal vibrations far more quickly than the whole metal atoms do. It’s the electron cloud that brings the thermal energy up to the top of the spoon so quickly, heating up the rest of the metal as they go. Copper is best of all the metals for doing this, by a long way; it conducts heat about five times faster than a steel spoon. That’s why cooking pans are sometimes made with copper bases but iron handles. You want the copper to share the heat out through the food quickly and evenly, but you don’t want the thermal energy to find its way up the handle.

Once you have proved the existence of atoms, you naturally ask what they’re up to in different situations. And that leads directly to an understanding of what heat energy actually is. We often talk about heat as though it’s a fluid that can flow through or into or out of the objects around us. But really, it’s just movement energy, shared around as different objects come into contact with one another. Temperature is a direct measurement of that movement energy. We can control how the energy is shared by using materials that are good conductors of heat, like metals, or poor conductors of heat, like ceramics. When you look at the control of heat and cold in our society, one system stands out above all the others for the difference it makes to our lives. We humans spend a lot of time making sure that we keep warm. But when it comes to food and pharmaceutical drugs, we have a huge invisible infrastructure for keeping things cold. Let’s finish this chapter by having a look at fridges and freezers.