Heat and temperature are not the same; then what is temperature?

The molecules inside any object constantly move and collide with each other. The energy produced from this movement is called heat. But heat and temperature are not the same. The movement or collisions of molecules mean the molecules are in motion. However, measuring exactly how fast each molecule moves is extremely difficult. Molecules are tiny particles, and their positions vary some may barely move and collide lightly, while others may travel longer distances before bumping into another molecule. The situation is completely random.

Take a container filled with some liquid or gas. Can you tell how many molecules are inside? Actually, there are trillions upon trillions of molecules inside. It’s practically impossible to measure the speed of each molecule individually. But does that mean we can’t know the state or properties of the molecules in that container? Of course, we can. Scientists have long calculated how many molecules exist in a certain amount of matter. French scientist Amedeo Avogadro showed this through what is now called Avogadro’s constant. According to this, the number of molecules in one mole of any substance is always the same 6.02214076 × 10²³. This means one mole of any substance contains 602,214,076,000,000,000,000,000 molecules or atoms.

With so many particles in a small space, collisions are inevitable, making the molecules constantly move. But whether this movement is in any specific direction or not is difficult to say. Imagine Gulistan, a busy area in Dhaka, during the evening rush. It gets extremely crowded. Moving straight through the crowd is almost impossible. Say a boy named Sayem is walking through this crowd. After taking two steps, he lightly bumps into someone in front of him. To bypass that person, he moves two steps to the right. But going straight there isn’t possible either because another person blocks the way. So, Sayem moves a bit more to the right again. After walking four steps, he encounters someone facing him, so he moves back three steps. But even that path gets blocked after three steps by another person. Then Sayem turns left and walks six steps. This way, he spends the whole evening just wandering around but can’t get out of Gulistan’s corner.

In 1592, Galileo invented the thermoscope, which could measure temperature. Later, in 1714, German scientist Daniel Gabriel Fahrenheit and in 1742, Swedish scientist Anders Celsius invented the modern thermometer.

Sayem is surprised by the jogging app on his mobile phone. According to the app, he walked 14 kilometers in four hours by taking small steps here and there. But how could he cover 14 kilometers inside an area only 200 meters wide? If he hadn’t bumped into so many people, he might have reached the Gabtoli Bus Terminal from Gulistan in much less time.

Sayem had a jogging app, so he knew how far he walked, but others didn’t notice this and ended up just wandering inside the crowd. That’s why the exact speed or movement of the crowd remains unknown to us.
 

2.

Suppose scientist Mokbul Haque stands at Gulistan Zero Point to observe the movement of people. Assume that during those four hours, about one million people gathered in Gulistan. Is it possible for Mokbul Haque to keep track of how many steps each person took or what their speed was?

Whether this is possible or impossible, we will discuss shortly. Now, imagine instead of Gulistan, you have a closed container of one-liter volume. Inside it is a gaseous substance containing trillions upon trillions of moles. And each mole has 6.02214076×10²³ molecules. So, think about how many molecules are in that container! Every molecule is running around and bumping into each other as well as against the walls of the container. Since the container is closed, the molecules cannot escape. The situation is exactly like the corner of Gulistan.

Is it possible to find out the speed of each molecule inside that container?

It is difficult — extremely difficult. It would be more feasible to try to find the average speed of the molecules. But even that is quite challenging. However, there is one thing that is relatively easier to measure: energy. Since the molecules collide with one another, energy is produced — thermal energy or heat. The faster a molecule moves, the more energy it produces upon collision. In other words, heat is generated. The average value of this energy is called temperature. That is, the average kinetic energy of the molecules is what we call temperature.

But the question is, how did scientist Galileo discover this method to find the average value 433 years ago?

Galileo was able to measure temperature using a thermoscope in 1592. Later, in 1714, German scientist Daniel Gabriel Fahrenheit and in 1742, Swedish scientist Anders Celsius invented the modern thermometer.

But rather than focusing on history, let’s try to understand temperature measurement in a simple way. For that, we return to Gulistan’s corner, where our scientist Mokbul Haque is present. Now he knows how many people are there, but he does not know their individual speeds.

Molecules don’t have feet, nor can they wear shoes. So, flying balloons isn’t an option. But scientists know exactly how many molecules are there. In fact, if you know the gas density, you can calculate how many molecules exist in a given volume.

Suppose today, very early in the morning, Mokbul Haque comes to Gulistan’s corner. With the help of several thousand volunteers, he makes everyone who comes there wear special shoes in pairs. Inside the sole of each shoe is a small generator and a tiny gas chamber. Also, there are 20 balloons attached. When the people walk, energy is produced with every step. This energy activates the generator, which produces electricity. The produced electricity opens the gas chamber, releasing gas to fill the balloons. Suppose after every 1 kilometer walked, a balloon gets fully filled with gas and automatically detaches from the shoe to rise into the sky.

Assume some drones are operating in the sky. Their job is to keep count of the balloons flying. Each balloon has a tiny chip with the name of its owner written on it. When the balloon rises, the drones can read this chip.

 

Mokbul Haque connected a computer to the drones. As soon as the data was received, it was transferred to the computer. After evening, he looked at the results on the computer. He saw that there were a total of 1 million people today. A total of 9 million balloons had flown. Since one balloon flies for every 1 kilometer walked, it means on average each person flew 9 balloons. In other words, each person walked 9 kilometers. Of course, not everyone can walk at the same speed. But the computer had all the calculations. He saw that 780 people flew 20 balloons each. No one flew more than that. At least 2,000 people flew the fewest balloons; not even one balloon flew from them.

Here, we can consider each balloon as a unit of kinetic energy of a person. Those who flew 20 balloons had kinetic energy of 20, and those who flew 5 balloons had kinetic energy of 5. In other words, the number of balloons flown corresponds to the amount of kinetic energy produced. The average kinetic energy for everyone was 9.

Now, imagine instead of Gulistan, there is a small bottle containing gas. The gas contains 1 million molecules. The average kinetic energy of those molecules is 9. That means the gas temperature is 9 degrees Celsius.

3.

Molecules don’t have feet, nor can they wear shoes. So, flying balloons is not an option for them. But scientists know exactly how many molecules there are. Actually, if you know the gas density, you can find out how many molecules are in a given space. Now you apply heat to the bottle; the molecules move faster, become more kinetic, and produce kinetic energy. You can find out the average kinetic energy. But how?

Your friend has another thermometer. It doesn’t match either of your thermometers. You put that thermometer into the bottle, and you see the mercury level rise to mark 122. You know that when heat is applied, objects expand. So, you pour mercury into a glass tube and seal its mouth tightly. Then, you mark lines at regular intervals on the glass tube. Next, you dip the bottom end of the tube into the gas bottle. You watch the mercury expand inside the tube and rise quickly upward. You see the mercury level reach the 50th mark and then stop. You assign each mark a value of 1 degree Celsius. So, you find that the average kinetic energy of the molecules in the bottle corresponds to 50 degrees Celsius. This is actually the temperature of the gas inside the bottle. And the glass tube is called a thermometer.

While you are doing this experiment, your friend may also have made a thermometer for the same purpose. He also poured mercury into a glass tube and marked lines at equal distances, but his marks are different from yours. The spacing of the marks is different. You put his thermometer into the bottle and see the mercury level rise to 323. He, however, does not assign degrees to the marks. He says the temperature of your gas is 323 Kelvin.

You have another friend’s thermometer. It doesn’t match either of yours. You put his thermometer into the bottle and see the mercury level rise to 122. He says the temperature of this bottle is 122 degrees Fahrenheit.

The three of you measured the temperature with three different thermometers and got different values. But actually, these are equal. The difference lies in how the scales are marked and some calculations. That calculation can be done another day. The bottom line for today is: the temperature measured by a thermometer is actually the average kinetic energy of molecules.