The movement of particles creates kinetic energy. The average of all the kinetic energy in an object gives us its temperature. Heat is a comparison between temperatures. The sum of all the kinetic energy in an object gives us its thermal energy. The difference between kinetic energy, temperature, heat, and thermal energy isn’t necessary to understand in order to understand electromagnetic radiation, but this can be confusing, so here are some ways to think about these concepts.
- Kinetic vs thermal: You can refer to kinetic energy without referring to thermal energy. You can refer to the kinetic energy of a car racing down a road; you are not referring to the kinetic energy of particles within the car, the sum of which would give you the thermal energy of the car. In other words, kinetic energy can refer to the movement of the object itself as well as the movement of particles within the object. Since we’re only talking about the movement of particles within an object right now, we’re not going to talk about kinetic energy any more after this.
- Temperature vs thermal: Imagine a cup of hot water. If you add more water of the same temperature, you do not increase the temperature of the water. This is because the average kinetic energy of the water is the same. When you add more water of the same temperature you do, however, increase the thermal energy present in the cup. By adding more particles of water, you've increased the sum of the kinetic energy present in the cup. Thermal energy is the sum of kinetic energy and temperature is the average of the kinetic energy, so for temperature it doesn't matter how many particles there are, whereas with thermal energy if you have more particles of the same kinetic energy you have more thermal energy.
- Now, imagine that cup of hot water and a lake that is one degree above freezing. The temperature outside drops to zero, and you put the cup of hot water outside. The water in the cup will freeze before the water in the lake. Intuitively, this makes sense--there is more water in the lake than in the cup, so we understand the lake will take longer to freeze. Scientifically, we explain this using the concept of thermal energy. The particles in the cup have higher kinetic energy than those in the lake, meaning the water in the cup has a higher temperature. However, the cup has a lower thermal energy than the lake. Even though the kinetic energy of the particles in the cup is really high, there aren't many particles so when you add all the kinetic energy you get a relatively low number. Even though the kinetic energy of the particles in the lake is very low, there are so many particles that when you add all their kinetic energy together, you get a higher total than the sum of kinetic energy in the cup. The thermal energy in the cup is lower than the lake so the water in cup will freeze faster than the lake.
- Heat vs everything else: If everything in the universe had the same temperature, there would be no heat, because heat is a comparison. A cardboard box sitting on a table has heat because the box is a slightly different temperature than the air around it, or even if they are the same temperature, they are a slightly different temperature than you who are perceiving it. You, the box, and the table are in a room and the room is in a house and the house is a different temperature than the world outside; the house has heat, and the outside has heat because it is hotter than outer space, etc.
- Because heat is a comparison, most scientists won’t say that objects “have” heat. Objects have thermal energy, and if the objects are in or near something with different thermal energy, one object will heat the other. Heat is therefore really only used as a verb in a scientific sense.