Heat and Temperature

We often refer to infrared radiation as being primarily heat (or thermal) radiation. But what exactly is heat, and how does it differ from temperature?

Heat is the energy that an object has because of the motion of its molecules - which are continuously jiggling and moving around. When energy is added to an object, its molecules move faster, creating more heat. Compared to a warm object, the molecules in a cold object have less molecular motion. Heat is the total energy of molecular motion in a substance.

Heat can be transferred from one place to another by three methods: conduction in solids, convection of fluids (liquids or gases), and radiation through anything that will allow radiation to pass. The method used to transfer heat is usually the one that is the most efficient.

In infrared astronomy, we measure heat which has travelled by radiation. Infrared radiation (also called heat or thermal radiation) is a type electromagnetic radiation (or light). Radiation is a form of energy transport consisting of electromagnetic waves traveling at the speed of light.

Temperature is a measure of the average heat or thermal energy of the molecules in a substance. The atoms and molecules in a substance do not always travel at the same speed. This means that there is a range of energy (the energy of motion) among the molecules. In a gas, for example, the molecules are traveling in random directions at a variety of speeds - some are fast and some are slow. Sometimes these molecules collide with each other. When this happens the higher speed molecule transfers some of its energy to the slower molecule causing the slower molecule to speed up and the faster molecule to slow down. If more energy is put into the system, the average speed of the molecules will increase and more thermal energy or heat will be produced. So, higher temperatures mean a substance has higher average molecular motion. We do not feel or detect a bunch of different temperatures for each molecule which has a different speed. What we measure as the temperature is always related to the average speed of the molecules in a system.

Since temperature is an average measurement, it does not depend on the number of particles in an object (or its size). Every object that has a temperature above absolute zero has some heat energy. Temperature is not energy, but a measure of it. Heat is energy.

See our education module Heat and Temperature to learn more about this topic.