Thermal energy Thermal energy and temperature Heating Heat Using thermal energy Remember - Heat is not a form of energy

"It's a hot day for Spring, you can really feel the heat! I hope the air-conditioner is working this summer, as the heating system did not work this winter and I was really cold. I had to wear my thermals to school just to keep warm!"

This is the way that we generally use words like hot, cold and heating and thermal in everyday life. In science though we need to be more precise with our language to get hold of the concepts of heat, heating and thermal energy as understood by scientists. Do not worry, the normal language is pretty close, you just have to be careful how each word is used.

Thermal energy
Thermal energy is the energy in a body due to its temperature. If you are hot because you have been out in the Sun, then scientifically you have increased thermal energy. This energy within the body or object includes kinetic energy from vibrations and movement of atoms and molecules and potential energy associated with the state (solid, liquid or gas) of the body. In short any energy that is temperature dependent.

Thermal energy and temperature
When something is cold it has less thermal energy than when it is hot, but thermal energy is not temperature! Temperature is a measure of the average kinetic energy of the particles in a body. Thermal energy is the heat energy in the whole body. This takes into account the temperature, type of material and mass of the body. For example, a lump of steel with a mass of 100kg will have twice the thermal energy of a 50kg piece of steel, if their average temperatures are the same. Thermal energy is a function of temperature, type of material and mass.

Thermal energy also is dependent on the type of material the object is made of. Some substances, like water require a lot more energy to heat up than others like fat. The measure of this is known as "Heat Capacity". The Heat Capacity of a substance is the energy required to heat 1 gram of the substance by 1°C.

Heating
Heating is the process of transferring energy from one body to another due to a difference in temperature between the bodies. Heating can occur by conduction, convection and radiation. For example, when someone sits out in the sunshine, they are undergoing a heating process, increasing their thermal energy. The three processes of thermal energy transfer are discussed in detail in the topic - Heating.

Heat
Heat is the quantity of energy that has been transferred during the heating process. If you stay out in the Sun for an hour instead of 5 minutes, you will be heated for longer and your body will have increased its thermal energy more. The quantity of energy transferred to you, heat, will be greater.

Using thermal energy
Here are a few examples of how thermal energy can be described:

• Your body transforms stored potential energy in the food you eat into thermal energy which keeps our bodies at 37.5°C.
  
  
• The daily temperature usually rises as radiant energy from the Sun is absorbed by the atmosphere increasing the thermal energy of the air.
  

Remember - Heat is not a form of energy
The correct uses of the word "Heat" are:

• As a verb, describing the process of
  

  • Transfer of energy from one body to another due to a difference in temperature. eg.
    "Heat up the water for the tea, please."
    "You're cold, sit by the fire and heat yourself up a bit."

   

  • As a noun, describing the quantity of energy that has been transferred during a heating process.
    "By sitting next to the fire I was able to feel the heat."

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