Thermal Energy – Fundamentals of Engineering Module #6

what is thermal energy

What is heat?

Time to once again rack your brains people. We have already gone over quite a bit in just over a month of internet existence. Earlier we discussed molecules. Remember how all substances are composed of very small particles called molecules? The energy associated with these crazy, gringo molecules is called thermal energy.

Thermal energy, like his beefier bro mechanical energy, exists in two stored forms and in one transitional form.

The two stored forms of thermal energy are:

  • internal potential energy and
  • internal kinetic energy

Thermal energy in transition is called heat.

thermal energy

The bonds of a water molecule.

Although molecules are too small to be seen, they behave in some ways pretty much like the larger objects we considered in the discussion of mechanical energy.

Molecules have energy of position, called internal potential energy, because of the forces that attract molecules to each other. Confused? Excellent. No worries, this is easy. You may have just read “internal potential energy” and gone huh? Recall gravity, earth, & Newton’s Laws that we considered before.

Molecules have energy of motion, internal kinetic energy, because they are constantly in motion. Damned molecules – never sittin’ still. So, the two stored forms of thermal energy, internal potential energy and internal kinetic energy, are in some ways similar to mechanical potential energy and mechanical kinetic energy, except everything is on a smaller, molecular scale. Sneaky.

Since we are not groundhog scientists, we don’t need to distinguish between the two stored forms of thermal energy. So let’s quit referring to internal potential energy and internal kinetic energy, and simply use the term internal energy. Internal energy then, will mean the total of all internal energy stored in the substance or system because of the motion of the molecules and because of the forces of attraction between molecules.

You probably know more about internal energy than you realize.

Because molecules are constantly in motion, they exert a pressure on pipe walls, tanks, your skin, or other objects in which they are contained. Also, the temperature of any substance arises from, and is directly proportional to, the activity of the molecules. Every time you read a thermometer or pressure gauge, you are finding out something about the amount of internal energy contained in the substance. High pressures and temperatures indicate that the molecules are moving rapidly and that the substance therefore has a lot of internal energy.

Heat is a more familiar term than internal energy, but may be more difficult to define properly. Remember – heat is thermal energy in tansition — that is, it is thermal energy that is moving from one substance or system to another.


Kind of lost? Maybe an example will be helpful. We have two equal lengths of pipe made of identical materials and containing steam at the same pressure and temperature. One pipe is well insulated; the other is not insulated at all.

From everyday experience you know that more heat will flow from the uninsulated pipe than from the insulated pipe. When the two pipes are first filled with steam, the steam in one pipe contains exactly as much internal energy as the steam in the other pipe. We know this is true because the two pipes contain equal volumes of steam at the same pressure and at the same temperature.

thermal energy definition

Steam piping

After a while, the steam in the uninsulated pipe will contain much less internal energy than the steam in the insulated pipe. We could measure the temperature and pressure but common sense should tell us that they will be lower.

What happened? Stored thermal energy (again, internal energy) has moved from one system to another, first from the steam to the pipe, then from the uninsulated pipe to the air. This flow or movement of thermal energy from one system to another is called heat.

A good deal of confusion exists concerning the use of the word heat. Sometimes people say that a hot object contains a lot of heat when they really mean that it contains a lot of internal energy. Or you might hear that heat is added to or removed from a substance. Since heat is the flow of thermal energy, it can no more be added to a substance than the flow of water could be added to a river. You might add water, and this might increase the flow, but you could hardly say that you added flow. The only thermal energy that can in any sense be added to or removed from a substance is internal energy.

Here is the link to the fundamentals of engineering module #7 – energy transformations.  Don’t stop now!  Your brain is filling up. If you think you can ace the quiz on thermal energy, click here and let’s see!