# Heat Energy

Heat energy in a material consists of the disordered motions of its atoms or molecules. In any interactions of atoms or molecules, the statistical odds are that they will end up with less order than earlier, with the result that the heat energy is spread out more evenly. With huge numbers of atoms and molecules, greater disorder is almost certain.

The Law of Conservation of Energy states that the total amount of energy in the universe is constant. We will always have the same amount of energy in the universe. Note that this does not mean that we will always have the same amount of energy on Earth. In various ways, energy from the Earth is given off into space so it is lost to us, but is still in the universe.

Heat is a form of energy, and like all forms of energy, it is interchangeable with the other forms.

One obvious way to transfer energy (so that nature can reach an equilibrium) is to do work on something. Work is the transfer of energy. This is a mechanical process.

Another way is the transfer of energy by means of microscopic collisions between atoms or molecules. Conduction is an example of this.

Energy can also be transferred as follows: mechanical waves (ex sound); electrically (electricity); electromagnetic radiation (light, etc., that travel without a medium present).

In general, energy can be transferred by one of three methods: conduction (actual contact between substances – particles collide and transfer energy), convection (bulk movement of a fluid), and radiation (only process that can occur in a vacuum). The exchange of heat energy is a method for obtaining thermal equilibrium. Two objects in thermal equilibrium are at the same temperature.

Temperature is defined as a measure of the average kinetic energy of the substance’s particles. The formula which relates temperature and energy is:

Kavg = 3/2 kb T

where Kavg is the average kinetic energy measured in Joules

kb = Boltzmann’s constant (1.3806503 × 10-23 m2 kg s-2 K-1)

T is the temperature in Kelvin

Temperature can be measured on several scales. The Celsius scale is used in the SI system as is the Kelvin scale. To convert between the two, use the relationship:

K = oC + 273

To convert between Fahrenheit (an Enlgish unit) and Celsius, use:

o F = 1.8 oC + 32

Now connect all of this with the Kinetic Theory which states that all matter is composed of small particles that are in random motion. This explains why conduction occurs between objects that are not in thermal equilibrium and are not at the same temperature.

As the particles move, they will collide with each other. During the collisions, energy is transferred from the particle with lower energy to the particle with higher energy.

## Specific Heat

When energy is transferred, you must also be concerned with the specific heat of the substance. This is the amount of energy (heat) needed to raise the temperature of a 1 kg of a material by 1 K. Because energy is required to change the phase of a pure substance, the amount of energy needed for these changes can be calculated by using the formula

Q = mc (Δ T)

Q = change in energy
m = mass of substance
c = specific heat of substance
T = temperature

If heat is measured in calories (not fat), 1 calorie = 4.19 J.

Since heat is a form of energy, it can be produced by mechanical work. As a matter of fact, in the real world, it is hard to NOT produce heat when work is done. This also led to the conclusion that the different forms of energy could be transformed into other forms of energy. The study of the quantitative relationships between heat and other forms of energy is called thermodynamics. More about that will be presented in a later unit.

However, if you take into consideration all forms of energy, including heat and internal energy (the total available potential energy and kinetic energy of the particles of a substance), then energy is, again, conserved.

Remember that the various forms of energy can be transformed among themselves and that all are (regardless of the form) measured in J.

E stored by your body (CPE) is convert it to KE.   KE is used to do work.
By definition the mechanical equivalent of heat is equal to 4.186 J/cal.

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