Heat and Motion (Kinetic Theory of Heat)

Under normal conditions matter exists in three forms:

solid

liquid

Matter is made up of atoms and molecules. These atoms in molecules are in a state of continuous motion. For example, molecules and atoms of a solid can have only vibrational motion, because they are held in their positions by strong intermolecular forces. In liquids, the intermolecular attraction is not strong enough to hold the molecules in their positions. Therefore, the molecules of a liquid can have random motion within the liquid surface. (Here, the intermolecular attraction holds the molecules and atoms bound to the surface. In the case of gases, intermolecular attraction is very weak. Hence, the molecules move randomly within the container. The intermolecular attraction provides potential energy and the energy of motion of the molecules gives them kinetic energy. The sum of potential energy and kinetic energy is known as internal energy or thermal energy of matter. This internal energy expresses itself as the temperature of the substance.

Thus, if a solid is heated, its molecules vibrate more vigorously within the intermolecular space. Its internal energy increases and hence, its temperature increases. If the solids are heated further, at a certain stage their kinetic energy increases so much that many molecules can overcome intermolecular attraction and break free. At this stage, the solid melts into liquid. Thus, heat can bring about a change of state.

In liquids the molecules are in a state of continuous random motion within the surface. If heat is supplied to a liquid, molecules move faster. Their kinetic energy increases and hence, the temperature increases. Here, some molecules may gain enough kinetic energy to overcome the surface attraction and move out. Thus, the liquid evaporates. As the temperature rises, rate of evaporation also increases. Thus, supply of heat energy causes change of state from liquid to vapor state.

In a gas the molecules are free to move randomly. When heat is supplied to a gas, its molecules absorb heat energy and move faster, their kinetic energy increases. Hence, then internal energy increases and the temperature rises.

Sub Topics

What happens when heat is taken away from substances?
Thermal Equilibrium
Mechanical work and heat

What happens when heat is taken away from substances?

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When a liquid is cooled, the temperature of the liquid decreases. This means the liquid molecules move with lesser velocity. When the liquid is cooled further, the temperature decreases. At a certain stage the kinetic energy of the molecules become so less that intermolecular attraction dominates and the liquid freezes into solid.

Thermal Equilibrium

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When two substances at different temperatures are brought together, heat energy flows from a substance at higher temperature to a substance at lower temperature. The heat flow continues till their temperatures are equalized. At this stage, the substances are said to be in thermal equilibrium.

Thus, 'Heat energy always flows from a hotter body to a colder body till their temperatures are equalized'.

This can be interpreted in another way - 'A cold body does not become hot on its own. i.e., in order to rise its temperature, external heat energy must be supplied to it'.

Also, we can say, 'temperature is a condition which determines the direction of heat flow between two bodies when they are mixed together'.

Mechanical work and heat

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If we rub our hands for a while, the hands become warm. Let us examine the process. Rubbing hands is mechanical work and in this process heat is generated. We can say heat energy is produced as a result of mechanical work.

Let us examine another example - when you light a matchstick, you strike it against a strip coated with chemicals. Heat produced due to friction lights the matchstick.

From the above examples, we can infer, mechanical work done, results in production of heat energy or simply, mechanical energy can be converted to heat energy.

During 1840-1850, James Joule conducted a series of experiments on conversion of mechanical energy into heat energy. He discovered that mechanical work done (W) is proportional to heat produced (H)

Thus W

W = JH

Here J is known as the mechanical equivalent of heat.

The mechanical equivalent of heat is defined as 'the work done to produce unit quantity of heat'.

In CGS. System, J = 4.2 X 10⁷ ergs/cal

In SI system, J = 4.2 kJ or kcal

Later, Joule's work lead to a new branch of physics known as thermodynamics conversion of heat energy into mechanical energy.

In fact any form of energy, mechanical, electrical, chemical energy can be very easily converted to heat energy. Similarly, heat energy may also be converted to other forms of energy such as light, mechanical, electrical energy etc.