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This is bold, italic & underline. Friday, September 5, 2008
Thermal Physics- Chapter 9

Chapter 9- Thermal Properties of Matter

Temperature and Internal Energy

What is internal energy?

The total energy of these particles is called the internal energy. Internal energy comprises two components- kinetic and potential energy.

When a body is heated, its temperature increases.
(a) For a solid:
1. The potential energy of the atom increases because the separation between atoms increases.
2. The kinetic energy of the atoms increases because the atoms vibrate faster and with larger amplitudes.

(b) For a liquid:
1. The potential energy of the atoms increases because the separation between atoms increases.
2. The kinetic energy of the liquid molecules increases because the molecules move faster.

(c) For a gas:
1. The potential energy of atoms is zero and is unaffected by change in temperature.
2. The kinetic energy of the atoms increases as they move faster at higher temperatures. Hence the internal energy of a body increase as the temperature increases.

Melting and solidification

When a solid melts, it changes from the solid to the liquid state at a certain temperature known as its melting point.

During solidification, a liquid changes into a solid. Latent heat of fusion is released but the temperature remains unchanged.

For a pure substance, both melting point and solidification point are the same.

Factors Affecting the Melting Point of Water
- Addition of impurities, such as salts, lowers the melting point of water to below 0 degree Celsius.
Hence, antifreeze is added to the water in the cooling system of a car to prevent it from freezing in the winter and salt is added to ice as freezing mixture to obtain temperatures below 0 degree Celsius.
- Increase in pressure reduces the melting point of ice.

Boiling and condensation

Boiling is the process in which a liquid changes to gas at a constant temperature which is known as the boiling point of the liquid.

When a liquid boils thermal energy is transferred to the liquid without a change in temperature.

The heat required by a liquid to change to vapour at its boiling point is known as the latent heat of vapourisation.

Condensation is the process when a vapour changes into liquid. During condensation heat is lost by the vapour at constant temperature.

Evaporation

Evaporation is the process in which a liquid changes to vapour at temperatures below the boiling point of the liquid.

During evaporation, liquid molecules that have high kinetic energy escape from the liquid surface.
The molecules that remain in the liquid have less kinetic energy. Hence the temperature of the liquid drops.

Factors affecting the rate of evaporation:
1. Temperature
2. Humidity of the surrounding air
3. Surface area of the liquid
4. Movement of air
5. Pressure
6. Boiling point of the liquid

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