SS2 Second Term Physics Lesson Note – Heat Energy IV

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WEEK 9

TOPIC: HEAT ENERGY

CONTENT:

• Evaporation: Meaning, Factors that affects evaporation and applications of evaporation and latent heat.
• Boiling point: Meaning, determination of boiling point,
• Effect of pressure and impurities on boiling and freezing point
• Relative humidity, mist, cloud, dew point

SUB-TOPIC 1:  Evaporation: Meaning, Factors that affects evaporation and applications of Evaporation and latent heat.

EVAPORATION

Evaporation is a change of state from the liquid to the gaseous state. It takes place at all temperatures and from the surface of the liquid. Volatile liquids such as Freon, ether, alcohol, vaporize easily. Evaporation causes cooling because latent heat is taken from the body to change the liquid to the gaseous state. Example the human utilizes the process of evaporation in cooling after exercise.

FACTORS AFFECTING THE RATE OF EVAPORATION

Temperature: The rate of evaporation increases as temperature increase.

Pressure: The rate of evaporation decreases with increase in pressure.

Area of liquid surface exposed: The greater the surface area of liquid exposed, the more rapid will be the evaporation.

The nature of the liquid: The lower the boiling point of the liquid, the greater will be the rate of evaporation.

Wind and dryness of the air: Dryness of the air around the body causes rapid evaporation from the body. Wind blows away liquid vapour and causes rapid evaporation.

APPLICATION OF COOLING EFFECT OF EVAPORATION AND LATENT HEAT

1. Refrigerator
2. Air conditioner

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