Heat energy is transferred from a higher temperature region to a lower temperature region until the temperatures are balanced.
There are three methods by which heat is transmitted from one place to another.
Conduction is defined as the process by which heat is transmitted through a medium from its hotter part to its colder part.
At the hotter part, the molecules vibrate actively. They collide with neighbors. Then, the vibration is transferred from the hotter part to the colder part.
Thermal conductivity depends on the materials. For example;
- Air, wool, cotton, wood, water, glass and plastic are bad conductors.
- Metals (e.g. steel, iron, copper, silver) are generally good conductors.
The relative order is as follows;
(higher conductivity) Metal > Non metal solid > Liquid > Gas (lower conductivity)
Convection is defined as the process by which heat is transmitted from one place to another by the movement of heated particles of a gas or liquid.
- When we heat a fluid (a gas or a liquid), it expands and its volume increases. Its density is therefore reduced. Hotter fluid surrounded by cooler fluid (higher density) will tend to float. The warmed fluid will move upwards, and it carries heat energy with it.
- The movement of fluid is called convection current.
- Convection only occurs in a fluid
- Convection current rises vertically from the source of heat where the fluid is hottest.
Radiation is defined as the flow of heat energy in the form of electromagnetic waves.
- This process does not require any medium.
- It can occur in a vacuum space.
- These electromagnetic waves are called infra-red ray.
- Infra-red rays are invisible.
- An object which receives infra-red rays is called an Absorber.
- An object which releases infra-red rays is called an Emitter.
Applications of heat transfer
Vacuum flask (Thermos flask) It can keep liquid hot or cold.
- Vacuum space prevents conduction and convection. (If there is no particles, conduction and convection don’t occur)
- Silvered wall prevents radiation.
Hot water system
Water is heated in the boiler. It rises by convecion to the hot water tank, while colder water flows from the tank to the boiler. The convection current keeps the water in the tank hot. When you open a tap, hot water flows out.
Sea and Land Breeze
The sun can cause very large convection currents of air. This flow of air is wind. In daytime, The land has higher temperature than the sea. The warm air rises over the land and it is replaced by colder air from the sea. This is called Sea breeze
At night, the reverse occurs, because the land cools down faster than the sea. This is called Land breeze .
Car engines are cooled by convection current in the water pipe. The radiator is a heat exchanger where water gives up its heat to the air.
A greenhouse is a building made of glass or transparent plastic for growing plants. Radiation from the sun enters through the glass but radiation from the plants can’t get out of the glass. It keeps the temperature inside warm.
In sunny countries, warm water can be produced using solar panels. In one type of panel, a metal tube is welded to the metal plate which is painted dull black. The plate absorbs the sun’s radiation, and shares this energy with the water by conduction.