- Heat energy can be transmitted from one point (or place) to another point (or place) by three different methods.
- The following are the methods of transmission of heat: conduction, convection and radiation.
- The process in which heat is transformed from one point to another point in a body by handing over from one particle to another particle without actual movement of the particles of the body (or medium) is called conduction.
If a metal rod is heated at its one end, after sometime, it is felt that the other end of the rod also gets heated.
- When the rod is heated at one end, the molecules at the end get heat energy.
- The molecules at the end begin to vibrate rapidly. In other words, their kinetic energy of vibration increases.
- When the molecules vibrate about their mean position, they collide with their neighboring molecules.
- In the collisions, the kinetic energy of vibration is transmitted to the neighboring molecules.
- Due to kinetic energy, the neighboring molecules begin to vibrate and their temperature rises.
- When the new molecules vibrate about their mean positions, they lose their kinetic energy to the other molecules.
- In this way, all the molecules in the rod start vibrating and the heat is transmitted from one end to the other end without actual movement of the molecules.
- This method of transmission of heat is called conduction.
Conductors and Insulators
- All substances do not allow the heat to pass (or to conduct) through them easily.
- Thus, the materials (or substances) which allow the heat to pass through them easily are called good conductor of heat.
- For examples: most of the metals like copper, aluminium, iron, silver, etc. are good conductors of heat.
- The materials or substances which do not allow heat to pass through them easily are called insulator or bad conductor of heat.
- For examples: wood, sand, cotton, wool, glass, etc. are bad conductor or insulator of heat.
The Conditions for the Conduction of Heat between Bodies
Heat is transmitted from one body to another body by the method of conduction if the following two conditions are met:
i) If the two bodies are in actual contact with each other,
ii) If there is difference of temperature between two bodies.
- Utensils used for cooking purposes are made up of good conductors like brass, aluminium, iron, copper, etc. the utensils of good conductors are quickly heated up and hence the cooking is effective.
- The handles of the cooking utensils are made up of bad conductors like wood, ebonite, etc. the bad conductors do not get heated. Therefore, the handles of utensils made up of bad conductors are comfortable to hold during cooking.
- In winter, people wear warm woolen clothes. The woolen clothes are bad conductor of heat. Hence, they prevent the conduction of heat between our bodies and the atmospheres and keep our bodies warm.
- The process of transfer of heat in which the heated particles actually move from one place to another place to carry out the heat is called convection.
- For example, heat is transferred in liquids and in gases by this method.
Some water is taken in a beaker and placed over Bunsen burner. A small quantity of potassium per-magnate is dropped into the middle of the beaker. When the water is heated, it is observed that the coloured water is moving up. Circulation of coloured water is observed.
- When heating started, the water molecules at the bottom of the beaker are heated.
- When they are heated, they become lighter than the fresh molecules. The heated molecules then move up.
- To fill the space of the already heated molecules, fresh molecules of water come.
- The fresh molecules are again heated. When they are heated, they move up and other molecules come.
- The process goes on until all the water in the beaker is heated. The heating of a substance in this way is called convection.
- Ventilations are provided in the kitchen or in the rooms. When air is heated in the room, it moves up and stays around the roof.
The ventilations are provided at the top of the kitchens or room so that the heated air pass out.
To fill the space of the hot air, fresh and cold air from outside comes into the kitchens or the rooms through doors and windows.
- Land Breeze and Sea Breeze
- The process of transfer of heat from one place to another without any medium is called radiation.
- For example, tremendous heat energy from the sun is approaching us by radiation.
- The heat energy by this process travels in straight line in the form of electromagnetic waves.
- The heat radiation travels with the speed of light. It is due to such high speed that the heat from the sun is able to reach the earth surface even if it is about 150 million km from the sun.
- The other examples of radiation are the heating of a room by an electric heater, heating by fire, etc.
- White and light coloured clothes are preferred in summer. It is because the white and light coloured clothes absorb little amount of sun’s radiation of heat and hence keep our bodies cold.
- Dark coloured clothes absorb most of the heat radiations emitted by the sun. If dark coloured clothes are put in winter, they absorb the heat radiation from the sun and keep our bodies warm. Therefore, dark coloured clothes are preferred in winter.
Methods of Heat Transmission