Transfer of Heat
If heat energy is supplied to any substance, it will be transferred from one part of the substance to another part. It takes place in different ways depending on the state of the substance. Three ways of heat transfer are:
Take some hot water in a cup and put a silver spoon in it. Leave the spoon inside the water for some time. Now touch the other end of the spoon. Do you feel the heat?
(i) Yes, we feel hot.
(ii) It is because heat in the hot water is transferred from one end to other end of the spoon.
(iii) In solid substances such as silver spoon, atoms are arranged very closely.
(iv) So heat transfer takes place from the higher temperature region to lower temperature region.
(v) This is due to conduction.
How did the other end of the spoon become hot? It is because heat in the hot water is transferred from one end to other end of the spoon. In solid substances such as silver spoon, atoms are arranged very closely. Hot water molecules which are vibrating transfer the heat energy to the atoms in the spoon and make them vibrate.
Those atoms make other atoms to vibrate and thus heat is transferred to the other end of the spoon.
In conduction heat transfer takes place between two ends of the same solid or through two solid substances that are at different temperatures but in contact with one another. Thus, we can define conduction as the process of heat transfer in solids from the region of higher temperature to the region of lower temperature without the actual movement of atoms or molecules.
All metals are good conductors of heat. The substances which does not conduct heat easily are called bad conductors or insulators.Wood, cork, cotton, wool, glass, rubber, etc are insulators.
Conduction in daily life
* We cook food in vessels made up of metals. When the vessel is heated, heat is transferred from the metal to the food.
* When we iron dresses, heat is transferred from the iron to the cloth.
* Handles of cooking utensils are made up of plastic or wood because they are poor conductors of heat.
* The temperature inside igloo (snow house) is warm because snow is a poor conductor of heat.
Take some water in a vessel and heat it on a stove. Touch the surface of the water. It will be cold. Touch it after some time. It will be hot now. How did the heat which was supplied at the bottom reach the top?
(i) When water in the vessel is heated, water molecules at the bottom receive heat energy and move upward.
(ii) Then the molecules at the top comes down and get heated.
(iii) This kind of heat transfer is known as convection.
When water in the vessel is heated, water molecules at the bottom receive heat energy and move upward. Then the molecules at the top comes down and get heated.
This kind of heat transfer is known as convection. This is how air in the atmosphere is also heated. Thus, the form of heat transfer from places of high temperature to places of low temperature by the actual movement of molecules is called convection. Convection takes place in liquids and gases.
Convection in daily life
* Formation of land breeze and sea breeze is due to convection of air.
* Wind flows from one region to another region by convection.
* In hot air balloons heat is transferred by convection and so the balloon raises.
* In refrigirators, cool air moves downward and replaces the hot air because of convection.
Radiation is the third form of heat transfer. By conduction, heat is transferred through solids, by convection heat is transferred through liquids and gases, but by radiation heat can be transferred through empty space even through vacuum. Heat energy from the Sun reaches the Earth by this form of heat transfer. Radiation is defined as the way of heat transfer from one place to another in the form of electromagnetic waves.
Radiation in daily life
* Heat energy from the Sun reaches the Earth by radiation.
* While standing near fire we feel the heat which is transferred as radiation.
* Black surfaces absorb heat radiation. So that the bottom of the cooking vessels are painted black.
* White colour reflects heat radiation. That’s why we are advised to wear white cloth during summer.
Heat transfer by radiation is visible to our eyes. When a substance is heated to 500°C the radiation begins to become visible to the eye as a dull red glow, and it is sensed as warmth by the skin. Further heating rapidly increases the amount of radiation, and its perceived colour becomes orange, yellow and finally white.