Transfer of Heat
Heat flows from hotter objects to colder objects. When an object is at a temperature different from its surroundings, heat transfer takes place such that the body and the surroundings reach the same temperature. For example, when milk is boiled and the flame is off, the milk slowly transfers heat to the surroundings and becomes cooler. There are three modes of heat transfer: Conduction, convection and radiation.
Conduction is the process of transfer of heat from the hotter part to the colder part of an object without the movement of its particles. Also, in conduction, heat gets transferred between substances that are in direct contact with each other. For example, when you pop corn in a cooker on a flame, heat is transferred from the flame to the corn by conduction. Based on their ability to conduct heat, materials are classified into conductors and insulators.
Conductors: Materials that allow the flow of heat are called conductors. Examples: copper, steel, silver and iron.
Insulators: Materials that do not allow the flow of heat are called insulators. Examples: wood, paper, rubber, cork, glass, bakelite and ceramic.
Convection is the process of transfer of heat by the movement of particles of a medium from one place to another. It takes place only in liquids and gases. Occurrence of wind currents, the lower floor of a building being cooler than the upper floor, water being warmer at the surface of a swimming pool or lake, and the atmosphere at the sea shore is always pleasant are due to convection.
The process in which heat flows from one object to another either through a medium or through vacuum is called radiation. The heat absorbed from the surroundings by a body increases its temperature. The sun warms up the earth through radiation. A camp fire, microwave oven and a light bulb are all examples of radiation.