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. In solids heat transfer takes place by conduction. 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. Materials that allow heat to flow through them are called conductors. Examples: copper, steel, silver, iron and mercury. Materials that do not allow heat to flow through them 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, sea breeze, land breeze, 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 pleasant atmosphere at the sea shore are due to convection.
The process in which heat flows from one object to another either through a medium or through vacuum, without heating up the space in between, 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, a microwave oven and a light bulb are a few examples of objects that transfer heat by radiation.