Heat transfers from hotter objects to colder objects. When an object is at different a temperature from its surroundings, then heat transfer takes place so the body and surrounding reaches the same temperature. Also, heat from a hotter object is transferred to the particles of the surrounding air that are comparatively cooler. For example, when milk is boiled and the flame is off, the milk slowly transfers heat and becomes cooler.
There are three modes of heat exchange: Conduction, convection and radiation.
Conduction is the 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. The better the conductor, the more rapidly does the heat transfer take place.
For example, when you pop corn in a cooker on a flame, heat is transferred from the flame to the corn by conduction.
Materials that allow the flow of heat are called conductors. Examples of good conductors are copper, steel, silver and iron.
Materials that do not allow the flow of heat are called insulators. Examples of insulators are wood, paper, rubber, cork, glass, Bakelite and ceramic.
Convection is the transfer of heat by the movement of particles of a medium from one place to another. It takes place only in liquids and gases.
Examples of convection are wind currents, the lower floor of a building is cooler than the upper floor, and water is warmer at the surface of a swimming pool or lake. Due to convection, the atmosphere at the sea shore is always pleasant.
The process in which heat flows from one object to another either through a medium or vacuum is called radiation. The heat absorbed from the surroundings by a body increases its temperature.
The sun warms the earth through radiation. A camp fire, microwave oven and a light bulb are all examples of radiation.