Physical Science:

It is a process of the transfer of heat from one location to another due to temperature difference. Heat can only flow from high temperature to low temperature. There are three processes involved in heat transfer. They are conduction, convection, and radiation. Conduction: In this process heat is transferred through a body by physical contact. In microscopic level atoms and molecules are always in random motion at any temperature above absolute zero. When heated, these particles (atoms and molecules) gain thermal energy and begin vibrating at even higher amplitude knocking the neighboring particles. By doing so, they impart some of their energy to their neighboring particles, and so on. Eventually heat is carried away from one end of a substance to another. In conduction, heat carrying particles do not physically move from their place rather they transfer their energy to neighbors when they knock one another. Every time one particle knocks onto another it passes some of its energy to the next one then returns to its original place. Heat transfers from the burner of a stove through the bottom of a pan to food in the pan is transferred by conduction. Conduction occurs in all states of substances solid, liquid, and gas. In solids, conduction happens due to molecular vibration. In gases and liquids, it is due to collision and diffusion of the molecules during their random motion.

Convection: In this process heat transfer occurs by the actual movement of fluid particles. Convection is heat transport due to bulk fluid motion. Convection happens in gases and liquids. When a fluid is heated, its molecules which are in contact with the hot surface get hot and expand. That is the fluid above a hot surface expands, becomes less dense, and rises. It carries the thermal energy along when rising. The relatively cold fluid on the upper surface is heavier and sinks down near the heat source. Hence a current of heat cycle develops in the convection process, called convection cycle.

Radiation: It is the process in which heat transfers without the need of an intervening medium. Heat transfer occurs when electromagnetic waves are emitted or absorbed. The warming of the earth surface by the Sun is due to radiation. Heat transfer by this process is the fastest one. All bodies above absolute zero emit thermal radiation. At room temperature the radiation is in the infrared range, wavelengths longer than those of the visible spectrum. In an atomic domain, the radiation is caused because of oscillating ions and electrons in a warm solid that are accelerating electric charges, such charges radiate. Different substances radiate with different efficiencies. A good radiator of radiation is a good absorber of radiation. A perfect absorber is called a black body (such perfection is not found in nature, but some things are close). Hence a black body is also a perfect radiator. A black body absorbs and emits radiation of all most all frequencies. It was found experimentally that heat energy radiated per unit time per unit surface area of a perfect black body is proportional to the fourth power of its absolute temperature.