Modes of Heat transfer

Heat is a form of energy which transfers between bodies which are kept under thermal interactions. When a temperature difference occurs between two bodies or a body with its surroundings, heat transfer occurs.There are  different modes of heat transfer  which are:

1. Conduction
2. Convection and
3. Radiation

1. conduction : Conduction with the mode of heat transfer in which the transfer of heat takes place only when the physical contact between the two surfaces is present. i.e.  in this, only the heat energy transfer from one point to another within a given body whereas the particles physically remain at same place(in case of solids). this could be understood by taking a simple example of a hot one piece pan.  In such case when the pan is kept on the flame,  even the handle of the pan gets heated up after awhile. This is because the heat gets conducted from the bottom of the pan to handle of the pan.The heat transfer occurs here by the two mechanisms happen.

1. By the transfer of free electrons. (Good conductors like metals have a plenty of free electrons to make conductive heat transfer.

2. The atoms and molecules having energy will pass those energy they have with their adjacent atoms or molecules by means of lattice .

Fourier Law of Conduction:

Q = -kAdT/dx

Where: Q is the heat flow rate by conduction

K is the thermal conductivity of the material

A is the cross sectional area normal to direction of heat flow and

2. Convection:  it is the transfer of heat by the actual movement of the warmed matter. Heat leaves the coffee cup as the currents of steam and air rise. Convection is the transfer of heat energy in a gas or liquid by movement of currents. (It can also happen is some solids, like sand.) The heat moves with the fluid. Another example is:  macaroni rise and fall in a pot of heated water because of convection. The warmer portions of the water rises because they are less dense. Meanwhile, the cooler portions of the water fall because they are denser.Convection can be of two types:

1. Natural convection: In this type of convection, the movement of particles which constitutes convection occurs by the variation in densities of the fluids. As we already know, as temperature increases, the density decreases and this variation in density will force the fluid to move through the volume. This cause convection to occur.
2. Forced Convection: The difference between natural convection and forced convection is that in forced convection, a work is done to make movement in the fluid. This is done using a pump or blower.

3. Radiation : it is the transfer of heat by means of electromagnetic waves. To radiate means to send out or spread from a central location. Whether it is light, sound, waves, rays, flower petals, wheel spokes or pain, if something radiates then it protrudes or spreads outward from an origin. The transfer of heat by radiation involves the carrying of energy from an origin to the space surrounding it. The energy is carried by electromagnetic waves and does not involve the movement or the interaction of matter. Thermal radiation can occur through matter or through a region of space that is void of matter (i.e., a vacuum). In fact, the heat received on Earth from the sun is the result of electromagnetic waves traveling through the void of space between the Earth and the sun.

All objects radiate energy in the form of electromagnetic waves. The rate at which this energy is released is proportional to the Kelvin temperature (T) raised to the fourth power.

Radiation rate = k•T4