# Heat Transfer: Thermal Conductivity, Equations on Thermal Conductivity (For CBSE, ICSE, IAS, NET, NRA 2022)

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# Heat Transfer: Thermal Conductivity

• Thermal conductivity often denoted by k, , or κ
• It refers to the intrinsic ability of a material to transfer or conduct heat. It is one of the three methods of heat transfer, the other two being convection and radiation.
• The molecules in two bodies at different temperatures have different average kinetic energies
• It is also defined as the amount of heat per unit time per unit area that can be conducted through a plate of unit thickness of a given material, the faces of the plate differing by one unit of temperature.
• It occurs through molecular agitation and contact, and does not result in the bulk movement of the solid itself.
• Heat moves along a temperature gradient, from an area of high temperature and high molecular energy to an area with a lower temperature and lower molecular energy.
• The transfer will continue till thermal equilibrium is reached.
• The rate at which heat is transferred is dependent upon the magnitude of the temperature gradient, and the specific thermal characteristics of the material.

## Equations on Thermal Conductivity

Where:

• Q = heat flow (W)
• L = length or thickness of the material (m)
• A = surface area of material ( ​​)
• = temperature gradient (K)

## Factors Affecting Thermal Conductivity

The rate of thermal conductivity depends on 4 basic factors;

• It is a physical quantity that illustrates to us in which direction and at what rate the temperature changes the most rapidly around a particular location.
• It tells that the temperature difference between places and the direction of transfer due to it. It is important to remember that heat always flows from the hottest to coldest spot.
• This flow will continue till the temperature difference disappears and a state of thermal equilibrium is reached.

### Physical Dimensions of the Body

• Cross section and path length are dependent on the physical dimensions of the body.
• If the size of the body is large, then the heat required to heat it is also larger.
• Having large bodies, we also have to consider the heat loss to the environment.
• A greater surface area between the hot and the cold body implies a greater rate of heat transfer.

### Properties of the Body

• Not all bodies have the same thermal behavior.
• We measure the rate of transfer of heat through the material using a parameter called the Thermal Conductivity of the material (K) .
• The more the value of K, more easily and quickly it can conduct heat. The SI Unit of K is .
• The thermal conductivity of a material in measured on a scale.
• Scale has two extremes; one the end of high thermal conductivity we have Silver with a perfect score of a 100 in heat conduction.
• Other end of the scale we have vacuum, which is absent of molecules and hence is incapable of conducting heat.
• Everything else is ranked between this, for example, Copper (92) , Iron (11) , Water (0.1) , Air (0.006) and Wood (0.03) .
• Materials that are poor conductors of heat are called insulators.

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