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 Îș
  • 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
Thermal Conductivity
  • 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

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;

Temperature Gradient

  • 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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