Heat Transfer: Thermal Conductivity

What is Heat Transfer?

Heat is a very curious form of energy. It helps us stay warm, prepare hot and tasty food but its applications far exceed the domestic uses mentioned here. Understanding the properties of heat  and heat transfer is the key to many fields of science. Thermodynamics is a massive field that deals only with the flow of heat through a system that is heat transfer through a system. Even nuclear energy uses the heat developed by the atom to create electricity. So it is clear that heat is quite important to us. That makes it imperative for us to take a closer look at heat.

Heat transfer can occur only through three means:

  • Conduction
  • Convection
  • Radiation

Heat Transfer: Conduction

heat transfer

Heat Conduction refers to the transfer of heat between bodies due to physical contact between the bodies. The transfer of heat by conduction actually occurs at a molecular level. Absorption of heat by a body caused the molecules of that body to gain excess energy. What do you do when you’re too energetic? You get very jittery and shaky, don’t you? You just want to move around to expend this energy. That is exactly what molecules do too.

In the process of gaining energy and vibrating excessively they bump into their neighbours and transfer a little bit of its extra energy to them. This extra energy appears in the neighboring molecules and heats them up too. This is how heat is transferred as long as heat is still being supplied.

Factors affecting thermal conductivity

The rate of thermal conductivity depends on four basic factors;

  • Temperature Gradient: This 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 basically tells us about 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.
  • 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. With large bodies we also have to consider the heat loss to the environment. Also a greater surface area between the hot and the cold body implies a greater rate of heat transfer.
  • The physical properties of the body play an immense role in thermal conductivity through the body. Not all bodies are blessed with the same thermal behaviour. 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 JS-1m-1K-1. The thermal conductivity of a material in measured on a scale. This scale has two extremes; one the end of high thermal conductivity we have Silver with a perfect score of a 100 in heat conduction. On the 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.

To understand heat transfer in a better way, watch our video “Visualizing Heat” given below:

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Practise This Question

Two different metal rods of the same length their ends kept at the same temperatures θ1 and θ2 with θ2 > θ1 If A1 and A2 are their cross-sectional area and K1 and K2 their thermal conductivities, the rate of flow of heat in the two rods will be the same if