Hello students, heat transfer can be divided in to three types, viz. Conduction, Convection and Radiation etc. Heat transfer from one part of solid surface to its part due to temperature difference is called as conduction. Conduction and convection requires material medium. The fastest mode of transfer of heat is with the help of electromagnetic radiation, known as radiation.
Radiation is the fastest mode of transfer of heat directly from one place to another without any material medium for the transfer of heat. Electromagnetic waves like ultraviolet waves, infra red waves are of different wavelength carries energies without necessity of any medium and hence heat transfer in vacuum is possible due to radiation.
In this article we are going to learn about the concept conduction in details.
Let’s understand concept of conduction in details….!
Definition
Mode of transfer of heat from point of higher temperature of a body to the point of lower temperature is without actual change in position of particles of medium is known as conduction.
Conduction is the slowest mode of transfer of heat which requires material medium.
Suppose that the metal rod XY of length ‘L’ and cross-sectional area ‘A’ having four holes along its length and thermometer placed in the holes to measure the temperature of rod at different points as shown in fig. below,
Consider that the rod is given a constant heat ‘Q’ (from source like steam) such that there is no loss of heat is take place in surrounding. The temperatures T1, T2, T3 and T4 in the thermometers A, B, C and D are recorded in fixed intervals of time till the temperatures in the thermometers attains certain constant value. The following observations can be drawn from the above experiment are,
- After some time it is observed that the temperatures in the thermometers have attained certain constant values (each thermometer shows different value) after some time. The state of thermometers here is known as steady state.
- During steady state, the ratio of difference in temperatures of thermometers at two points and the distance between the two points is always constant. This ratio is known as temperature gradient.
∴ Temperature gradient = dT/dL OR
∴ Temperature gradient = (T1 – T2)/L
The quantity of heat ‘Q’ that transfer from the hot end X to the cold end Y is
i) directly proportional to the cross-sectional area A of the face
∴ Q ∝ A
ii) directly proportional to the temperature difference between the two faces
∴ Q ∝ (T1 – T2)
iii) directly proportional to time t (in seconds) for which heat flows
∴ Q ∝ t
iv) inversely proportional to the length of rod
∴ Q ∝ 1/L
Then from all above conditions we can write that,
From the above equation, “Coefficient of thermal conductivity of a material defined as the rate of flow of heat per unit area per unit temperature gradient when the heat flow is through faces of a thin parallel-sided slab of material.”
SI unit of coefficient of thermal conductivity is,
J s-1 m-1 °C-1 or J s-1 m-1 K-1
Just check the following example to understand the concept in detail….!
Ex: The temperature difference between two ends of iron rod 20 cm long is 18°C. Heat is transferred through rod at the rate of 450 kcal per minute per square metre at steady state. Find the thermal conductivity of iron rod.
Solution: