Modes of Heat Transfer

There are three Modes of Heat Transfer from one place to another-

  1. Conduction
  2. Convection
  3. Radiation

Read More- Heat and Mass Transfer: Objective Questions with Answers


What do you understand by Conduction?

  • In this mode of heat transfer, heat is transferred from one place to another without the net motion of the molecules of the medium.
  • Conduction generally takes place in solids.
  • It is a slow process compared to convection and radiation.
  • It is experimentally observed that at a steady state.

The rate of heat flow (∆Q/∆t) is proportional to (T1-T2)

T1, T2 are temp. at hot and cold ends at steady states

Also (∆Q/∆t) is proportional to Area

and inversely proportional to the length

Thus, (∆Q/∆t) = KA (T1-T2)/L

K= thermal conductivity of a material

SI Unit of K is watt/mK   or watt/mC

  • K depends only on the type of materials.


  • Length is always taken along the Flow of Heat
  • Cross-Sectional area (A) is taken perpendicular to the flow of heat.

Read More- Radiation: What is Shape Factor?

Important Basic Formulas

Series Combination of Rods

  • If the rate of heat flow is the same in each rod then the combination is called series combination.

Let R1 = L1/K1A & R2 = L2/K2A

Then, Req = R1 + R

  • Rate of Heat Flow (Heat Current)

∆Q/∆T = (T1-T2)/ Req

  • Equivalent Thermal Conductivity (Keq)

Req = R1 + R2

Then, (L1 + L2)/KeqA = (L1/K1A1) + (L2/K2A2)

(L1 + L2)/Keq = (L1/K1) + (L2/K2)

Since A=A1=A2

  • For n rods combination

(L1 + L+L3 +….Ln)/Keq = (L1/K1) + (L2/K2) + (L3/K3) +…….. (Ln/Kn)

Read More- Convection Heat Transfer Process

Read More- Radiation Heat Transfer Process

Conceptual Knowledge

At 4 degrees Celsius density of water is maximum. So as water is heated from 0 to 4, the density of water increases and there is no motion of water molecules. So during heating from 0 to 4 degrees Celsius, heat transfer takes place only due to conduction.

If water is further heated from 4 degree then the transfer of heat takes place due to convection.

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