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

  1. Conduction
  2. Convection
  3. Radiation


  • 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 as compared to convection and radiation.
  • It is experimentally observed that at 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 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.



  • If the rate of heat flow is 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 Process- ‘Click Here

Read More- Radiation Process- Click Here’

Conceptual Knowledge

At 4 degree Celsius density of water is maximum. So as water is heated from 0 to 4, density of water increases and there is no motion of water molecules. So during heating from 0 to 4 degree 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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