## Modes of Heat Transfer

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

1. Conduction
2. Convection

### 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.

NOTE-

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

### 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