**Modes of Heat Transfer**

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

**Conduction****Convection****Radiation**

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

**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 (T_{1}-T_{2})

T_{1}, T_{2} 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 (T_{1}-T_{2})/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.

**Read More- Radiation: What is Shape Factor?**

**IMPORTANT BASIC FORMULAS**

__SERIES COMBINATION OF RODS__

__SERIES COMBINATION OF RODS__

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

Let R_{1} = L_{1}/K_{1}A & R2 = L_{2}/K_{2}A

Then, R_{eq} = R_{1} + R_{2 }

*Rate of Heat Flow (Heat Current)*

∆Q/∆T = (T_{1}-T_{2})/ R_{eq}

*Equivalent Thermal Conductivity (K*_{eq})

R_{eq} = R_{1} + R_{2}

Then, (L_{1} + L_{2})/K_{eq}A = (L_{1}/K_{1}A_{1}) + (L_{2}/K_{2}A_{2})

(L_{1} + L_{2})/K_{eq} = (L_{1}/K_{1}) + (L_{2}/K_{2})

Since A=A_{1}=A_{2}

*For n rods combination*

(L_{1} + L_{2 }+L_{3} +….L_{n})/K_{eq} = (L_{1}/K_{1}) + (L_{2}/K_{2}) + (L_{3}/K_{3}) +…….. (L_{n}/K_{n})

**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.