# Heat Transfer

## 1. Introduction

• We already know that heat is the energy transferred from from one systm to another or from one part of the system to its another part , arising due to temperature difference.
• Heat can be transferred from one place to other by through three different modes conduction,convection & radiation.

## 2. Thermal Conduction

• Conduction of heat takes place in a body when diffrent part of body are at diffrent temperature.
• To notice conduction of heat put one end of metal rod on flame and another end on your hand. After some time you will feel hotness in your hand also.
• Here heat transfer takes place from hot end on flame to cold end in your hand through conduction.
• How heat transfers through conduction is given in steps below
1)Molecules at hot end of the rod begin to vibrate as there is an increase in the energy of vibration as temperature of the end of rod on flame increases.
2)These vibrating molecules then collides with the nearest neighbour sharing their energy with them and increasing their energy.
3)These neighbouring molecules further pass their energy to molecules on colder end of the rod i.e farther from the end put on flame.
4)This way energy of thermal motion is passed along from one molecule to the next keeping their original position fixed.
• Metals are good conducters of electricty as well as heat.

## 3.Thermal Conductivity

• Ability of Material to conduct heat is measured by thermal conductivity of that material.
• Consider a slab of uniform crossection A and length L also one face of slab is kept at Temperature T1 and another at T2 and remaining surface area is covered with a non conducting material to avoid transfer of heat.

• After sufficient time slab reaches steady state temperarure at every point will remain unchanged.
• In steady state, rate of flow of the heat through any crosssection of slab is
a) directly propertional to area A
b) directly propertional to temperature diffrence (T2-T1)
c) inversely propertional to length
• Thus if H is the quantity of heat flowing through slab per unit time then

where k is a constant whose numerical value depends on the material and is called thermal conductivity of the material.
• S.I. unit of thermal conductivity is Js-1m-1K-1.
• For small amount of heat dQ flowing between two faces of slab in small time interval dt,
• Materials for which K is large are good conductors of heat, while small value of K for a material implies material is poor conductor of heat.