TOPIC 6:

HEAT

FIZ1801

LEARNING OUTCOMES

At the end of each subtopic, students should be able to:

1) Define heat and explain the mechanism of heat transfer

2) Use rate of heat conduction, = − to solve problems.

3) Define thermal expansion and factors affecting the thermal expansion.

4) Solve problems involving linear, area and volume thermal expansion.

WHAT IS

Heat is a form of energy that is transferred from one body to

another as the result of a difference in temperature.

• The more movement of molecules, the more heat energy

• Heat energy has the ability to do work.

MODE OF HEAT TRANSFER

Conduction

Convection

Radiation

Conduction

Conduction is heat transfer within solids or

between contacting solids.

Convection

Convection is heat transfer by the movement

of liquids or gasses.

Radiation

Radiation is heat transfer through space or air

without any aid of material medium.

5

LET’S GUESS

Conduction

Convection

Radiation

Conduction

Convection

Radiation

Conduction

Convection

Radiation

Conduction

Convection

Radiation

Conduction

Convection

Radiation

Conduction

Convection

Radiation

Conduction

Convection

Radiation

Conduction

Convection

Radiation

https://kahoot.it/

Heat Conduction

If one end of a metal rod is at a higher temperature, then energy will be

transferred down the rod toward the colder end because the higher speed

particles will collide with the slower ones with a net transfer of energy to the

slower ones.

After certain period of time, the rod reached a steady state – 16

the temperature at each points remains constant with time.

For conduction between two plane

surfaces , the rate of heat transfer is

dQ k A dT steady-state

dt dx

Q k A TH TC

t L

T is temperature (in Kelvin),

A is the exposure area (meters squared),

L is the depth of the solid (meters), and

k is a constant that unique for different materials know

as the thermal conductivity (Watts/meters*Kelvin).

17

Material Thermal conductivity k A good conductor has a

(W.m-1.K-1) large value of k.

diamond 2450

Cu 385 18

Al 205

Brick 0.2

0.8

Glass 0.2

Body fat 0.6

0.2

Water 0.01

Wood 0.024

Styrofoam

Air

EXAMPLE

Calculate the rate of heat transfer on a cold day through a rectangular

window that is 1.2 m wide and 1.8 m high, has a thickness of 6.2 mm, a

thermal conductivity value of 0.90 W/mK. The temperature inside the home

is 21°C and the temperature outside the home is -4°C.

Rate of heat conduction = ( 1− 2)

(0.90 −1 −1)(1.2 1.8 )(294.15 − 269.15 )

= 6.2 10−3

= 7838.7

HEAT CONDUCTION THROUGH A

COMPOUND SLAB

1 2

1 1 2 2

steady-state 1 ( 1 − ) = 2 ( − 2)

1 2

EXAMPLE

The figure shows two metal rods M and N with lengths 30 cm and 10 cm respectively.

The cross section for both rods are the same and they joined end-to-end as shown.

The ends are maintained at 100 ⁰C and 0 ⁰C respectively. The rods are perfectly

insulated. Calculate the temperature of the junction.

(Given :Thermal conductivity of M= 30W/m ⁰C and that of N = 60 W/m⁰C

?

At steady state; 1 ( 1 − ) = 2 ( − 2)

1 2

30 A(100⁰C − T) = 60 A(T − 0⁰C)

⁰C ⁰C

0.03 0.01

T= 14.3 ⁰C

THERMAL

EXPANSION

FACTOR AFFECTING

THERMAL EXPANSION

Temperature Type of Material Original

Dimension

LINEAR EXPANSION

Coefficient of linear thermal

expansion of some common solids

EXAMPLE

A metal rod of length 7 m is being heated to 35o C. If the length of rod expands to 7.12 m

after some time, calculate the expansion coefficient (Room temperature is 27o C).

THERMAL EXPANSION COEFFICIENTS

L L0 T A A0 T V V0 T

2 3

EXERCISE

#Text book

Check Point 13.2