7.2HEAT FLOW AND

ITS EFFECT

EXPANSION OF METALS CAUSES BY HEAT

Metal expand when they are heated. Metal contract when they are cooled.

The iron ball can pass through the ring

before heating

After heating, the metal ball cannot

pass through the ring

When a metal is heated, its particles receive more energy and vibrate faster.

The particles occupy more space and move further apart from one another. So the metal expands.

On cooling, the vibrations slow down and the particles return to their original position. So the metal contracts.

Metal expands when heated

Metal 1 : ironMetal 2: copper

Different metal expand different amount when heated

AluminiumBrassCopperIronZinc

Expands most

on heating

EXPANSION OF LIQUID CAUSED BY HEAT

Liquid expand when they are heated. Liquid contract when they are cooled.

When a liquid is heated, its particles receive more energy and vibrate faster.

The water molecules occupy more space and move further apart from one another. So the liquid expands.

On cooling, the vibrations slow down. So the liquid contracts.

Coloured water

Water level before heating

Water level after heating

The water level in the glass tube falls a little at first because the flask expands before the water.

Liquid expand more than metals for the same rise in temperature.

Different liquids expand different amount when heated.

AlcoholOilWater

Expands moston heating

Different liquid expand different amount when heated

Different liquid expand different amount when heated

Water

Oil

Alcohol

EXPANSION OF GAS CAUSED BY HEAT

Gas expand when they are heated. Gas contract when they are cooled.

When the air is warmed, the gas particles receive more kinetic energy and move outwards very quickly in all direction.

Gases expand very fast because there is:a) Plenty space between the particlesb) Force of attraction between particles are

very weak. All gases expand at about the same rate

when heated.

Principle of heat transfer

1. Conduction : through solid2. Convection : through fluid (liquid or gases)3. Radiation : through empty space

High temperature Low temperature

3 ways of heat transfer

CONDUCTION Definition: movement of heat through a

solid from a hot part to a cooler part. Metals are good conductors of heat. Non-metal are bad conductors (insulators)

of heat.

CONDUCTION When the hot end of a metal rod is

heated, the particles receive more energy and vibrate faster.

The more active vibrating particles knock their neighbours. So heat is transferred along the metal rod.

Conductors Insulators

SilverCopperGoldAluminiumIronTinLeadMercury

Very good

Good

VacuumAirCorkPlasticWoodGlassAsbestosWater

Application in daily life

CONVECTION Definition:

movement of heat through a fluid by the movement of the fluid particles.

Liquid and gases are referred as fluid.

When a fluid is heated, it expands and become less dense.

The heated fluid rises, and cooler fluid flows down.

Convection cannot take place in solid because the particles in solid cannot move freely.

Convection in a liquid Convection in a gas

Application of convection Electric kettle Refrigerator

Application of convection Ventilation

system Car radiator

Application of convection Sea breeze Land breeze

Sea breeze During the day, the

land is much hotter than the sea.

Hot air above the land rises, and cooler air from the sea moves in to take its place.

Land breeze At night, the land

become colder than the sea.

Hot air above the sea rises, and cooler air from the land moves in to take its place.

Radiation Radiation is the transfer of heat without

using a medium. Radiated heat can travel through a

vacuum. Ex: 1. Heat from the sun travels through a

vacuum in outer space to the Earth.2. Scouts sitting around a camp fire.

Radiation of heat

Application of heat transfer- the thermos flask-

Stopper: made up of cork/plastic (heat insulator).

Glass wall: inner surface of the wall are silvery and reduce heat transfer by radiation.

Vacuum: stop conduction and convection.

Base: made up of cork (heat insulator).

Silvered on inside

Contents

Vacuum

Outer container

Screwed top