heat for form 1 a

8/22/2019 Heat for Form 1 A

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HEAT for F1A( Revision)


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HEAT AS A FORM OF ENERGY

The sun as a source of heat

1. The sun is a very hot object

2. Its get its heat energy from nuclear reactions takingplace in its center.

3. The temperature at its center is about 15 000 000C

and its surface temperature is between 10 000C and

500 000C.

4. Only a small portion of the suns energy reaches the

earth. Without heat energy from the sun, the earth will

be a very cold place, forever in darkness and no living

things on the earth.


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Heat as a form of energy

1.Heat is a form ofenergy

2.Heat can travel through a vacuum

3.Heat causes things to become hot

4.Heat moves from a hot place to a cooler place

5.Heat is measure in unit calledjoules (J)

6.Heat has different effects on different

substances


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1.Heat cause metals to expand

2.Heat causes air to expand and rise

3.Heat can cause things to burn and give outlight

4.Heat can cause a change of state e.g. ice to

melt and water to boil5.Heat can cause a substance to decompose.


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Ways to produce heat

Heat can be produced in many ways from

different form of ENERGY.

From kinetic energy.

Rubbing our hands together produces heat.

This is due to friction between the hands

Friction changes kinetic energy into heat.


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From electrical energy.

Heat is produced whenever electrical energy passes

through a wire.

A thin high resistance wire produces a lot of heat when a

electric current passes through it.

This principle is used in an electric iron, electric toaster

and electric kettle.

The filament in a bulb becomes a white hot and gives out

light when an electric current passes through it.


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From chemical energy

Heat can be produced from chemical energy

during a chemical reaction

Heat is produced when a fuel burns.

Carbon + oxygen carbon dioxide + energy (heat) + light

Heat is produced during respiration in the cells of the body.

Glucose + oxygen carbon dioxide+ water + energy ( heat)


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From sound energy.

Heat is produced when sound energysuch as a loud noise is absorbed by the

wall of a room.

The heat produced is so little that it isdifficult to detect.


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From nuclear energy.

A tremendous amount of heat is produced

when a nuclear explosion takes place.The heat produced during a nuclear explosion

is so strong that it melt metals

The heat produced in a nuclear reactor is usedto produced steam to drive turbines and

generators for producing electricity.


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From solar energy.

Solar energy is converted to heat for used in some hot

water system in Malaysia.

Solar cells are used to produce electrical energy


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Uses of heat in daily life.

1.For keeping warm---- in cold industries,

electric radiator and coal fires are used to

warm houses.

2.For cooking---- electric cookers, toaster and

kettles are used to generate heat for cooking.

3.For producing light -----the filament in anelectric bulb becomes so hot that it gives out

light.


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4. For heating----- gas burner in school

laboratories are used to supply heat for

heating substances and things.

5. For washing and cleaning

6. For melting

7. For producing steam


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8. For sterilizing

9. For making substances

10. For treatment of injured muscle.

E.g. infra red heat.


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Differences between heat and temperature

Temperature Heat

1. Temperature measure how hot a

substance is.

2. The temperature of an object rises

when heat is added to it, but falls when

heat is removed from it.

3. Temperature is usually measured in

degree Celsius. (C)

4. A hot object has higher temperature

than a cooler object.

1. Heat is a form of energy.

2. An object become hotter when heat is

added to it ( temperature rises), and

cooler when heat is removed from it (temperature falls).

3. Heat is measured in joule (J)

4. The amount of heat in an object

depends on

Its temperature

Its mass ( sizes)

Its capacity to hold heat


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Heat flow and its effect

EXPANSION OF METAL CAUSED BY HEAT

1. Metals expand when they are heated

2. Metals contact when they are cooled


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1. In the ball and ring experiment, the metal ball can pass through the ring

before it is heated.

2. The metal ball can not pass through the ring after it is heated

3. However , the metal ball passes through the ring when it is cooled.

4. This shown that the metal ball expands when it is heated and contracts

when it is cooled.


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Explanation When a metal is heated, its particles (atoms)

receive more energy and vibrate faster.

The particles occupy more space and more

further apart from one another, keeping thesame relative positions. So the metal

expands.

On cooling, the vibrations slow down and the

particle return to their original position. So the

metal contracts.


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EXPANSION OF LIQUID CAUSED BY HEAT.

1. A liquid expand when it is heated.

2. A liquid contract when it is cooled.

3. This can be shown by the following experiment.


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1. The flask containing coloured water and carrying a glass tube is set up

and placed in a basin.2. The water level in the glass tube is marked.

3. Hot water is poured into the basin

4. It is observed that the water level in the glass tube falls a little and then

rises.

5. The water level drops back to its original position when the flask is taken

out of the basin and cooled.

6. The result show that water expands on heating and contract on cooling.


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Explanation

When the water is heated, its particles

receive more energy and move further apart

form one another in all direction.

The water takes up more space and soexpands.

The water level in the glass tube falls a little

at first because the flask expands before the

water.


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EXPANSION OF GASES CAUSED BY HEAT

1. Gases expand on heating and contact on cooling.

2. This can be shown by experiment below.


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1. The flask containing air is set up and clamped upright.

2. The position of the drop of coloured water is marked.

3. The flask of air is warmed by placing both hands on it.

4. It is observed that the drop of coloured water rises very fast when the air

is warmed.

5. When the flask of air is allowed to cool, the drop of coloured water falls

very fast.

6. The result show that air expands very fast on heating and contacts very

fast on cooling.


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Explanation.

When the air is warmed, the gas particles receive

more kinetic energy and move outwards very

quickly in all directions. So the air expands very

fast.

All gases expand at about the same rate when

heated.

Gases can expand very fast because there is:

1. Plenty of space between the particles.

2. Hardly any force of attraction between the particles.


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Principle of heat transfer

Heat flow from a place of high temperature to

a place of lower temperature.

The bigger the difference in temperature, the

faster is the heat flow.

High temperature Low temperatureHeat flow


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Heat is transferred in three ways:

By conduction

By convection

By radiation


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Comparing the method of heat transfer


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Conduction of heat

If one end of a metal rod is heated, the otherend will become hot.

Heat has been transferred from the hot end to

the cooler end through the metal rod(conductor).


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Explanation

When the hot end of a metal rod is heated,

the particles (atoms) receive more energy

and vibrate faster.

The more active vibrating particles knocktheir neighbours. In this way the stronger

vibration ( heat ) are passes along the metal

rod. So heat is transferred along the metal

rod.


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Metals are good conductors of heat. They are

allow heat to flow through them quickly.

Non metals are bad conductors of heat. They

are also called insulator of heat.

Very good

Good

Conductors

Silver

Copper

Gold

AluminiumZinc

iron

Tin

Lead

mercury

insulators

Vacuum

Air

Cork

PlasticWood

Glass

Asbestos

Water


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Heat transfer by convection

Convection is defined as the transfer of heat

through a fluid by the movement of the fluid

particle.

Liquid and gases are referred to as fluid.


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Explanation..

When a fluid is heated, it expand and

becomes less dense.

Then heated fluid rises and cooler fluid flows

in to take its place. As a result, a convectioncurrent is set up in the fluid.

Heat is carried upwards by the fluid particles

when they move upwards.

This is called convection.

Convection cannot take place in a solid

because the particles in a solid cannot move

freely.


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Application of convection


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Land breeze


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Sea breeze


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Heat transfer by radiation

Radiation is defined as the transfer of heat

without using medium.

Heat from the sun travels through a vacuum

in outer space to the earth by radiation. Radiated heat:

-Can be absorbed by surface

-Can be reflected from a surface-Travels at the speed of light

-Can travel through a vacuum


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Heat form the sun travels through a vacuum

in outer space to the earth by radiation

Scouts sitting around a camp fire receive a lot

of heat by radiation. In cold countries, electric radiators are used

to radiate heat to warm room.


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Effect of heat on the state of matter

Physical processes that change the state of matter.

Absorption of heat Release of heat

Melting Freezing

Boiling Condensation

Evaporation Sublimation

Sublimation


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Melting

Melting takes place when a solid is heated and

changes into liquid.

Heat is absorbed during melting.

When ice is heated, its particles receive more energy

and vibrate faster. The energetic particles move away

from their originally arranged positions. When this

happens the ice melts.

A solid melts at a definite temperature called its

melting point.

- Ice melts at 0C

- Iron melts at 1540C


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Boiling

Boiling takes place when a liquid is heated until it

change to a gas at its boiling point.

Heat is absorbed during boiling

When water is heated, its particle (molecules) receive

more energy and move very actively. When they get

suffient energy, they escape into the air as gas

particle and boiling takes place.

A liquid boils at a fixed temperature called its boiling

point.

- Water boils at 100C

-Ethanol (alcohol) boils at 78C


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Evaporation

Evaporation as the process of changing a liquid into agas at a temperature below the boiling point of liquid.

Heat is required for evaporation occur.

When a little alcohol is exposed to the air, its particlesreceive more energy from the heat in the air. Theparticles escape into the air when they gain sufficientkinetic energy. This happen below the boiling point ofthe liquid.

How fast a liquid evaporates depends on

- Wind movement

- Amount of water vapour in the air

- Temperature of the air

- Surface area of liquid exposed to the air.


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Condensation

Condensation is the process of cooling a gasinto a liquid

Heat is removed from the gas duringcondensation.

When steam (gas) is cooled, its particles(water molecules) lose kinetic energy andcome closer to one another. Eventually theparticles group together to form water.

Oxygen gas can be cooled and condensed toform liquid oxygen.


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Freezing

Freezing is the process of cooling a liquid to form a

solid

Heat is removed from the liquid during freezing.

When a liquid is cooled, its particles lose energy to

the surroundings, move more slowly and come very

close to one another. When the particles arrange

themselves in an orderly manner, the liquids turns

into solid.

- Freezing point of naphthalene , 80C

- Freezing point of water, 0C

- Freezing point of nitrogen, -210C


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Sublimation

Sublimation is the process of changing a solid to a vapour andthen from vapour into a solid without going through the liquidstate.

Heat absorbed when the solid change to vapour, heat is lostwhen the vapor changes to a solid.

Ammonium chloride sublimes when it is heated. When heated

the particles of ammonium chloride gain energy and overcomethe attracting forces between them.

Eventually the particles have sufficient energy to escapedirectly into the air without forming a liquid

When the vapour particles are cooled, they loose energy andregroup themselves into a solid.

Example of other substance which sublimes are- Iodine

- Solid carbon dioxide (dry ice)


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APPLICATIONS OF EXPANSION

AND CONTRACTION OF MATTER


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SOLVING PROBLEMS CAUSEDBY EXPANSION AND

CONTRACTION


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