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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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Uses of heat in daily life.
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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Uses of heat in daily life.
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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