7.2 The Effects of Heat Flow on Matter

1. When objects are heated, their volume generally increases. This increase in size called expansion.

2. When objects are cooled, their volume generally decreases. This decrease in size is called contraction. In some cases, the expansion or contraction is very small and not noticeable.

A. Expansion of Metals caused by heat.

1. Expansion and contraction of solids.

a) In the ball and ring experiment, the metal ball can pass through the ring before it is heated.

b) The metal ball cannot pass through the ring after it is heated.

c) However, the metal ball passes through the ring when it is cooled.This shows that the metal ball expands when it is heated and contracts when it is cooled.

Explanation.a) When a metal is heated, its particles (atoms) receive more energy and vibrate faster.b) The particles occupy more space and move further apart from one another, keeping the same relative positions. So the metal expands.

c) On cooling the vibrations slow down and the particles return to their original positions. So the metal contracts.

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2.a) Different metals expand different amounts on heating.

3.a) The strong force of a contracting metal can be shown by the following experiment.

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b) This can be shown by heating a bimetallic strip.

c) A heated copper-iron bimetallic strip bends with the copper on the outside and the iron on the inside.

d) This shows that copper expands more than iron.

(* Important facts!)Aluminium (Expands most on heating)BrassCopperIronZincInvar (Expands least on heating)

b) The steel rod is strongly heated so that it expands. The nut is screwed tightly so that the cast iron bar is held tightly by the steel rod. Then water is poured on the heated rod to cool it.

Observationsa) On cooling, the steel rod contracts and breaks the cast iron bar into two.

ConclusionThis shows that the steel rod exerts great force on cooling.

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B. Expansion of Liquids caused by heat.

1. A liquid expands when it is heated and contracts when it is cooled.

1. Expansion and contraction of liquids.

a) The flask containing coloured water and carrying a glass tube is set up and placed in a basin.b) The water level in the glass tube is set up and placed in a basin.

c) Hot water is poured into the basin.

Observations.The water level in the glass tube fall a little and then rises.The water level drops back to its original position when the flask is taken out of the basin and cooled.

Conclusion.The result show that water expands on heating and contracts on cooling.

Explanation.a) When the water is heated, its particles (molecules) receive more energy and move further apart from one another in all directions.b) The water takes up more space and its volume expands.c) The water level in the glass tube falls a little at first because the flask expands before the water.

2. Different liquids expand different amounts when heated.

2. Different liquids expand different amounts when heated.

a) The three glass bulbs of the same size are filled with different liquids and clamped in an upright position in a basin.

b) The liquid level in each tube is marked, and hot water is poured into the basin to heat the liquids.

Observations.The liquid levels rise to different height. Alcohol rises the highest, followed by oil and water.

The liquid levels return to their original positions when the glass bulbs are taken out and cooled.

Conclusion.The result shows that different liquids expand different amounts on heating.

C. Expansion of Gases caused by heat.

1. Gases expand on heating and contract on cooling.

Observations.

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1. Expansion and contraction of gases.

a) The flask containing air is set up and clamped upright. Zb) The position of the drop of coloured water is marked.c) The flask of air is warmed by placing both hands on it.

The drop of coloured water rises very fast when the air is warmed.When the flask of air is allowed to cool, the drop of coloured water falls very fast.

Conclusion.The result shows that air expands very fast on heating and contracts vary fast on cooling.

Explanation.a) When the air is warmed, the gas particles (molecules) receive more kinetic energy and move outwards very quickly in all directions. So the air expands very fast.

b) Gases can expand very fast because there is : i. Plenty of space between the particles.ii. Hardly any force of attraction between the particles.c) All gases expand at about the same rate when heated.

D. Principle of Heat transfer.

1. Heat flows from a place of high temperature to a place of lower temperature.

2. The bigger the difference in temperature, the faster is the heat flow.3. A hot cup of coffee becomes cold after a while because heat is transferred from

it to the air. This heat transfer will go on until the cup of coffee and the air are the same temperature.

E.

Conduction of Heat1. a) If one end of a metal rod is heated, the other end will become hot.

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High temperatur

e

heat flow

Lowtemperatur

e

Three ways in transferring

the Heata) By conduction b) By convection

c) By radiation

b) Heat has been transferred from hot end to the cooler end through the metal rod. This method of heat transfer is called conduction.

2. Conduction is defined as the movement of heat through a solid from a hot part to a cooler part.

3. Explanation :a) Conduction of heat can be explained by the particle theory.

b) When the hot end of a metal rod is heated, the particles (atoms) receive more energy and vibrate faster.

c) The more active vibrating particles knock their neighbours. In this way the stronger vibrations (heat) are passed along the metal rod. So heat is transferred along the metal rod.

4. Metals are good conductors of heat. They allow heat to flow through them quickly.

5. Non-metals are bad conductors of heat. They are also known as insulators of heat.

6. Different metals conduct heat at different rates i.e. some metals conduct heat faster than others.

F. Experiments on Conduction of Heat

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Conductors Insulators

Good

Very Good

SilverCopper

GoldAluminium

IronTin

LeadMercury

VacuumAir

CorkPlasticsWoodGlass

AsbestosWater

Conduction of heat

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Different materials conduct Heat at Different Rates.

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Water as a Conductor

Heat Insulators.

G. Uses of Heat conductors in Daily Life

H. Uses of Heat Insulators in Daily Life

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Uses of Heat

Conductors in Daily Life

a) Mercury in thermometers- Mercury is good conductor of heat suitable for use in thermometer.- It is warmed and cooled very quickly.

b) Cooking utensils- Pots and pans are made from aluminium / steel so that they can quickly conduct heat from fire to the food.

c) Soldering Irons- The head of a soldering-iron is usually made of copper so that it can rapidly transfer heat to the metals being soldered.

d) Car radiators- Car radiators have copper tubes in front. These tubes conduct heat away from the hot water flowing through them.

e) Refrigerators- Refrigerators have copper pipes for carrying the cooling agent. The pipes remove heat from the agent.

f) Fins on motor cycle engines- The ‘fins’ around the engine of a motor cycle are made of stainless steel to conduct heat away from the engine.

Uses of Heat

Insulators in Daily Life

a) Handles of pots and pans- They are made of wood / plastics so that they cannot get too hot for holding.

d) Styrofoam- Ice is sometimes put into a Styrofoam container to prevent it from melting. The styrofoam prevent prevents heat from going into the container.

c) Saw Dust- Ice is covered with saw dust to prevent heat from melting the ice quickly.

e) Blankets- A blanket traps air among its fur and also air between it and the body. Warm air under the blanket cannot escape and cold air is kept away from the body by the blanket.

f) Fibre glass - Fibre glass is placed in refrigerators and oven to prevent heat transfer by conduction.

b) Igloos - Is made of ice which is heat insulator. It prevents warmth in the igloo from escaping to the air outside it.

I. Heat Transfer by Convection

Convection1. Liquids and gases are referred as fluids.2. Liquid and gases are poor conductors of heat because their particles are far apart, except for mercury.3. Liquids and gases transfer heat by a process called convection,4. Convection is defined as the transfer of heat through a fluid by the movement of the fluid particles.5. Convection can be explained by using the particle theory.

J.Experiments to Show Convection

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a) When a fluid (liquid or gases) is heated, it expands and becomes less dense.

b) The heated fluid rises and cooler fluid flows in to take its place. As a result, convection current is set up in the fluid.

c) Heat is carried upwards by the liquid particles or gas particles when they move upwards. This process is called convection.

d) Convection cannot take place in a solid because the particles in a solid cannot move about freely.

Convection in Air

Convection in Water

K. Applications of Convection

1. Convection currents are made use of in daily life, and are observed in natu ral phenomena such as sea breezes.

2. Electric kettle.

3. Refrigerator.

4. Car Radiator.

5. Ventilation in a building.

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a) The heating element is fixed to the bottom of the kettle. This allows heated water to rise and cool water to come down, setting convection current.

b) In time the water becomes very hot and boils.

a) The freezer compartment is at the top of the refrigerator. Dense cold air from the top sinks and warmer air from below rises and creates convection current.

b) In this way the air in the refrigerator is cooled.

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a) The heat generated when the car engine is running heats up the water in the radiator. The hot water rises and flows down the copper tubes in front of the radiator. Here the water is cooled by air and by conduction of heat by the copper tubes.

b) The water forms convection current in the radiator and moves heat from the engine.

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a) Convection currents are used in ventilating a building.

b) Hot air which is less dense rises and leaved through top openings in the building.

c) Cool air enters the building from below through windows, doors and other openings.

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6. Land Breeze.

7. Sea Breeze.

L. Transfer of Heat by Radiation

1.

2. Radiation does not need the help of a medium; it can travel through a vacuum.

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a) At night the land becomes colder than the sea. (Land is a better conductor of heat than the sea).

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

c) This causes a land breeze.

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a) During the day, the land becomes hotter than the sea. (Land is a better conductor of heat than the sea).

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

c) This causes a sea breeze.

a) When a thermometer is placed below a lighted bulb, the thermometer registers a rise in temperature, showing that heat from the bulb has reached it.

b) The heat could not have reached the thermometer so quickly by conduction (since air is a bad conductor of heat) or by convection (since hot air rises).

c) Heat has moved to the thermometer by a process called radiation.

3. Radiation is defined as the transfer of heat without using a medium.4. Radiated heat :

a) can be absorbed by a surface.b) Can be reflected from a surface.c) Travels at the speed of light.d) Can travel through a vacuum.

5. Heat from the Sun travels through a vacuum in outer space to the Earth by radiation.

6. Scouts sitting around a camp fire receive a lot of heat by radiation.

7. In cold countries, electric radiators are used to radiate heat to warm a room.

M. The thermos Flask

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1. The thermos flask is used to keep hot things inside it hot and cold things inside it cold.

2. It is designed to reduce heat flowing into it or out of it.3. Stopper

The stopper is made of cork or plastic. Both substances are bad conductor of heat. So the stopper reduces heat transfer by conduction and convection (heat is stopped from rising)

4. Glass wall The glass wall is a bad conductor of heat. The inner surface surfaces of the walls are silvery and reduce heat transfer by radiation.

5. VacuumThe vacuum between the glass walls stops conduction and convection.

6. BaseThe base is made of cork which is a bad conductor of heat.

N. Comparing the methods of heat transfer

Conduction Convection Radiation

Book is passed from person to person.

Book is carried by the first person to the last person. Book is thrown by first person to

the last person.1. Heat is transferred from

particle to particle.1. Heat is carried by particles

moving away.1. Heat is set free without using

any particles.

2. Process usually takes place in solids, but can also take place in liquids and gases.

2. Process takes place only in liquids and gases.

2. Process can be carried out by solids, liquids and gases.

3. Process cannot take place in a vacuum.

3. Process cannot take place in a vacuum.

3. Process can take place in a vacuum.

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4. Process is slow. 4. Process is fast. 4. Process is very fast (speed of light).

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