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Form 1 Chapter 7: Heat
Transcript of Form 1 Chapter 7: Heat
At the end of the chapter.......
7.2 The Effects of Heat Flow on Matter
When objects are heated, their volume generally increases.
This increase in size called expansion.
When objects are cooled, their volume generally decreases.
This decrease in size is called contraction.
7.3 Effect of Heat on Matter
Difference between heat and temperature
Two objects in a room have the same temperature (the room temperature) but may contain different amounts of heat.
When a metal is heated, its particles (atoms) receive more energy and vibrate faster.
The particles occupy more space and move further apart from one another, keeping the same relative positions. So the metal expands.
On cooling the vibrations slow down and the particles return to their original positions. So the metal contracts.
Experiment: Expansion and contraction of solids.
In the ball and ring experiment, the metal ball can pass through the ring before it is heated.
The metal ball cannot pass through the ring after it is heated.
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.
Different metals expand at different rates on heating.
This can be shown by heating a bimetallic strip.
A heated copper-iron bimetallic strip bends with the copper on the outside and the iron on the inside.
This shows that copper expands more than iron.
Alluminium (Expands most on heating)
Invar (Expands least on heating)
The flask containing coloured water and carrying a glass tube is set up and placed in a basin.
The water level in the glass tube is set up and placed in a basin.
Hot water is poured into the basin.
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.
The result show that water expands on heating and contracts on cooling.
When the water is heated, its particles (molecules) receive more energy and move further apart from one another in all directions.
The water takes up more space and its volume expands.
The water level in the glass tube falls a little at first because the flask expands before the water.
Expansion of Liquids caused by heat
Different liquids expand different amounts when heated
The three glass bulbs of the same size are filled with different liquids and clamped in an upright position in a basin.
The liquid level in each tube is marked, and hot water is poured into the basin to heat the liquids.
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.
The result shows that different liquids expand different amounts on heating.
Expansion of Gases caused by heat
The flask containing air is set up and clamped upright
The position of the drop of coloured water is marked.
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.
The result shows that air expands very fast on heating and contracts vary fast on cooling.
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.
Principle of Heat transfer
Heat flows from a place of high temperature to a place of lower temperature.
The bigger the difference in temperature, the faster is the heat flow.
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.
Conduction of Heat
What is conduction?
If one end of a metal rod is heated, the other end will become hot.
Heat has been transferred from hot end to the cooler end through the metal rod. This method of heat transfer is called conduction.
Conduction is defined as the movement of heat through a solid from a hot part to a cooler part.
Conduction of heat can be explained by the particle theory.
When the hot end of a metal rod is heated, the particles (atoms) receive more energy and vibrate faster.
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.
Metal and Non Metal
Metals are good conductors of heat. They allow heat to flow through them quickly.
Non-metals are bad conductors of heat. They are also known as insulators of heat.
Different metals conduct heat at different rates i.e. some metals conduct heat faster than others.
Heat Transfer by Convection
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.
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.
c) 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.
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.
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.
Liquids and gases are referred as fluids.
Liquid and gases are poor conductors of heat because their particles are far apart, except for mercury.
Liquids and gases transfer heat by a process called convection,
Convection is defined as the transfer of heat through a fluid by the movement of the fluid particles.
Convection can be explained by using the particle theory.
At night the land becomes colder than the sea. (Land is a better conductor of heat than the sea).
Hot air above the sea rises and cooler air from the land moves in to take its place.
This causes a land breeze.
During the day, the land becomes hotter than the sea. (Land is a better conductor of heat than the sea).
Hot air above the land rises and cooler air from the sea moves in to take its place.
This causes a sea breeze.
Transfer of Heat by Radiation
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.
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).
Heat has moved to the thermometer by a process called radiation.
Radiation does not need the help of a medium; it can travel through a vacuum.
Radiation is defined as the transfer of heat without using a medium.
Radiated heat :
can be absorbed by a surface.
can be reflected from a surface.
travels at the speed of light.
can travel through a vacuum.
Heat from 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 a room.
The thermos flask is used to keep hot things inside it hot and cold things inside it cold.
It is designed to reduce heat flowing into it or out of it.
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)
The glass wall is a bad conductor of heat. The inner surface surfaces of the walls are silvery and reduce heat transfer by radiation.
The vacuum between the glass walls stops conduction and convection.
The base is made of cork which is a bad conductor of heat.
7.4 Application of Expansion and Contraction of Matter
a) When a gas is cooled, its particles release heat and lose energy.
b) When the gas particles lose energy, they move nearer to one another and become liquid
c) Examples of condensation
I. Water droplets form on the outer wall of a glass containing ice cubes.
II. Dew forms on grass during a cold night.
a) When a liquid is heated or exposed to heat at any temperature below its boiling point, evaporation occurs.
b) When the liquid particles absorb enough energy, they move further away from one another and become gas particles.
c) Examples of evaporation
I. Alcohol evaporated from the skin when applied on it.
II. Water evaporated from wet clothes.
a) When a solid is heated, the solid particles absorbs heat and become more energetic. This will cause the solid particles to vibrate and rotate at a higher speed.
b) When the solid particles gain enough energy, they move further away from one another and become liquid particles. The temperature at which the solid turns into a liquid is known as melting point.
c) Examples of melting
I. A candle melts when it is heated.
II. Ice melts into water when it is heated.
a) When a liquid is cooled, its particles release heat and lose energy.
b) When the liquid particles lose energy, they move nearer to one another and become solid
c)The temperature at which the liquid turns into a solid is known as freezing point.
d) An example of freezing is that water turns into ice when it is frozen in a freezer.
a) When a gas releases heat and turns into a solid or when a solid absorbs heat and turns into a gas without going through the liquid state, sublimation occurs.
b) During sublimation, solid particles gain enough energy to become gas particles without going through the liquid state while gas particles release enough energy to become solid particles without going through the liquid state.
c) Examples of sublimation
I. Mothballs in a cupboard undergo sublimation.
II. Iodine crystals sublimate to iodine vapour when they are heated.
7.5 Absorbing and Giving Out Heat
The rate of absorption and radiation of heat for an object is determined by the
a) Temperature of the surrounding
b) Colour and type of its surface
The principle of heat flow can be used to improve air circulation. a) Opening windows of buildings. b) Installing air conditioner at the top part of a wall.
c) Installing air suction devices or air turbines on the roof of buildings.
d) Ensuring that building have ventilation holes.
7.6 The Benefits of Heat Flow
Benefits of heat flow
a) Ensuring that the air is cool and clean
b) Preserving our health and the comfort of our lives.