8I Heating and Cooling

KS3 Physics

8I Heating and Cooling 1 of 20 44

© Boardworks Ltd 2005 2004

Contents

8I Heating and Cooling Heat and temperature Conduction and convection Radiation Changes of state and evaporation Summary activities 1 2 of 20 44


What is heat? Heat is a type of energy. Heat is the name for the type of kinetic energy possessed by particles. Heat energy is measured in joules (J). How many joules are there in a kilojoule (kJ)?

If something gains a lot of heat energy, it becomes hot – so what is temperature?

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What is temperature? Temperature is a measure of how hot or cold an object is. (It is not the total amount of energy contained in the object.)

Temperature is measured in degrees Celsius (ºC). The freezing point of water is defined as 0ºC (at 1 atm.). The boiling point of water is defined as 100ºC (at 1 atm.). Temperature can be measured by a variety of different thermometers. These include liquid in glass, digital, thermocouple and bimetal strip thermometers.

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Heat transfer The correct phrase for heat transfer is ‘thermal transfer’. Heat energy can be transferred (moved) by four processes:

1. conduction 2. convection 3. evaporation 4. radiation In each process, thermal energy is also transferred in the way:

HOT

COLD

Heat energy only flows when there is a temperature difference. 1 5 of 20 44


The particle model The differences between solids, liquids and gases can be explained by the particle model:

1.  All substances are made up of particles (atoms, ions or molecules).

3.  These particles are attracted to each other, some strongly and others weakly.

4.  These particles move around (i.e. have kinetic energy). 5.  The kinetic energy of particles increases with temperature.

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Temperature and energy What happens to the gas as the temperature increases? Would this be the same for solids and liquids?

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Contents

8I Heating and Cooling Heat and temperature Conduction and convection Radiation Changes of state and evaporation Summary activities 1 8 of 20 44


Conduction experiment Conduction of heat in different materials can be investigated in an experiment. Apparatus:

8 cm strip of copper 8 cm piece of wood Bunsen burner stopwatch blob of wax

Take care whilst doing this experiment: l  Wear safety glasses. l  Do not touch mot metal objects! l  Wash any burn with cold water for ten minutes. 1 9 of 20 44


Conduction experiment

l  Add a blob of wax to one end of the copper and hold the other end in the yellow Bunsen flame. l  How long does it take for the wax to melt and drop off the metal strip? l  Now do the same for wood. l  Why are the times very different? 1 10ofof20 44


Conduction in metals

Metals are good conductors of heat because: l Metals have atoms inside them and lots of free electrons. l The free electrons can move around and vibrate. l The heat energy is passed on by neighbouring particles vibrating along the metal.

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Conduction – compare a metal and a non-metal Which material feels warmer if you touch a piece of wood and a piece copper metal, both at room temperature (i.e. both at 25ºC)?

The wood feels warmer because it is a poor conductor. It cannot conduct heat away from your hand as quickly as the copper which is a good conductor. 1 12ofof20 44


Conduction in non-metals Non-metals are poor conductors of heat. In a non-metal, heat energy is only passed on by neighbouring particles vibrating along the non-metal (no free electrons). This allows a flow of energy from hot to cold.

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Conduction in liquids Metals are good conductors of heat and non-metals are poor conductors of heat (insulators). Are liquids good at conducting heat? l Use some gauze to hold an ice cube at the bottom of a tube of water. l Carefully heat the water at the top of the tube only, until this water is boiling.

100ºC

0ºC

l If the liquid is good at conducting, the ice should quickly melt – it doesn’t. l Liquids are poor conductors of heat (i.e. good insulators). 1 14ofof20 44


Conduction in gases Liquids are poor conductors of heat (good insulators). Are gases good at conducting heat? l Carefully hold a safety match 1 cm away from a Bunsen burner flame. (Do not put the match in the flame!) l If a gas is a good conductor, the air between the flame and the match should conduct heat and light the match – it doesn’t. l Gases are poor conductors of heat (i.e. good insulators). 1 15ofof20 44


Conduction – summary table

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Material

Conductor or Insulator?

metals

very good conductors

non-metals

insulators

liquids

insulators

gases

good insulators

vacuum

excellent insulator


Convection and density To understand how heat can be transferred by convection, the idea of density is important. If water, oil and air are mixed up, they will settle out in order of density – which one will rise to the top? The air is least dense and rises to the top, the water is the most dense and sinks to the bottom – it depends on how far apart the particles are.

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Convection current in a liquid The movement of hotter areas in a liquid can be seen using potassium permanganate as a dye: This cycle is called a convection current.

heat

heat

Can you explain how the convection current moves using the idea of density? 1 18ofof20 44


Convection current in a gas Convection currents cannot occur in solids because the particles are held in fixed positions – but can they occur in gases? l Place a candle at one side of a litre beaker. l Place a piece of card down the centre, leaving a gap of 2cm at the bottom. l Make some smoke with smouldering spills and watch the path of the smoke. Can you explain why this happens? 1 19ofof20 44


Convection currents in coal mines When shaft mining was first used to mine coal, convection currents caused by an underground fire were used to ventilate the shafts:

Why do you think miners don’t use this method anymore? 1 20ofof20 44


Convection currents in a pan of boiling water

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Contents

8I Heating and Cooling Heat and temperature Conduction and convection Radiation Changes of state and evaporation Summary activities 1 22ofof20 44


Radiation Heat can move by travelling as infrared waves. These are electromagnetic waves, like light waves, but with a longer wavelength.

This means that infrared waves act like light waves: l  They can travel through a vacuum. l  They travel at 300,000,000 m/s. l  They can be reflected. l  They cannot travel through opaque materials. 1 23ofof20 44


Radiation experiment – instructions Paint four thermometer bulbs with the following colours:

black

white

silver

red

l  Place each thermometer into very hot water for one minute. l  Take it out of the water, start the stopwatch and read the temperature. l  Read the temperature every 30 seconds and record the results in a table.

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Radiation experiment – results table

0

0.5

1

1.5

2

2.5

3

3.5

4

Temp

Colour min min min min min min min min min change

black white White

silver red Which colour of the thermometer bulb radiated most heat? 1 25ofof20 44


Radiation experiment – results analysis Either: a) Draw a bar chart of your results. Put temperature change on the y axis and colour on the x axis. Or: b) Draw a line graph of your results. Put temperature on the y axis and time on the x axis. Draw four lines on the graph, one for each colour.

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Radiation investigation A cup of tea takes up to 30 minutes to go cold. This depends on the colour and shape of the cup.

Design an investigation to find out the best shape and colour of a cup to keep tea warm for longer.

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Contents



States of matter and temperature Water can be a solid, liquid or gas.

l  At a cold enough temperature, even substances that are normally gases will become solid. l  At higher temperatures, solids change to become liquids or gases – as long as they don’t catch fire or decompose first. 1 29ofof20 44


Changes of state Each change of state is given a different name:

melting

solid freezing

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liquid

boiling

gas

condensing


Changes of state activity

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Changes of state – heating curve If a solid is heated, its temperature rises until it reaches the melting point of the solid. At the melting point, the temperature stops rising whilst the solid melts. This is because heat energy is going into separating the particles rather than raising the temperature. Once all the solid has melted, the temperature starts to rise again until it reaches the boiling point. At the boiling point the temperature again stays the same as energy goes into further separating the particles.

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Changes of state – heating curve

temperature

boiling ⇒ liquid ⇔ gas

melting ⇒

condensing⇐

solid ⇔ liquid

freezing⇐

time 1 33ofof20 44


Changes of state – cooling curve activity

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What is evaporation? Evaporation occurs when the particles in a liquid escape to form a vapour. Evaporation can occur at any temperature but it occurs most rapidly at a liquid’s boiling point. The particles that escape take some energy from the remaining particles and so the temperature of the liquid falls. Evaporation is another method of heat transfer.

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Evaporation experiment Evaporation from different liquids can be investigated using this experiment: l  Take four equal masses of cotton wool and soak each one in a different liquid – ethanol, water, propanol and octanol. l  Wrap each piece of cotton wool around the bottom of a thermometer and secure it with a rubber band. l  Read the temperature every 0.5 minutes and record the results in a table. 1 36ofof20 44


Evaporation experiment – results table

Liquid

Temp. change 0 0.5 1.0 1.5 2.0 2.5 3.0 (ºC) min min min min min min min Temperature (ºC)

ethanol water propanol octanol

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Evaporation experiment – results analysis Either:

1. Draw a bar chart of your results. Put temperature change on the y axis. Put type of liquid on the x axis. Or:

2. Plot a line graph of your results. Put temperature on the y axis. Put time on the x axis. Plot four lines, one for each liquid. Are any of the results anomalous? 1 38ofof20 44


Contents



Glossary l  conduction – The way that heat energy travels through

solids because their particles are close together. l  conductor – A material that allows heat energy to flow through it. l  convection – The way that heat energy travels through liquids and gases because their particles are free to move.

l  heat – A form of energy, measured in joules (J). l  heating – The transfer of heat energy. l  insulator – A material that does not allow heat energy to

flow through it. l  radiation – The transfer of heat energy by electromagnetic waves, and which does not need a medium. l  temperature – How hot or cold an object is, measured in degrees Celsius (°C). 1 40ofof20 44


Anagrams

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Heat transfer questions 1. How does a cup of tea lose heat by conduction, convection, evaporation and radiation?

2. Why does take-away food often come in aluminium containers?

3. Why do elephants have big ears?

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Heat transfer – energy losses Name the three processes that cause energy to be lost from the home.

Which areas of the house lose energy?

What can be done to prevent this energy loss?

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Multiple-choice quiz

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