8i heating and cooling -...

© Boardworks Ltd 2004 1 of 20 © Boardworks Ltd 2005 1 of 44

KS3 Physics

8I Heating and Cooling



Contents

Heat and temperature

Conduction and convection

Changes of state and evaporation

Summary activities

Radiation


Heat is a type of energy.

Heat is the name for the type of kinetic energy possessed by particles.

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

Heat energy is measured in joules (J).

How many joules are there in a kilojoule (kJ)?

What is heat?


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.

What is temperature?


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:

Heat energy only flows when there is a temperature difference.

Heat transfer

HOT COLD


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.

The particle model

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


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

Temperature and energy



Contents



Summary activities

Radiation



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.

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

Conduction experiment

Apparatus: 8 cm strip of copper

8 cm piece of wood

Bunsen burner

stopwatch

blob of wax


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?

Conduction experiment


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.

Conduction in metals


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

Conduction – compare a metal and a non-metal

the copper which is a good conductor.


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.

Conduction in non-metals


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.

l If the liquid is good at conducting, the ice should quickly melt – it doesn’t.

0ºC

100ºC

Conduction in liquids

l Liquids are poor conductors of heat (i.e. good insulators).


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).

Conduction in gases


Material Conductor or Insulator?

metals very good conductors

non-metals insulators

liquids insulators

gases good insulators

vacuum excellent insulator

Conduction – summary table


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.

Convection and density


Can you explain how the convection current moves using the idea of density?

The movement of hotter areas in a liquid can be seen using potassium permanganate as a dye:

heat

This cycle is called a convection current.

Convection current in a liquid

heat


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?

Convection current in a gas


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?

Convection currents in coal mines


Convection currents in a pan of boiling water



Contents



Summary activities

Radiation Changes of state and evaporation


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.

Radiation


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.

Radiation experiment – instructions


Colour 0 min

0.5 min

1 min

1.5 min

2 min

2.5 min

3 min

3.5 min

4 min

Temp change

black

White

silver

red

Which colour of the thermometer bulb radiated most heat?

Radiation experiment – results table

white


Either:

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.

Radiation experiment – results analysis

a) Draw a bar chart of your results.

Put temperature change on the y axis and colour on the x axis.


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.

Radiation investigation



Contents



Summary activities

Radiation



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.

Water can be a solid, liquid or gas.

States of matter and temperature


Each change of state is given a different name:

solid liquid gas melting

freezing

boiling

condensing

Changes of state


Changes of state activity


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.

Changes of state – heating curve


melting ⇒

freezing⇐

solid ⇔ liquid

boiling ⇒

condensing⇐

liquid ⇔ gas

tem

pera

ture

time

Changes of state – heating curve


Changes of state – cooling curve activity


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.

What is evaporation?

Evaporation is another method of heat transfer.


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.

Evaporation experiment

Evaporation from different liquids can be investigated using this experiment:


Temperature (ºC) Liquid

0 min

0.5 min

1.0min

1.5 min

2.0 min

2.5 min

3.0 min

Temp. change

(ºC)

ethanol

water

propanol

octanol

Evaporation experiment – results table


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?

Evaporation experiment – results analysis



Contents



Summary activities

Radiation



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).


Anagrams


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

3. Why do elephants have big ears?

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

Heat transfer questions


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?

Heat transfer – energy losses


Multiple-choice quiz

8i heating and cooling -...

Documents