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Energy: Heat Transfer

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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Temperature measurement
Temperature is a measure of the hotness or coldness of something, not the total amount of energy contained. Temperature is measured in degrees Celsius (or centigrade) - C.

The freezing point of water is DEFINED as 0oC (at 1 atm.).

The boiling point of water is DEFINED as 100oC (at 1 atm.). The temperature can be measured in a variety of different thermometers. These include liquid in glass, digital, thermocouple and bimetal strip thermometers.
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Energy : heat transfer

The correct phrase is thermal transfer. Heat energy can be transferred (moved) in 4 ways: 1. 2. 3. 4. Conduction Convection Evaporation Radiation

Which ever way heat moves, it always moves from

HOT to COLD.
Heat energy only flows when there is a temperature difference.
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The Particle Model

The existence of solids, liquids and gases is explained by the following ideas: 1. All substances are made up of particles (atoms, ions and molecules). 2. That these particles are attracted to each other, some strongly and others weakly. 3. That these particles move around (i.e. Have kinetic energy). 4. That their kinetic energy 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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Energy : heat transfer

1. Conduction Take care whilst doing this experiment:

ALWAYS LET GO OF THE METAL QUICKLY WASH ALL BURNS WITH COLD WATER FOR 10 MINS

You need an:

8cm strip of Cu

8cm piece of Wood

Bunsen Burner Stopwatch
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Energy : heat transfer

1. Conduction

WEAR SAFETY GLASSES

Hold the piece of Cu in the YELLOW Bunsen Burner flame. How long does it take you to feel the heat? Now do the same for wood. Why are the times very different?
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Energy : heat transfer

1. Conduction in metals Metals have atoms inside them and lots of free electrons.

The free electrons can move around and vibrate. The heat energy is passed on by neighbouring particles vibrating along the metal. There are too few free electrons in a non-metal for this to happen.
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Energy : heat transfer

Which would feel warmer if we picked up a piece of wood and a piece copper metal both at room temperature (i.e. both at 250C)?

The wood feels warmer because it is a POOR conductor. So it would not conduct heat away from your hand as quickly as the copper.

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Energy : heat transfer

1. Conduction in non-metals

The heat energy is 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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Energy : heat transfer

1. Conduction Metals are good conductors of heat & non-metals are poor conductors of heat [insulators]. What about liquids? Use some gauze to hold an ice cube at the bottom of a tube of water. Carefully heat the water at the top of the tube until boiling. If the liquid was good 0C at conducting, the ice would quickly melt - it doesnt. WEAR SAFETY GLASSES

100C

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Energy : heat transfer

1. Conduction Liquids are poor conductors of heat [insulators].

What about gases?

Carefully put your finger 1cm away from a bunsen burner flame. If gases were good conductors, youd burn your finger - you dont. Gases are good insulators.

WEAR SAFETY GLASSES

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Energy : heat transfer

1. Conduction Copy this summary table into your book :

Material
Metals Non-metals Liquids Gases Vacuum

Conductor or Insulator ?
very good conductors Insulators Insulators Good insulators Excellent insulator

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Energy : heat transfer

2. Convection 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 the water is the most dense - it depends on how far apart the particles are.
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Energy : heat transfer

2. Convection The movement of hotter areas in a liquid can be seen using potassium permanganate as a dye: This cycle is called a convection current. Can you explain how the convection current moves using the idea of density?

H E AT

The diagrams on the left will help you.

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Energy : heat transfer

2. Convection Convection currents cant occur in solids because the particles are held in fixed positions - but can they occur in gases? Place a candle at one side of a litre beaker. Place a piece of card down the centre, leaving a gap of 2cm at the bottom. Make some smoke with smouldering spills and watch the path of the smoke. Can you explain why this happens?
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Energy : heat transfer

2. Convection 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 dont use this method anymore?

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Convection currents in a pan of boiling water.

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Energy : heat transfer

3. Evaporation Evaporation is when the particles in a liquid escape to form a vapour. Evaporation can occur at any temperature but it occurs most rapidly at a liquids boiling point. The particles that escape take some energy from the remaining particles and so the temperature of the liquid falls. Take 4 equal masses of cotton wool and soak them in the 4 different liquids provided. Wrap the cotton wool around the bottom of a thermometer and secure it with an rubber band. Take the temperature every 0.5 minutes and record your results in the table on the next slide
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Energy : heat transfer

3. Evaporation

Temperature after x mins Liquid

0

Temp. Change [C] 0.5 1.0 1.5 2.0 2.5 3.0

Ethanol Water Propanol Octanol

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Energy : heat transfer

3. Evaporation Either :

1.

Draw a bar chart of your results.

Put temperature change on the y axis Put 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 4 lines, one for each liquid Are any of the results anomalous?
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Energy : heat transfer

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

This means that infra red waves act very much like light waves: They can travel through a vacuum They travel at 300,000,000 m/s They can be reflected They cant travel through opaque materials.
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Energy : heat transfer

4. Radiation Paint 4 thermometer bulbs with the following colours

Black

White

Silver

Red

Place the thermometers into very hot water for 1 minute.

Take it out of the water, start the stopwatch and read the temperature.
Take the temperature every 30 seconds and put the answers in the results table on the next page.
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Energy : heat transfer

4. Radiation
Colour Temp 0 0.5 1 1.5 2 2.5 3 3.5 4 min min min min min min min min min Change

Black Black

White Silver Red

Which colour radiated most heat?
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Energy : heat transfer

4. Radiation 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 4 lines on the graph, one for each colour.
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Energy : heat transfer

4. Radiation

1.

How does a cup of tea lose heat by conduction, convection, evaporation and radiation?
Why does take-away food often come in aluminium containers?

2.

3.

Why do elephants have big ears?

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Energy : heat transfer

4. 5. Radiation 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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Energy losses
Name the three processes which cause energy to be lost from the home. Which spots on the house diagram lose energy? What can we do to prevent this energy loss?

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Changes of state
At cold enough temperatures even things that are normally gases become solid. At higher temperatures solids change to become liquids or gases as long as they dont catch fire or decompose first.

Water can be solid, liquid or gas

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Changes of state
Each change of state is given a different name. Gas

Boiling Condensing

Melting

Solid
Freezing

Liquid

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Changes of state heating curves

If a solid is heated its temperature rises until it reaches its melting point.

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
Boiling Condensing

Melting Freezing
Solid Liquid

Liquid Gas

Time
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 Boardworks Ltd 2003