© boardworks ltd 2003 energy: heat transfer heat is the name for the type of kinetic energy...

© Boardworks Ltd 2003

Energy: Heat Transfer

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]?


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) - ºC.

Temperature measurement

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.


Energy : heat transfer

The correct phrase is ‘thermal transfer’.

Heat energy can be transferred (moved) in 4 ways:

1. Conduction

2. Convection

3. Evaporation

4. Radiation

Which ever way heat moves, it always moves from

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


The Particle Model

The existence of solids, liquids and gases isexplained 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 kinetickinetic energy).4. That their kinetic energy increases with

temperature.


Temperature and energy

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


1. Conduction

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?

WEAR SAFETY GLASSES



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.



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.



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.



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

at conducting, the ice would quickly melt - it doesn’t. WEAR SAFETY GLASSES

0ºC

100ºC



Liquids are poor conductors of heat [insulators].

WEAR SAFETY GLASSES

What about gases?

Carefully put your finger 1cm away from a bunsen burner flame.

If gases were good conductors, you’d burn your finger - you don’t.

Gases are good insulators.

1. Conduction



Copy this summary table into your book :

Material Conductor or Insulator ?

Metals very good conductors

Non-metals Insulators

Liquids Insulators

Gases Good insulators

Vacuum Excellent insulator

1. Conduction



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.



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

H E A T

This cycle is called a convection current.

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

The diagrams on the left will help you.

2. Convection



Convection currents in a pan of boiling water.


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 liquid’s 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



Temperature after x minsLiquid

0 0.5 1.0 1.5 2.0 2.5 3.0

Temp.Change

[C]

Ethanol

Water

Propanol

Octanol

3. Evaporation



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 can’t travel through opaque materials.



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

3. Why do elephants have big ears?

4. Radiation


1. How does a cup of tea lose heat by conduction,

convection, evaporation and radiation?


5. A cup of tea takes up to 30 minutes to go cold.

This depends on the colour and shape of the cup.

4. Radiation



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?


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 don’t catch fire or decompose first.

Water can be solid, liquid or gas


Each change of state is given a different name.

Solid Liquid

Gas

Melting

Freezing

Condensing

Boiling

Changes of state


• 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.

Changes of state – heating curves


Solid Liquid

Liquid

Liquid Gas

Gas

Solid

Tem

pera

ture

Time

Melting Freezing

Boiling Condensing

Changes of State

© boardworks ltd 2003 energy: heat transfer heat is the name for the type of kinetic energy...

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