Heat can be transferred by convection and conduction7.1 This continues until the two objects are the same temperature (equal amounts of thermal energy

Heat can be transferred by

convection and conduction7.1

This continues until the two objects are the

same temperature (equal amounts of thermal

energy).

Consider what happens when we heat a

saucepan of water on a gas burner.

1 When the gas burns, thermal energy is

released.

2 The hot molecules in the gas flame move

quickly and occasionally bump into atoms

of the relatively cold metal of the saucepan.

3 Kinetic energy passes to the slowly vibrating

atoms in the saucepan so that they vibrate

faster.

4 The quickly vibrating atoms in the

saucepan bump into other nearby metal

atoms, transferring thermal kinetic energy

to them. This heats the saucepan.

5 When the saucepan heats up, the thermal

kinetic energy is transferred to the water

inside it.

Although the thermal energy moves through

the metal of the saucepan and into the water, the

atoms in the metal do not change their positions

– metal atoms do not move into the water.

Conductors and insulators

A thermal conductor is any material that

allows thermal energy to flow easily through it.

All metals are conductors of thermal energy,

although some are better conductors than

others. Thermal insulators are materials

that slow down the transfer of thermal energy

because the molecules don’t allow the thermal

kinetic energy to flow very easily. Insulators

such as socks, jumpers and blankets keep us

warm in cold weather. They make it difficult

for our ‘body heat’ to escape, insulating us

against the cold. Insulation in the roof and

walls of a house prevents heat gain and loss

during summer and winter. Insulation can hold

thermal energy in or keep it out.

Modelling energy transfer

In Year 8 you discovered how energy can be

transferred from one substance or medium

to another. The way the energy is transferred

can vary depending on the type of energy that

is being transferred. This can be modelled

using wave or particle models to explain the

movement of the energy. Throughout this

chapter we will look at the different ways to

model thermal (heat) energy, sound energy,

electrical energy and light energy.

Modelling thermal energy

The particle model suggests that all things are

made up of particles of atoms or molecules that

have kinetic (movement) energy. Solid objects

have vibrating particles that are bonded closely

together. If thermal energy is added to a solid

object, the particles start vibrating faster until

they can move around one another.

This increase of energy and movement

turns the solid into a liquid. If more

energy is added to the liquid, particles

will start moving faster until they are

able to break free and move freely as a

gas (Figure 1).

When modelling thermal energy, it

is important to consider the movement

of the heat. Cold objects have less

thermal energy and hot objects have

more thermal energy.

Heating by conduction

Heating an object by conduction involves

the transfer of thermal energy between

two objects that are in contact with each

other. The energy is transferred from an

object with high thermal energy to an

object with low thermal energy (from hot

objects to cooler objects).

Key ideas

• The energy from heat moves spontaneously from a hot material to a cool material.

• Conduction occurs when the kinetic energy of particles is transferred.

• Convection occurs when a particle with high kinetic energy moves to another space.

Learning intentions

By the end of this

topic, you will be

able to:

• explain how

heat energy

is transferred

by conduction

and convection

in terms of

the motion of

particles.

Figure 1 Increasing or decreasing

thermal energy can change the

movement of particles and the state

of an object.

Increase

thermal

energy

(heat up)

Solid

Liquid

Gas

Decrease

thermal

energy

(cool down)

conduction

the transfer of thermal

energy from hot objects

to cooler objects by direct

contact with no movement

of material

thermal conductor

a material that allows

thermal energy to flow

through

thermal insulator

a material that prevents or

slows down the transfer of

thermal energy

CHAPTER 7 PARTICLES AND WAVES137OXFORD UNIVERSITY PRESS

1 Thermal energy transfers by conduction

from the hot saucepan to the water

molecules that are touching the metal.

2 The water molecules in contact with the

metal gain kinetic energy and move faster

than the molecules in the water above.

Because they are moving faster, they take

up more space. They are less dense.

3 As a result, the heated (less dense) water

molecules near the bottom of the saucepan

begin to rise, leaving room for the cooler

(more dense) water molecules to take their

place (Figure 3).

4 The heated water molecules take thermal

energy with them as they move.

We heat liquids from below because most of

the energy transfer in liquids (and gases) takes

place by convection. This process happens in the

air. The Sun heats the ground and the warmed

ground then heats the air next to it by conduction.

The warmed air, being less dense than the cooler

air above, rises, taking the thermal energy with it.

This distributes the energy through a much deeper

layer of air than could occur just by conduction

from the ground. This process of convection in

the air is what drives the weather on Earth.

You can test the thermal conductivity of

the different materials around you. If you put

your hand on a metal object, it will feel cold

to touch. This is because the metal conducts

heat away from your hand, making it feel cold.

If you touch a wood object, it will feel warmer

than the metal object. In reality, both objects

will be exactly the same temperature, but the

wood acts like a thermal insulator, preventing

the thermal energy from being conducted away

from your hand. Because your hand is not

losing heat, it will feel warm.

Heating by convection

The particles in liquid and gas materials are

able to move more freely than in solid objects.

In these materials, thermal energy moves by

convection. As the particles gain thermal

kinetic energy, they are able to move away

from the heat source. Tiny currents, called

convection currents, carry the particles and

their thermal energy across the liquid or gas

until the heat is evenly spread.

When we heat a saucepan of water on a gas

flame, the following occur.

Silvered wall

Vacuum

Padding

The plastic stopper is an

insulator – it prevents heat loss

or gain through convection and

conduction.

The glass walls are insulators –

they prevent heat loss or gain

through conduction.

The silvered wall

prevents heat loss

by radiation.

The vacuum between the walls is

an insulator – it prevents heat

loss or gain through conduction

and convection.

Figure 2 Vacuum flasks are

designed to keep hot substances

hot and cold substances cold.

To do this they must prevent the

contents from losing or gaining

heat – conduction and convection

must be minimised. Careful

choice of materials and clever

design make this possible.

7.1A: Investigating heating by

convection

Go to page 205.

7.1B: Testing insulating

materials

Go to page 206.EXPERIMENT EXPERIMENT

convection

the transfer of thermal

energy by the movement of

molecules in air or liquid

from one place to another

convection current

the current or flow of air or

liquid that results from the

transfer of thermal energy

through convection

Figure 3 Convection

currents are created in a

saucepan of water when

it is heated. The heated

water molecules (shown

in red) rise while the

cooler ones (shown in

blue) sink.

Retrieve

1 Identify two examples of

situations where thermal energy

is transferred by:

a conduction b convection.

Comprehend

2 Identify one example of where

good thermal insulators and

conductors are needed in everyday

life. Describe the materials that

are used in each situation.

3 Some modern saucepans have a

copper bottom, steel sides, a plastic

handle and a glass lid. Explain

why each of these materials is used

for particular parts of a saucepan.

4 Think of a situation where you can

see expansion due to heating of a

solid, liquid or gas. Explain what

the molecules or atoms are doing

to cause the expansion.

Analyse

5 Consider why scientists are

happy to refer to thermal energy

transfer as heating, even though

in every case something is

being cooled.

7.1 Check your learning

Quiz me

Complete the Quiz me to check

how well you’ve mastered the

learning intentions and to be

assigned a worksheet at your level.138OXF ORD SCIENCE 9 AUSTRALIAN CURRICULUM OXFORD UNIVERSITY PRESS

Vibrating particles pass

on sound7.2

pushing them closer together in one place and

forcing them further apart in another. In this

way, the air around the drum is made to vibrate

too. This causes the particles further away to

vibrate, and so on, until the air close to your ears

eventually vibrates and causes your eardrum to

vibrate too. And that’s when you hear the sound.

The region with the particles forced close

together is called a compression, and the less

dense region where the air particles are further

apart is called a rarefaction. Sound waves

travel as a longitudinal wave because the air

particles move back and forth parallel to the

wave as the vibration passes through the air.

The distance a particle of air moves is called the

amplitude of the wave (Figure 1). Sound waves

with a large amplitude mean the air particles

move with greater kinetic energy. This makes

the sound feel louder to our ears. An example

of this is when musicians use amplifiers to

increase the loudness of their music. Amplifiers

increase the distance air particles move during

compression and rarefaction.

A sound wave moves out in all directions

from the place where the vibration began

(Figure 2).

Modelling sound waves

We know that sound energy travels because we

can often hear it a long way from its source.

Consider the example of a drum being played.

The drum skin vibrates (moves up and down)

when it is hit. The kinetic energy of the vibrations

is transferred to the surrounding air particles,

Waves

in air

Less

amplitude

Greater

amplitude

Figure 1 Red arrows indicate how far a particle in

a sound wave moves.

compression

part of a