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

Physics

Heat is a type of energy because it can make things move

If something gains heat energy, it becomes hotter – particles move more!

Heat energy is measured in joules (J)

What is heat?

How many joules are there in a kilojoule (kJ)? 1000 J

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

Temperature and energy

Temperature is a measure of how hot or cold an object is.

Temperature is measured in degrees Celsius (ºC).

Temperature can be measured using a thermometer.

What is temperature?

Measuring the temperature of different substances

Substance Temperature

Yogurt from fridge

My hand

Water from the tap

A cup of coffee

Boiling water

Melting ice

- 22 0C

6000 0C

Heat energy will always flow from an area of high temperature to an area of lower temperature

Energy and temperatureWhich direction is the heat flowing in the examples below?

What is the rule that tells you the direction heat will move?

How would putting a coat on this snowman effect how quickly he’ll melt?

What happens to solids when they are heated?

Solids expand when heated and contract when cooled

What happens to liquids when they are heated?

The results of the experiment… Liquids expand when heated and contract when cooled

1. Heat the gas inside the round bottomed flask

2. What do you observe?3. Allow the gas to cool

again.4. What do you observe?

Investigating what happens to gases when they are heated and cooled..

Gases expand when heated and contract when cooled

Water expands when frozen – an exception

At a cold enough temperature, even substances that are normally gases will 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 a solid, liquid or gas.

States of matter and temperature

Each change of state is given a different name:

solid liquid gasmelting

freezing

boiling

condensing

Changes of state

How does pressure effect the boiling point of a liquid

1. Get some hot water in a beaker and measure the

temperature2. Pull the water into the syringe.3. Keep the syringe closed, and

pull the handle back to decrease the pressure.

4. What happens to the water

Results?Conclusions?

Changes of state activity

What happens to the temperature of a substance as it changes state?

Results – the cooling curve for napthelene

Changes of state – cooling curve activity

melting

freezing

solid liquid

boiling

condensing

liquid gas

liquid

Changes of state – heating curve

Latent heat is the heat change that happens when a substance changes state

Heat energy can be transferred (moved) by three processes:

1. conduction

2. Convection

3. Radiation

Heat energy only flows when there is a temperature difference.

Heat transfer

HOT COLD

1. Conduction

• Conduction is when heat energy is passed along a material without the particles moving position.

• Heat energy is only passed on by neighbouring particles vibrating.

Movement of heat by conduction

Add a blob of wax to one end of the copper and hold the other end in the yellow Bunsen flame.

How long does it take for the wax to melt and drop off the metal strip?

Now do the same for wood.

Why are the times very different?

Conduction experiment

Metals are good conductors of heat because:

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.

Conduction in 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.

Conduction in non-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.

Todays objectives

• To investigate conduction in liquids and gases

Homework Write up todays experiment in your notes

Are liquids good at conducting heat?

Use a weight to hold an ice cube at the bottom of a tube of water.

Carefully heat the water at the top of the tube only, until this water is boiling.

If the liquid is good at conducting, the ice should quickly melt

100ºC

Conduction in liquids

Liquids (like water) are poor conductors of heat

Are gases good at conducting heat?

Carefully hold a safety match 1 cm away from a Bunsen burner flame.

If a gas is a good conductor, the air between the flame and the match should conduct heat and light the match

Gases are poor conductors of heat

Conduction in gases

Are gases good at conducting heat?

Carefully hold a safety match 1 cm away from a Bunsen burner flame.

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!!!

Gases are poor conductors of heat

Convection

• Convection is when heat energy is passed along a material by the particles moving and changing position.

• What types of materials do you think convection happens in?

This cycle is called a convection current.

how do convection currents flow in a liquid?

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:

This cycle is called a convection current.

Convection current in a liquid

Convection currents in a pan of boiling water

What 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

Radiation is when heat can move by travelling as waves..

They can travel through a vacuum.

They travel at 300,000,000 m/s.

They can be reflected.

Radiation

All hot bodies radiate heat

Objectives for today

• What insulators are and some examples

Homework

Make mind map on Heat. Test on Thursday!!!

Heat insulators

• Heat insulators do not allow heat to pass through them easily

Can you think of any examples of good insulators?

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

Glossary

conduction – The way that heat energy travels through solids because their particles are close together.

conductor – A material that allows heat energy to flow through it.

convection – The way that heat energy travels through liquids and gases because their particles are free to move.

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

flow through it.radiation – The transfer of heat energy by electromagnetic

waves, and which does not need a medium.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

Multiple-choice quiz

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

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