© Boardworks Ltd 20081 of 45. © Boardworks Ltd 20082 of 45.

3 of 45 © Boardworks Ltd 2008

Ideas about heat


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

Temperature is measured in degrees Celsius (°C).

The freezing point of pure water is defined as 0 °C (at sea level).

The boiling point of pure water is defined as 100 °C (at sea level).

Temperature can be measured by a variety of different thermometers. These include liquid in glass, digital, thermocouple and bimetallic strip thermometers.

It is determined by the average kinetic energy of the particles in the object.

What is temperature?


Heat is the energy transfer that takes place between objects of different temperatures.

Heat is measured in joules (J).

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

What is heat?

Heat transfer only occurs when there is a temperature difference between substances. Heat always moves from an area of higher temperature to an area of lower temperature.


Heat transfer can take place in three different ways:

conduction, which mainly occurs in solids

Types of heat transfer

radiation, which occurs through any material or even through a vacuum.

convection, which only occurs in liquids and gases


The particle model

The differences between hot and cold objects – and between solids, liquids and gases – can be explained by the particle model. All substances consist of particles (atoms or molecules).

The kinetic energy of particles increases with temperature.

The particles move around, i.e. they have kinetic energy.

The particles are attracted to each other; some strongly and others weakly.

The particles are tightly packed in a solid or spaced further apart in a liquid or gas.


Designing a conduction experiment

Can you design an experiment to investigate how well different materials conduct heat? Remember to make your experiment a fair test to ensure you get reliable results. The available apparatus includes:

drawing pins

stopwatch

tripod

Bunsen burner

wax blobs

metal, glass andceramic rods


Investigating conduction


If you touch a piece of wood and a piece of copper metal that are both at room temperature (about 21 °C), which material feels warmer?

The wood feels warmer because it is a poor conductor.

Conduction in metals and non-metals

The wood cannot conduct heat away from your hand as quickly as the copper, which is a good conductor.


Conduction in non-metals


Conduction in metals

Metals are good conductors of heat because:

metals contain many free electrons

the free electrons can move between atoms at high speed

the free electrons carry energy from high-energy atoms to low-energy atoms further away from the heat source.


Conduction in metals


Metals are good conductors of heat and almost all non-metals are poor conductors of heat.

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 only until this water is boiling.

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

Conduction in liquids

Are liquids good at conducting heat?

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


Conduction in gases

Carefully hold a safety match 1 cm away from a Bunsen burner flame. (Do not put the match in the 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 (i.e. good insulators).

Are gases good at conducting heat?


Conductor or insulator?


This is important in explaining how heat is transferred by convection.

If water, oil and air are mixed together, 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.

Convection and density

The density of a substance depends on how far apart its particles are.


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 currents in a liquid

Can you use the concept of density to explain how the convection current moves?


How does convection in a liquid occur?


Explaining 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 don’t use this method anymore?

Convection currents in a coal mine


What is radiation?

We know that conduction mainly takes place in solids, and convection takes place in liquids and gases.

How does the Earth receive heat energy from the sun through the empty vacuum of space?

There is a third kind of heat transfer, which is called radiation, or ‘infra-red radiation’.


Absorbing radiation


Seeing infrared radiation

The police and fire service use infra-red cameras to find people in the dark.

Infra-red has no colour, but a computer can put a false colour on the camera image so we can ‘see’ the object.

The colours are put on a scale which corresponds to temperature. White, yellow and red usually represent the warmest areas; green, blue and black usually represent the coolest.


Infrared goggles


Changes of state activity


Changes of state – heating curve

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

Once the solid has melted, the temperature rises again until it reaches the boiling point.

At the melting point, the temperature stops rising while the solid melts, because energy goes into separating the particles.


Changes of state – heating curve

At the boiling point, the temperature again stays the same as energy goes into further separating the particles.


Changes of state – cooling curve


Evaporation occurs when the particles in a liquid escape to form a vapour.

Evaporation can take place 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, so the temperature of the liquid falls.

Evaporation is another method of heat transfer.

What is evaporation?


1. Take four equal masses of cotton wool and soak each one in a different liquid – ethanol, water, propanol and octanol.

2. Wrap each piece of cotton wool around the bottom of a thermometer and secure it with a rubber band.

3. Read the temperature every 30 seconds and record the results in a table.

Evaporation from different liquids can be investigated using this experiment:

Evaporation experiment


Evaporation experiment – results

ethanol

water

propanol

octanol

LiquidTemperature (°C) Temp.

change (°C)

0.5min

1.0min

1.5min

2.0min

2.5min

3.0min

0min


Draw a bar chart of your results:

with temperature change on the y axis,

with the type of liquid on the x axis.

Plot a line graph of your results:

with temperature on the y axis,

with time on the x axis,

and four lines plotted, one for each liquid.

Evaporation experiment – analysis

Are any of the results anomalous?


The dark wall areas are the coolest – these are the best insulated parts of the house.

Using thermograms to investigate heat loss

Thermograms are useful for examining heat lost from a house, as they show the temperature distribution over its surface.

The yellow and orange roof and window areas are the warmest – these are the worst insulated parts of the house.

A poorly insulated house loses more energy and so costs more to heat. It also means that more pollution, particularly carbon dioxide, is created in order to heat the house.


Heat transfer – energy loss

Where is the most heat lost from a poorly insulated house?

walls 36%

floor 28%

roof 20%

windows 12%

doors 4%


Examining insulation

What are the different types of home insulation?

Double glazing adds an extra pane of glass to each window. This means that there is a layer of air or a vacuum between the two panes.

This helps to prevent heat loss by conduction.

Loft insulation is plastic foam that contains trapped air and so acts as an insulator to stop conduction of heat through the roof tiles.

Draught excluders are hairy or spongy strips which are placed under doors to prevent the movement of convection currents.

side-viewof double glazing

insulating layer of air


Examining insulation

Curtains reduce heat loss by covering up gaps around windows through which convection currents sometimes pass.

Cavity wall insulation involves placing a layer of plastic foam insulation in between two layers of outside walls. This prevents heat loss by both conduction and convection currents.

Placing a sheet of shiny foil behind radiators prevents heat radiation reaching the wall and conducting outside. Instead, it is reflected back into the room.

convection currents

in the cavity

foam insulationprevents

convection currents


Glossary


Anagrams


Multiple-choice quiz

© Boardworks Ltd 20081 of 45. © Boardworks Ltd 20082 of 45.

Documents

Transcript of © Boardworks Ltd 20081 of 45. © Boardworks Ltd 20082 of 45.