# Energy Transfers

## How does Heat Energy Move?

Hot objects have heat energy. Heat energy always moves from something hot to something colder. There is no such thing as cold energy, so an object can only get colder by heat energy moving away from it.

There are three ways that heat energy can move:

1. Conduction.
2. Convection.

#### Conduction

When you first pour boiling water onto a Pot Noodle, the plastic container feels cool on the outside. Soon, the heat energy has worked its way through the plastic and the container starts to feel hot on the outside. Heat energy has travelled through the solid plastic container. This process is called conduction.

Atoms in a substance are always vibrating. If the substance gets hotter,the atoms vibrate more. The heat energy is given to the atoms, which makes them move about faster. Note: the atoms don't swap places, or move around they just vibrate more on the spot.

Have you ever danced next to someone really energetic? If so, you know that it makes you have to move about more - often just to get out of the way! It is like that for atoms passing heat energy on to each other.

Solids are better at conducting than liquids and gases because the atoms are closer together. If the atoms are too spaced out it makes it harder for the atoms to pass the energy along.

Metals are the best solids for conducting heat energy. In metals, there are free electrons that can move through the metal. These electrons are able to move from hot parts of the metal to colder parts, taking the heat energy with them. This is called electron diffusion.

The poorest conductors are gases as their molecules are too far apart to affect each other much. This means that air is a terrible conductor of heat energy.

#### Convection

Hot air rises in cold air. Hot water rises in cold water. This way of moving energy is called convection. When hot air rises, colder air has to move in to replace it. When hot water rises in a cup, colder water sinks to replace it.

This movement of a liquid or gas is called a convection current.

Convection cannot happen in solids, as the atoms aren't able to move around.

When a liquid or gas heats up, the particles move around more. This makes the particles spread out, so they have more room to move. This lowers the density of the substance. The hotter it gets, the lower the density goes. It is this lower density that makes the hotter substance rise. The cooler substance has a higher density, which makes it sink.

"How to toast bread!" When a piece of bread is put in a toaster the wires inside the toaster glow red hot on either side of the bread.

How does the heat energy get to the bread?

Is it by conduction?

No, the heat energy cannot conduct through the air to the bread because air is a very bad conductor.

Is it by convection?

No, hardly any of the heat energy could have travelled to the bread by convection, as the hot air particles would rise out of the toaster.

The heat energy must have reached the toast some other way. It travelled as radiated heat. This heat energy movement is sometimes called heat waves, but strictly speaking, it is infrared radiation.

Hot objects radiate heat to their colder surroundings. The weird thing is that the surface colour of an object makes a difference.

Black and dull surfaces emit (give out) and absorb radiation well.

Which of these surfaces is best at emitting radiation?

Put them in order of best to worst:

Marathon runners are wrapped in foil blankets at the end of the race. The shiny surface is a poor emitter of radiation and so prevents them losing too much precious body heat.

So why do we paint radiators white so often?

I guess people think it looks better.

Which of these surfaces is best at absorbing radiation?

Put them in order of best to worst:

Solar panels are always coloured black. They then absorb the maximum amount of the Sun's energy. This is very important considering the amount of sun we get in the UK. Wearing white in the summer should, in theory, be cooler than wearing black, as more of the incoming heat is reflected away.

## How can we Stop Heat Moving?

#### Insulation

If your house has a roof space, then it probably has insulation in it. That insulation is a thick layer of fibre glass. It's actually the air in between the fibreglass that makes it a good insulator.

Air is a very bad conductor. Hardly any heat energy can conduct through the trapped air in the fibreglass.

If air is so good why doesn't it work on its own?

Well, the air can convect the heat energy away from the house if it is able to move. If the air is trapped in small spaces between the fibres in the fibreglass it can't move so it doesn't convect the heat energy.

Many insulators work because they contain trapped air.

#### Ways to save energy in the home

There are two real reasons for reducing heat losses from a home:

1. Saving energy means that less energy needs to be produced, so there will be less damage to the environment. (Think about the pollution from a coal-fired power station.)
2. Saving energy means using less energy so it costs less to heat the house. It can cost hundreds of pounds a year to heat a family house!

Obviously it also costs money to install (put in) insulators and other energy-saving devices. The payback time is how long it takes for the savings to cover the cost of installation.

Each method has to stop either conduction, convection, radiation, or any combination of them.

See if you can identify how each method of insulation saves energy in the home, by dragging the coloured boxes into the correct place:

Of course you can do other things: Use low energy light bulbs, turndown heating thermostats, and fit draught excluders, for example.

## Non-Renewable Energy Sources

#### Generating electricity in power stations

All power stations use a similar process to produce electricity.

1. Fuel is used to produce heat energy.
2. The heat energy heats water and turns it into steam.
3. The steam is pushed at high pressure along pipes to the turbines.
4. The steam makes the turbines spin, turning a generator which then produces electricity.

The electricity is then supplied to houses, factories and schools via the national grid.

Drag the correct form of energy directly onto the actual source of the energy and mark you answer:

#### Fossil fuels

The fossil fuels are oil, gas and coal. They are non-renewable energy sources, which means when the existing supplies run out,they can't be replaced! We are very reliant on fossil fuels in the modern world.

Do you remember all the aggro during the fuel crisis of September 2000 when there was a sudden shortage of petrol and diesel (both made from oil)?

These fuels have many uses, but the main ones are for heating, transport and generating electricity. The USA uses more fuel per person than any other country and much of the developed world uses plenty. As developing countries become more industrial, they use more and more energy.

Fossil fuels are formed under the ground. Dead matter is squashed under extremely high pressure over millions of years. This is why we can't remake it when it runs out.

#### Environmental impact of fossil fuels

When you burn any fossil fuel, gases escape into the air. The main two gases released are carbon dioxide and sulphur dioxide.

(Note: One reason why petrol tax is high in the UK is because the government was trying to discourage car use so that less pollution is released into the air.)

Carbon dioxide is the most common of several gases that contribute to the greenhouse effect. I think the effect is best experienced under a duvet! The heat that exists underneath can't easily get out, so you get hotter and hotter. For the Earth, the result is global warming. Sadly, it means more heat energy in the atmosphere, so the weather is more extreme (not just hotter). Worse still, is that the ice-caps melt raising the sea levels. Literally millions would be affected in countries like Bangladesh, even parts of England could go under the water.

Sulphur dioxide causes acid rain. The gas dissolves in rainwater to form an acid. The acid rain harms plants, animals and stonework. It is an international problem because acid rain clouds created in one country can be blown over to another country.

#### Nuclear power

When your parents went to school, nuclear power stations were the great hope to solve all the world's energy problems. 1 kg of Uranium can produce the same amount of energy as 10,000 kg of coal. Since then, the negative side to nuclear power has changed many people's view. Accidents at power stations like Chernobyl showed us that although nuclear power has many advantages over the fossil fuels, it is also highly dangerous.

Uranium is the fuel used in many nuclear power stations. Uranium is not burnt like coal or gas. Instead nuclear fission takes place. Atoms of uranium are split up which releases large amounts of energy. Left uncontrolled this could cause an explosion as in nuclear weapons, but when controlled the energy can be used to heat water to produce steam, just like in other power stations.

Unfortunately, nuclear fission produces harmful radiation. Radioactive substances are produced that release alpha, beta and gamma radiation into the surroundings. This can be harmful to plant and animal life.

Accidents are rare, but can be serious. After the Chernobyl accident in the Ukraine, radioactive particles were carried in clouds across Europe contaminating land hundreds of miles away from the accident. 15 years later, there is still an area around the power station that people are not allowed into.

The waste from the reactors is also radioactive. It can be stored safely, but it stays radioactive for years. Unfortunately this waste has to be stored somewhere but nobody wants the nuclear waste buried near their town!

So where are we going to keep putting it?

## Alternative Energy Sources

Below is some general information about alternative energy sources, but there is a lot more on the web. Try the Centre for Alternative Technology or The Guardian Renewables page as a starting point if you are researching this area. For the exam, make sure that you know the advantages and disadvantages of each energy source.

Most of the alternative energy sources are renewable. This means there is either an endless supply of them so that they will not run out, or they can be easily replaced.

#### The power of water

Nothing new here! Water wheels were used at the start of the industrial revolution. Now we can use water running down a hill or falling over a dam to turn a turbine. This is called hydro-electric power (HEP).

Some developing countries get all their energy from hydro-electricity schemes on large dams. The Aswan dam was the first and most famous, in Egypt on the Nile. The down side is that the large lakes made behind a dam can drastically change the countryside, sometimes covering small villages.

Big waves at sea also have a lot of energy - too much energy really, as no large-scale scheme has been designed to cope with it! Also, the tide has a lot of energy. If you block in the water at high tide and then let it out through a turbine as the tide falls, you can generate electricity. But as with the dams, this alters the natural water levels, so the local habitat is affected.

#### Harnessing the weather

Solar power - The energy from the sun can be changed into electrical energy using solar panels. This is used in the UK even though it's not always sunny! Solar panels are often used alongside other energy sources, as it is not powerful enough to be used as the only source. Although the sun's energy is free the actual solar panels are very expensive to make which makes solar energy quite an expensive option.

This year, a company has started marketing roof tiles that are also solar cells, which can supplement your domestic electricity.

Wind power - Britain is a windy country! A lot of farmers make money by renting out land to build wind farms. This is a group of wind turbines that generate electricity from wind as slow as 5 miles per hour. It may only be a few years before over 10% of our electricity is wind generated. Look out too for the first wind turbines in back gardens. Unfortunately, some people don't like wind farms because they spoil the view or make a noise.

#### Other alternatives

Geothermal power uses the natural heat in volcanic rock under the ground to generate electricity. This is popular in Iceland but not likely to happen in the UK as the structure of the Earth under the surface is not as suitable.

Biomass - when dead plants and animals rot the bacteria involved produce methane gas. This gas can be collected and burnt as a fuel. It is often called biogas. Although this makes good use of natural waste, unfortunately burning methane produces pollution like the fossil fuels.

Burning waste - Burning rubbish is not a way to avoid pollution, but it does preserve fossil fuels as well as avoid rubbish having to be put in landfill sites.

Crops for fuel - This is particularly popular in third world countries, as it is cheaper than buying fossil fuels. In Brazil, they grow a lot of Sugar Beet. It is processed into alcohol and used instead of petrol in cars.

## Exam-style Questions

1. a) A wind powered generator is used to produce electrical power when the wind is blowing. The table shows the electrical power generated by the wind for different wind speeds.

 Power generated (watts) 0 0 140 900 1100 1160 1160 Wind Speed (km/h) 0 2 5 10 12 15 20

i) On the axes below draw a graph to show how the power generated changes with wind speed.

(3 Marks)

ii) What is the lowest wind Speed needed to generate power?

(1 Mark)

iii) What is the maximum power generated by the wind?

(1 Mark)

iv) Explain one disadvantage of using only a wind generator as the source of electrical power

(1 Mark)

b) Complete the sentence to show the energy transfer taking place in the wind powered generator

........... Energy is transferred to ............energy.

(2 Marks)

(Marks available: 8)

Answer outline and marking scheme for question: 1

a)

i) points plotted correctly;

smooth curve drawn

(3 Marks)

(1 Mark)

iii) 1160 watts

(1 Mark)

iv) not always windy/variable output/too much land needed

(1 Mark)

b) kinetic/movement; electrical

(2 Marks)

(Marks available: 8)

2. The main heat energy losses from a house are shown in the diagram.

a) Complete the diagram to show the percentage heat energy loss through the walls.

(1 Mark)

b) Complete the table below to show how the heat energy loss from each part of the house can be reduced. The first one has been done for you.

Part of the house Method used for reducing heat energy loss
roof glass-fibre insulation in the loft
walls ...............................................................
floor ...............................................................

(2 Marks)

(Marks available: 3)

Answer outline and marking scheme for question: 2

a) 65°

(1 Mark)

b) Walls: draught excluder/curtain/cavity walls insulation

Floor: carpets/wooden floors

(2 Marks)

(Marks available: 3)

3. a) name a renewable energy resource.

(1 Mark)

b) Describe three main advantages that renewable energy resources have over non-renewable resources for the generation of electricity.

(3 Marks)

c) You are a scientist who wants to use more renewable energy.

Suggest how you would persuade people to want more renewable energy sources

(4 Marks)

(Marks available: 8)

Answer outline and marking scheme for question: 3

a) Any of these: wind, wave, tidal, geothermal, solar , wood

(1 Mark)

b)

Will not run out;

No waste;

No fuel cost;

No transportation cost

(3 Marks)

c) present/communicate scientific evidence for use of:

renewable energy sources (1)

saves using other resources (1)

limitless supply (1)

no pollution / clean / environmental issues (1)

or arguments based on use of non-renewable sources.

(4 Marks)

(Marks available: 8)

4. This question is about keeping a house warm. A house has been insulated in these two ways.

Describe how each of these ways helps to keep the house warm. Use your ideas about conduction, convection and radiation.

a) Double glazing.

(2 Marks)

b) Putting shiny aluminium foil on the wall behind a radiator.

(2 Marks)

(Marks available: 4)

Answer outline and marking scheme for question: 4

a) trapped air

(1 Mark)

poor conductor/convector

(1 Mark)

b) reflects

(1 Mark)

(1 Mark)

(Marks available: 4)

5. a) The devices shown below transfer electrical energy in different ways.

The list gives the useful form of energy the devices are deisgned to produce.

Match words from the list with the devices numbered 1-4

Heat (thermal energy)

Light

Movement (kinetic energy)

Sound

(4 Marks)

b) Match words from the list with the numbers 1-4 in the sentences.

Conduction

Convection

Insulation

Energy travels from the Sun the Earth by ............1...............

In a kettle hot water rises by...............2.............

Thermal energy passes through the walls of a room by ..............3...........

To reduce heat loss from a house, the cavity walls are fitted with ...........4...............

(4 Marks)

(Marks available: 8)

Answer outline and marking scheme for question: 5

a)

• Light
• Movement (kinetic energy)
• Heat (thermal energy)
• Sound

(4 Marks)

b)

• Convection
• Insulation
• Conduction

(4 Marks)

(Marks available: 8)

6. a) The supply of energy from oil is decreasing. The supply from coal is increasing.

Why is this a problem for the environment?

(1 Mark)

b) We have relied on fossil fuels to supply most of our energy needs. Why must we find alternative energy sources?

(1 Mark)

c) On average, the energy use of each family in the UK relases over 25 tonnes of carbon dioxide and 4 kilograms of sulphur diosxide into the air every year.

i) State one environmental effect which is increased by releasing carbon dioxide into the atmosphere

(1 Mark)

ii) State a different environmental effect caused by releasing sulphur dioxide into the atmosphere

(1 Mark)

d) Electricity may be generated using nuclear fuels. Apart from the cost of the electricity, what are the advantages and disadvantages of doing this?

(5 Marks)

(Marks available: 9)

Answer outline and marking scheme for question: 6

a) coal gives more CO2 than oil (for same energy)

(1 Mark)

b)

the demand is rising (too rapidly)

(1 Mark)

resources insufficient to meet (future) demand

(1 Mark)

c)

i) greenhouse (effect)

(1 Mark)

ii) (produces) acid rain

(1 Mark)

d) any five from: (there must be at least 2 advantages and 2 disadvantages for full marks)

• no polluting gases released
• large energy
• so no increase in greenhouse effect/acid rain
• when running normally very little radiation escapes
• if an accident occurs (large) ammounts of radiation may be released
• waste has to be stored (safely) for a longtime/underground
• take a long time to start up

(5 Marks)

(Marks available: 9)

## Types of Energy Transfers

#### Energy

Energy is needed for us to do work. Energy is measured in Joules, J or kilojoules, kJ.

Many scientists believe that there is a certain amount of energy in the universe. As energy cannot be made this energy is just constantly moving around the universe being changed into different forms.

The Earth receives almost all its energy from the sun, but much of it leaves the Earth's atmosphere again and is lost in space.

#### Types of energy

Although energy cannot be made or destroyed it can be changed into different forms.

There are many types of energy, but the ones listed below are the most common:

• Kinetic energy is a more scientific name for movement energy.
• Potential energy is a more scientific name for stored energy.
• Potential energy can be divided into three types:
• #### Changing energy

There are many different ways for energy to change its form. We use Sankey Diagrams to show energy transfers:

The bigger the arrow is the larger the amount of that type of energy.

In the following diagram drag the correct energy labels to the appropriate arrows.

Values for energy are often added to the diagrams.

Always make sure that the total amount of energy coming out is equal to the energy put in!

## S-Cool Revision Summary

#### Types of energy

Energy can not be created or destroyed it can only change from one form into another.

There are many types of energy including,

• sound

• heat

• light

• kinetic

• nuclear

• potential energy

Kinetic energy is movement energy. Potential energy is stored energy. There are three main forms of potential energy including gravitational, chemical and elastic.

Sankey diagrams can be used to represent energy changes. The size of the arrows represents the amount of that type of energy.

Energy is measured in Joules, J or kilojoules, kJ.

#### Conduction

Heat energy always moves from hotter objects to colder objects.

Heat energy is conducted through solids by particles vibrating and passing on the movement to neighbouring particles.

Metals are best at conducting heat. As well as the vibrating particles, they move the heat energy by free electrons moving between their atoms.

The poorest conductors are gases as their molecules are too far apart to affect each other much.

Air is a very bad conductor. Most insulators work because of trapped air.

#### Convection

Convection is hot gases or liquids rising and cooler gases and liquids sinking to replace it. As substances heat up the density decreases, which is what makes them float.

This movement of molecules is called a convection current. It can only happen in a gas or liquid where the molecules are free to move around.

Black, dull surfaces are the best emitters of heat radiation. Lighter, shinier surfaces are poor radiators of heat.

Radiated heat can also be absorbed by cooler objects. Black is the best absorber. Surfaces coloured silver or white will reflect the radiated heat.

#### Ways to save energy in the home

Reducing heat losses from a home means less damage to the environment and lower heating bills.

Installing insulation costs money. The payback time is how long it takes for the savings to cover the cost.

Each strategy has to reduce conduction, convection, radiation or any combination of them.

Common strategies are double-glazing, loft insulation, tank lagging, lined curtains, cavity wall insulation, blocking up disused fireplaces and putting foil behind radiators.

Other conservation strategies include using of low-energy light bulbs, turning down heating thermostats, fitting draught excluders and switching off unattended appliances.

#### Non-renewable fuels and power stations

The fossil fuels are oil, gas and coal. They are non-renewable, which means that they can not be replaced. They will eventually run out.

These fuels have many uses but the main ones are heating, transport and generating electricity.

In power stations, the fuel is burnt and the heat turns water into steam. That steam pushes around a turbine that is connected to a generator. The generator produces electricity.

This process is the same for all power stations.

Nuclear power stations don't burn the fuel. Uranium fuel generates heat that turns water into steam just like in other power stations.

Nuclear accidents are rare, but can be serious. The waste from the reactors can be radioactive. It is easy to store it safely for now but it will stay radioactive for years.

#### Environmental impacts of burning fuels

Carbon dioxide is the most common of several gases that contribute to the greenhouse effect. The result is global warming. This would result in the weather being more extreme and the ice caps melting raising the sea levels.

Sulphur dioxide is the most common cause of acid rain. It dissolves in rainwater to form an acid. The acid rain harms plants, animals and stonework.

#### Alternative energy sources

Most of the alternative energy sources are renewable. This means there is either an endless supply of them so that they will not run out, or they can be easily replaced.

Hydroelectric power is only possible where the geology is right, such as Scotland. Water runs fast down an incline and turns a turbine.

Some developing countries get all their energy from HEP schemes on large dams. The large lake made behind the dam drastically alters the surroundings.

Waves and tides have a lot of energy. Few schemes exist because of technological problems and environmental objections.

Solar power converts the suns energy into electricity using solar panels. These panels are expensive to make.

Wind farms are groups of wind turbines that generate electricity from wind. Some people don't like wind farms because they spoil the view or make a noise.

Geothermal energy uses the natural heat in volcanic rock to generate electricity.

Gas called methane is produced when matter rots. This gas can be used to generate heat to produce electricity.

Burning rubbish is not a way to avoid pollution but it does preserve fossil fuels as well as avoid rubbish having to be put in landfill sites.

Crops can be grown to be burnt in a power station. Another version of this is to process the crops into alcohol and use it instead of petrol in cars.