Science GCSE

PHYSICS

Energy and the future

Energy transfer and efficiency

energy: The ability to do work or cause movement.

There are several different types of energy which can be transferred from one type to another. Energy transfer diagrams show these transfers in process. More efficient devices transfer the energy supplied to them into a greater proportion of useful energy.

Forms of energy

You should be able to recognise the main types of energy. Mainly there are 9 different types of energy. One way to remember the different types of energy is to learn this mnemonic sentence:

Most Kids Hate Learning GCSE Energy Names

where each capital letter is the first letter in the name of a type of energy.

Types of energy/Description/Example

Magnetic: Energy in magnets and electromagnets. Example: Magnet
Kinetic: The energy in moving objects. Also called movement energy. Example: A bullet cutting a playing card.
Heat: Also called thermal energy. Example: Burning match
Light: Also called radiant energy. Example: Sunlight
Gravitational potential: Stored energy in raised objects. Example: Skydivers
Chemical: Stored energy in fuel, foods and batteries. Example: Organic food
Sound: Energy released by vibrating objects. Example: Guitar
Electrical: Energy in moving or static electric charges. Example: Lightning
Elastic potential: Stored energy in stretched or squashed objects. Example: Catapult
Nuclear: Stored in the nuclei of atoms. Example: Nuclear fuel assembly

Energy transfers

In Key Stage 3 it is taught how to draw and interpret an energy transfer diagram.

We will revisit energy transfer diagrams as follows:

Different types of energy can be transferred from one type to another.

Energy transfer diagrams show each type of energy, whether it is stored or not, and the processes taking place as it is transferred.

Sankey diagrams also show the relative amounts of each type of energy.The thicker the line or arrow in a Sankey diagram, the greater the amount of energy involved.

Sankey diagram generic

Sankey diagram for a filament lamp

Source: http://www.bbc.co.uk/schools/gcsebitesize/science

In the Sankey diagram for a filament lamp above one can see the relative amounts of each type of energy. The electrical energy is 100J. While the useful light energy given is 10J, the waste energy is the heat energy which is 90J.

Energy transfer diagrams

The following three energy transfer diagrams show the useful energy transfer in a car engine. You can see that a car engine transfers chemical energy, which is stored in the fuel, into kinetic energy in the engine and wheels.

Diagram 1: Energy transfer diagram in a Car Engine

Diagram 2: Energy transfer diagram in a Car Engine

Diagram 3: Sankey diagram in a Car Engine

Process of using chemical energy

This diagram shows the energy transfer diagram for the useful energy transfer in an electric lamp. You can see that the electric lamp transfers or converts electrical energy into light energy.

Process of using electrical energy

Notice that these energy transfer diagrams only show the useful energy transfers. However, car engines are also noisy and hot, and electric lamps also give out heat energy.

Sankey diagrams

Sankey diagrams summarise all the energy transfers taking place in a process. The thicker the line or arrow, the greater the amount of energy involved. The Sankey diagram for an electric lamp below shows that most of the electrical energy is transferred as heat rather than light.

Sankey diagram for a filament lamp

Energy efficiency

You should know that energy can be 'wasted' during energy transfers, and you should be able to calculate theefficiency.

efficiency: The fraction of the energy supplied to a device which is transferred in a useful form. of a device.

'Wasted' energy

Energy cannot be created or destroyed. It can only be transferred from one form to another or moved. Energy that is 'wasted', like the heat energy from an electric lamp, does not disappear. Instead, it is transferred into the surroundings and spreads out so much that it becomes very difficult to do anything useful with it.

Electric lamps

Ordinary electric lamps contain a thin metal filament that glows when electricity passes through it. However, most of the electrical energy is transferred as heat energy instead of light energy. If you stroll down you will see the Sankey diagram for a typical filament lamp.

Sankey diagram for a filament lamp

Modern energy-saving lamps work in a different way. They transfer a greater proportion of electrical energy as light energy. This is the Sankey diagram for a typical energy-saving lamp.

Sankey diagram for a typical energy-saving lamp.

Source: http://www.bbc.co.uk/schools/gcsebitesize/science/aqa_pre_2011

Conclusive comparative statements:

From the Sankey diagram for a typical energy-saving lamp, you can see that much less electrical energy is transferred, or 'wasted', as heat energy in comparison with the energy loss in a filament lamp.The electrical energy is both cases is 100J. While the useful light energy given in the filament lamp is 10J, the useful light energy given in the energy-saving lamp is 75J. The waste energy in the filament lamp is the heat energy which is 90J while the waste energy thus the heat energy in the energy-saving lamp is 25J. The useful energy thus the light energy is much higher and the waste energy thus heat energy is much lower in the energy-saving lamp

Calculating efficiency

The efficiency of a device such as a lamp can be calculated using this equation:

efficiency = ( useful energy transferred ÷ energy supplied ) × 100

The efficiency of the filament lamp is (10 ÷ 100) × 100 = 10%.

This means that 10% of the electrical energy supplied is transferred as light energy (90% is transferred as heat energy).

The efficiency of the energy-saving lamp is (75 ÷ 100) × 100 = 75%. This means that 75% of the electrical energy supplied is transferred as light energy (25% is transferred as heat energy).

Note that the efficiency of a device will always be less than 100%.

Sankey Diagram: Calculating Efficiency in an electric motor

Test: file:///C:/Users/User/Desktop/AQA-PHYS-W-SQP-1F.PDF

Questions Efficiency and Sankey diagrams
Efficiency (%) = (Useful energy produced/Total energy used) x 100 

Q1. What is the efficiency of a wind up car that begins with 140 joules of elastic energy and produces 63 joules of kinetic energy? 

Q2. What is the efficiency of a buzzer if it is supplied with 235 joules of electrical energy and produces 92 joules of sound energy? 

Q3. What is the efficiency of a laptop that uses 12000 joules of energy and produces 3360 joules of light energy? 

Now questions get a little trickier. 

Q4. What is the efficiency of a light bulb that produces 8 joules of heat energy when it is supplied with 18 joules of electrical energy?
 
Q5. What is the efficiency of a yo-yo that begins with 84 joules of gravitational potential energy and produces 6 joules of heat energy as it falls? 

Sankey diagrams - You need graph paper

Draw a Sankey diagram to represent a light bulb that uses 14 joules of electrical energy and produces 6 joules of light and 8 joules of heat.
A mobile phone uses 40 joules of electrical energy and produces 25 joules of light and sound energy, while also producing 15 joules of heat.

Temperature

Metal takes in heat and emits energy

All objects take in and radiate heat (thermal) energy in the form of infrared radiation. For example on a hot day a metal bench in the park will absorb thermal energy and then it will give out (emit) thermal energy at night as it cools down. A system(meaning anything where a change can happen) must emit the same average power as it absorbs to be at a constant temperature.

Different materials and thermal energy

Some materials absorb and radiate thermal energy better than others and this can be tested by comparing different surfaces to see which heats up the fastest.

Thermal absorption in surfaces

Surface	Absorbs thermal energy	Emits thermal energy
shiny	poor	poor
matt	good	good
light	poor	poor
dark	good	good

Energy and the future

Energy transfer and efficiency

Test

1. How many forms of energy are there?

2. What is another term for movement energy?

Kinetic energy

Change energy

Transfer energy

3. If a torch takes in 50J of energy and transfers 20J to light. What is its efficiency?

20%

50%

40%

4. A fan takes in electrical energy and gives out movement, heat and sound. Which type of energy is useful?

Heat

Movement

Sound

5. If 90J of energy goes into a torch and 50J is given out as light, how much is given out as heat - the only other product?

40J

90J

50J

6. Which type of surface will absorb the most heat energy?

Shiny and light

Dark and matt

Shiny and medium coloured

Answers:

9, kinetic,movement, 40J, dark and mat

Conduction, convection and radiation

Source: http://www.bbc.co.uk/education/guides/zttrd2p/revision

Energy can be transferred by conduction, convection and radiation. Insulation is used to stop heat energy transfers from buildings and the human body.

Conduction

Heat is thermal energy. It can be transferred from one place to another by conduction.

Metals are good conductors of heat, but non-metals and gases are usually poor conductors. Poor conductors are called insulators.

Heat energy is conducted from the hot end of an object to the cold end.

Conduction in metals

The electrons in a piece of metal can leave their atoms and move about in the metal as free electrons. The parts of the metal atoms left behind are now positively charged metal ions.

The ions are packed closely together and they vibrate continually. The hotter the metal, the more kinetic energy these vibrations have. This kinetic energy is transferred from hot parts of the metal to cooler parts by the free electrons.

These move through the structure of the metal, colliding with ions as they go.

Conduction in metal

A metal bar is heated up. What is happening in 3 steps?

Investigating conductors

An experiment can be used to investigate which metal is the best conductor of heat. It involves some long thin strips of different metals (eg steel, aluminium and copper), wax, drawing pins and a Bunsen burner.

Method:

Fix the drawing pin to the end of the metal strip using drops of wax.
Position the other end of the metal strip into a Bunsen flame.
Record the time taken for the wax to melt and the drawing pin to drop off.

The fastest time shows the best conductor of heat.

Variables that affect the time taken for the drawing pins to fall include the distance they are from the flame and the thickness of the metal.

If you have controlled all of these variables, you should find that copper conducts better than aluminium, while aluminium conducts better than steel.

Convection

Heat can be transferred from one place to another by convection.

Fluids

Liquids and gases are fluids because they can be made to flow. Theparticles in these fluids can move from place to place. Convection occurs when particles with a lot of heat energy in a liquid or gas move and take the place of particles with less heat energy.

As the hot air above a radiator rises it pushes cooler air away from it. The cooler air eventually circulates back round to the radiator where it gets heated and the cycle continues.

Air current close to a radiator

Heat energy is transferred from hot places to cooler places by convection.

A beaker is heated and the coloured fluid inside shows convection currents

Liquids and gases expand when they are heated.

This is because the particles in liquids and gases move faster when they are heated than they do when they are cold.

As a result, the particles take up more volume. This is because the gap between particles widens, while the particles themselves stay the same size.

The liquid or gas in hot areas is less dense than the liquid or gas in cold areas, so it rises into the cold areas.

The denser cold liquid or gas falls into the warm areas.

In this way, convection currents that transfer heat from place to place are set up.

Convection currents can be seen in lava lamps. The wax inside the lamp warms up, becomes less dense than the liquid and so rises.

When it rises, it cools and becomes denser again, so it sinks. This same effect can be seen by putting a crystal of potassium permanganate in a beaker of water and gently heating it.

Convection explains why hot air balloons rise, and also why it is often hotter in the lofts of houses than downstairs.

As well as these examples, convection is seen on a much bigger scale in our weather and ocean currents.

Heat transfer by radiation

Heat can be transferred by infrared radiation. Unlike conductionand convection - which need particles - infrared radiation is a type ofelectromagnetic radiation that involves waves.

The sun seen from space with Earth in the foreground.

Light from the sun reaching earth

Because no particles are involved, radiation can even work through thevacuum of space. This is why we can still feel the heat of the Sun even though it is 150 million km away from the Earth.

Different surfaces

Some surfaces are better than others at reflecting and absorbinginfrared radiation. This table summarises some differences:

Surface	Absorption	Emission
Dull, matt or rough	Good	Good
Shiny	Poor	Poor

You can see that dull surfaces are good absorbers and emitters of infrared radiation. Shiny surfaces are poor absorbers and emitters (but they are good reflectors of infrared radiation).

If two objects made from the same material have identical volumes, a thin, flat object will radiate heat energy faster than a fat object.

This is one reason why domestic radiators are thin and flat.

Radiators are often painted with gloss paint, but they would be better at radiating heat if they were painted with matt paint instead. Also, despite their name, radiators actually transfer most of their heat to a room by convection, not radiation.

Heat radiation investigation

The transfer of infrared radiation from a hot object to cooler surroundings can be investigated using a piece of apparatus called Leslie’s cube.This is a metal cube with four side prepared in different ways: black, white, shiny, or dull. It can be filled with hot water or heated on an electrical hot plate so that all four sides are at the same temperature.

Method

Measure the temperature a fixed distance from each side of a Leslie's cube using thermometers (or using a thermocouple, an electrical device that produces a potential difference depending on the temperature).
Heat the Leslie’s cube with boiling water or with a hot plate.
Continue to measure and record the temperatures every 30 seconds for five minutes, then plot a graph of temperature against time for each side.
Compare the four graphs obtained. Remember that the four thermometers may vary in accuracy, so take this into account when analysing the results.

Reducing heat transfers – houses

Heat energy is lost from buildings through their roofs, windows, walls, floors and through gaps around windows and doors. However, there are ways that these losses can be reduced.

Heat escape routes

Take a look at this thermogram of a house. The roof and windows are the hottest, showing that most heat is lost from the house through those parts.

Thermogram of a house showing areas of heat loss

Heat energy is transferred from homes by conduction through the walls, floor, roof and windows.

It is also transferred from homes by convection. For example, cold air can enter the house through gaps in doors and windows, and convection currents can transfer heat energy in the loft to the roof tiles.

Heat energy also leaves the house by radiation - through the walls, roof and windows.

Ways to reduce heat loss

There are several different ways to reduce heat loss:

Simple ways to reduce heat loss include fitting carpets, curtains and draught excluders. It is even possible to fit reflective foil in the walls or on them.
Heat loss through windows can be reduced by using double glazing. These special windows have air or a vacuum between two panes of glass. If the double glazing has a vacuum there will be no conduction or convection. If the double glazing is made with air between the glass then convection is minimised because there is little room for the air to move. Air is a poor conductor so there will be very little heat loss by conduction.
Heat loss through walls can be reduced using cavity wall insulation. This involves blowinginsulating material into the gap between the brick and the inside wall. Insulating materials are bad conductors and so this reduces the heat loss by conduction. The material also prevents air circulating inside the cavity, therefore reducing heat loss by convection.
Heat loss through the roof can be reduced by laying loft insulation. This works in a similar way to cavity wall insulation.
Reducing heat transfers – the human body
Heat energy is lost from the human body. However, there are ways that these losses can be reduced.

This is a thermogram of a man, woman and child not wearing clothes. The white and red parts are the hottest and so are losing most heat. Orange and green represent medium temperatures, and blue and purple are the coldest parts.

It is easy to see that the heads of all three people are the hottest parts, followed by their torsos. Their arms and legs show medium temperatures, while their hands and feet are among the coldest parts. It is in these places that we sometimes get frostbite in very cold conditions.

To reduce heat transfers from our bodies, we can wear insulatingclothing - such as gloves, scarves, hats and thermal socks - in cold weather. The more layers we wear, the more warm air can be trapped and the warmer we will remain. Some clothes, such as wool, are excellent insulators, because they trap lots of air.Glossary
1. atomAll elements are made of atoms. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons.
2. conductionThe transfer of heat through a material by transferring kinetic energy from one particle to another.
3. conductorA material which allows charge to move easily through it.
4. convectionThe transfer of heat energy through a moving liquid or gas.
5. denseCrowded closely together.
6. electronSub-atomic particle, with a negative charge and a negligible mass relative to protons and neutrons.
7. frostbiteDamage to the skin and other tissues that happens because of freezing. Severe frostbite can lead to loss of body parts such a fingers and toes.
8. infrared radiationElectromagnetic radiation emitted from a hot object.
9. insulateTo help maintain the temperature.
10. ionElectrically charged particle, formed when an atom or molecule gains or loses electrons.
11. kinetic energyThe energy that moving objects have.
12. particleA general term for a small piece of matter. It can mean protons, neutrons, electrons, atoms, ions or molecules for example.
13. radiationEnergy carried by particles from a radioactive substance, or spreading out from a source.
14. thermogramImage made by detecting infrared radiation rather than visible light. Different colours are used to represent the different temperatures of the objects in the image.
15. vacuumA volume that contains no matter.

Torso

Head

Answers: radiation, conduction, gases and liquids, radiation, convection, black, black, convection, head.

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Monday 13 June 2016

Science GCSE PHYSICS Energy and the future Energy transfer and efficiency