Physics GCSE (Unit 1a)

Heat Transfer:

• Heat is transferred by conduction, convection and radiation.
• Conduction and convection involve particles but radiation does not.
• Dark, matt surfaces are good absorbers and poor emitters of heat radiation.
• Light, shiny surfaces are poor absorbers and poor emitters of heat radiation.
• The rate at which something radiates heat depends on its surface area and how much hotter it is than its surroundings.

Using energy:

• Energy cannot be created or destroyed, just transformed from one form to another.
• Not all energy transformations are useful, some energy is always 'wasted'.
• All energy is eventually transferred to the surroundings, which become warmer.
• Efficieny = useful energy transferred/ total energy supplied.

Electrical energy:

• Power is the amount of energy transferred each second.
• Power is measured in watts. Their symbol is W.
• Energy is measured in joules. Their symbol is J.
• Electricity is transferred around the country through the National Grid.
• Energy transferred = power x time.
• The kilo-watt hour is another unit of energy, which is used to measure the amount of electricty used at home.

Generating electricity:

• Some power stations are non-renwable sources of energy such as coal and natural gas.
• In nuclear power stations, energy is produced by nuclear fission.
• Renewable energy sources include:
  -wind
  -waves
  -tides
  -falling water
  -the Sun
  -heat from the gorund (geothermal energy).
• There are advantages and disadvantages to using both renewable and non-renewable energy sources. 

Key points- 

• Thermal radiation is energy transfer by electromagnetic waves.
• All objects emit thermal radiation.
• The hotter an object is, the more thermal radiation it emits.

Thermal or heat radiation is the transfer of energy by infra-red waves. These waves are part of the electromagnetic spectrum.

• All objects emit (give off) heat radiation.
• The hotter the object the more heat radiation it emits.
• Heat radiation can travel through a vacuum like space. This is how we get heat from the sun.

KEY WORDS: thermal, heat, radiation, infra-red, emit.

CHECK YOURSELF QUESTIONS:

1) Which part of the electromagnetic spectrum is concerned with heat energy?

2) How does heat from the Sun reach the Earth?

3) Do all objects give out the same amount of heat radiation? Explain your answer.

Key points-

• Dark, matt surfaces are better emittes of thermal radiation than light shiny surfaces.
• Dark, matt surfaces are better absorbers of thermal radiation than light, shiny surfaces.

Dark matt surfaces are good absorbers of radiation. An object painted dull black and left in the Sun will become hotter than the same object painted shiny, white.

Dark matt surfaces are also good emitters of radiation. So an object that is painted dull black will lose heat and cool down quicker than the same object painted shiny, white.

KEY WORDS: abosorber, emitter.

CHECK YOURSELF QUESTIONS:

1) Which surfaces are the best emitter of heat radiation?

2) Which surfaces are the best absorbers of heat radiation?

3) Why are the pipes on the back of a fridge usually painted black?

Key points- 

• Conduction in a metal is due mainly to free electrons trnasferring energy inside the metal.
• Non-metals are poor conductors because they do not contain free electrons.
• Materials such as fibreglass are good insulators because they contain pockets of trapped air.

Conduction occurs mainly in solids. Most liquids and all gases are poor conductors.

• If one end of a solid is heated, the paritcles at that end gain kinetic energy and vibrate more. This energy is passed to neighbouring particles and in this wat the heat is transferred throught the solid.

This process occurs in metals. 

• Poor conductors are called insulators.

• In addition, when metals are heated their free electrons gain kinetic energy and move through the metal transferring energy by colliding with other particles. Hence all metals are good conductors of heat.

KEY WORDS: conduction, conductor, insulator.

CHECK YOURSELF QUESTIONS:

1) Why are materials that trap air, such as fibreglass, good insulators?

2) Why are saucepans often made of metal with wooden handles?

3) Which materials are the best conductors of heat?

Key points-

• Convection takes place only in liquids and gases (fluids).
• Heating a liquid or a gas makes it less dense.
• Convection is due to a hot liquid or gas rising.

Convection occurs in fluids.

When a fluid is heated it expands, becomes less dense, and rises. The warm fluid is replaced by cooler, denser fluid. The resulting convection current transfers heat throughout the fluid.

Convection currents can be on a very small scaled (e.g. heating water in a beaker), or on a very large scale (e.g. heating the air above the land and sea).

Convection currents are responsible for onshore and offshore breezes.

KEY WORDS: fluid, convection, convection current.

CHECK YOURSELF QUESTIONS:

1) In which states of matter does convection occur?

2) What happens to the density of a fluid when it is heated?

3) Why doesn't convection take place in solids?

Key points-

• A radiator has a large surface area so it can lose heat easily.
• Small objects lose heat more easily than large objects.
• Heat loss from a building can be reduced by using:
  -aluminium foil behind radiators.
  -cavity wall insulation.
  -double glazing.
  -loft insulation.

In many situations we want to minimise heat loss. We do this by reducing the losses due to conduction, convection and radiation.

• We can reduce heat loss by conduction by using insulators, e.g. trapping a layer of air.
• Heat loss by convection can be reduced by preventing convection currents being set up, e.g. by trapping air in small pockets.
• We can reduce heat loss by radiation by using light, shiny surfaces, which are poor emitters.

Sometimes we need to maximise heat loss to keep things cool. To do this we may use things that are: 1.good conductors, 2.painted dull black, 3.have the air flow around them maximised.

KEY WORDS: maximise, minimise

CHECK YOURSELF QUESTIONS:

1) What typed of heat transfer is reduced by double glazing?

2) How does cavity wall insulation reduce heat loss from a building?

3) What is loft insulation usually made from?

1) Name three types of heat transfer.

2) Which type of heat transfer occurs mainly in solid?

3) How are convection currents set up in fluids?

4) Describe the process of heat transfer though a metal.

5) What factors affect the amount of heat radiated by a body?

6) Why are gases poor conductors?

7) How does surface colour affect the rate of conduction?

8) Why do central heating radiators have large surface areas?

9) Why does a concrete floor feel cooler to your feet than a carpeted floor at the same temp?

10) Why are hot water tanks often wrapped in glass fibre jackets?

1) What type of energy is stored in any object that can fall?

2) What is kinetic energy?

3) What energy transformation takes place in a microphone?

4) What is meant by the 'conservation of energy'?

5) What is the useful energy transformation in an electric motor?

6) What happens to the energy wasted by a device?

7) What are the units of efficiency?

8) What is the SI unit of energy?

Key points-

• Energy exists in different forms.
• Energy can change (transform) from one form into another form.

Energy exists in diff. forms such as: light, thermal (heat), sound, kinetic (movement), nuclear, electrical, gravitational potential (G.P.E), elastic potential and chemical.

The last three forms are stored energy.

FORM OF ENERGY: EXAMPLE:

Light-From the Sun or a lamp

Thermal (heat)-Flows from a hot object to a colder object

Sound -from a loudspeaker or your voice

Kinetic (movement)- Anything moving

Nuclear- From nuclear reactions

Electrical- Whenever an electric current flows

Gravitational Potential- Stored in any object that can fall

Elastic Potential (strain)- Stored in stretched objects such as elastic bands

Chemical Stored- in fuels, food and batteries and is released when chemical reactions take place. 

KEY WORDS: energy, kinetic, potential

CHECK YOURSELF QUESTIONS:

1) What energy do we give to a spring if we squash it?

2) Where does the chemical energy stored in your muscles come from?

3) What form of energy does a moving train have?

Key points-

• Energy can be transformed from one form to another or transferred from one place to another.
• Energy cannot be created or destroyed.

The total amount of energy is always the same (when transforming or transferring energy). This is called the 'conservation of energy'.

E.G. When an object falls, G.P.E is transformed into kinetic energy. Similarly, stretching an elastic band transforms chemical energy into elastic potential energy.

In a solar cell, light energy is transformed into electrical energy.

KEY WORDS: transform, transfer, conservation.

CHECK YOURSELF QUESTIONS:

1) What energy transformation takes place when you burn a fuel?

2) What energy transformations take place in a light bulb?

3) When you run, what type of energy is changed into kinetic energy? Where does this energy come from?

Key points-

• Useful energy is energy in the place we want it and in the form we need it.
• Wasted energy is energy that is not useful energy.
• Useful energy and wasted energy both end up being transferred to the surroundings, which become warmer.
• As energy spreads out, it gets more and more difficult to used for further energy transfers.

A device (or machine) is something that transfers energy from one place to another or transforms energy from one form to another.

The energy supplied to a device is called the 'input energy'.

The energy we get from the device consists of:

• Useful energy (transferred to the place we want and need it)
• Wasted energy (mostly it is converted as heat, frequently as a result of friction between the moving parts of the device)

From the conservation of energy we know that:

Input energy = useful energy + wasted energy

Both useful energy and and wasted energy will eventually be transferred to the surroundings (warming them up). As it does so, it becomes more difficult to use the energy.

KEY WORDS: device, input energy, useful energy, wasted energy.

CHECK YOURSELF QUESTIONS:

1) What is the useful energy transformation in a light bulb and what happens to the wasted energy?

2) Why does energy become more difficult to use as it spreads out?

3) Why do computers and televisions have vents?

Key points-

• Energy is measured in joules.
• The efficiency of a device= useful energy transferred by the device / total energy supplied to the device.
• Wasted energy causes inefficiency.

Symbol of a joule = J. This unit is used for all forms of energy.

The less energy that is wasted by a device, the more efficient the device is said to be.

We can calculate the efficiency of a device using the equations in the key points.

The efficiency can be left as a fraction or multiplied to give a %.

No device can be 100% efficient, except an electric heater, which usefully transforms all of the electrical energy supplied to it into heat.

KEY WORDS: joule, efficiency

CHECK YOURSELF QUESTIONS:

1) If a device is adjusted so that it wastes less energy as heat, what happens to its efficiency?

2) In a light bulb, for every 20 joules of energy input to the bulb, 5 joules are usefully transformed into light energy. What is the efficiency of the bulb?

3) In an electric motor 2000 joules of energy are given out as heat to the surroundings, for every 5000 joules of electrical energy supplied to the motor. What is the efficiency of the motor?

N.B.- Efficiency is a ratio. That means it does not have unit.

1) What energy transformation takes place in an electric iron?

2) What energy transformations take place in a television?

3) What is meant by the power of a device?

4) What is the unit of power?

5) What unit do electricity companies use to measure the amount of electrical energy used?

6) What is the equation that relates to energy, power and time?

7) What is the national grid?

8) What typed of transformers makes voltages larger?

Key points- 

• Electrical energy is energy transfer due to an electric current.
• Uses of electrical device include: heating, lighting, making objects move (using an electric motor), creating sound and visual images.

Electrical devices are extremely useful. They transform electrical energy into whatever form of energy we need at the flick of a switch.

Common electrical devices include:

• kettles (to produce heat energy)
• Lamps (to produce light energy)
• Electric mixers (to produce kinetic energy)
• Speakers (to produce sound energy)
• Televisions (to produce light and sound energy)

KEY WORDS: electrical devices

CHECK YOURSELF QUESTIONS:

1) What energy transformations take place in an electric drill?

2) How is electrical energy supplied to a torch?

3) What energy transformations take place in a vacuum cleaner?

Key points-

• The unit of power is the watt (W), equal to 1 J/s.
• 1 kilowatt (kW) = 1000 watts
• Power (in watts)= energy transferred (in joules) / time taken (in seconds)

The power of a device is the rate at which it transforms energy.

The unit of power if the watt (symbol W). A device with a power of 1 watt, transforms 1 joule of electrical energy to other forms of energy every second.

Often a watt is too small a unit to be useful, so power may be given in kilowatts (kW).

Power is calculated using the equation in the key points.

KEY WORDS: power, watt, kilowatt

CHECK YOURSELF QUESTIONS:

1) How many watts are in 30 kilowatts?

2) Which is more powerful, a 2.5 kW heater of a 3000 W heater?

3) An electric motor transforms 36 kJ of electrical energy into kinetic energy in 3 minutes. What is the useful power output of the motor?

Key points-

• Energy transferred (kW h) = power of device (kW) x time (h)
• Total cost of electricity = number of kW h x cost per kW h

Companies that supply mains electricity charge customers for the amount of electrical energy used.

The amount of energy is measured in kW h.

A kW h is the amount of energy transferred by a 1 kW device used for 1 hour.

The electricity meter in a house records the amount of kW h of energy used.

If the previous meter reading is subtracted from the current reading, the electrical energy used between readings can be calculated.

KEY WORDS: kilowatt-hour, kilowatt, hour

CHECK YOURSELF QUESTIONS:

1) What quantity is measured in kilowatts?

2) How much electrical energy, in kW h, does it take to use a 9kW shower for 20 minutes?

3) The price of 1 kW h of electricity is 8p. How much does it cost to use a 100 W electric light for 4 hours?

Key points-

• The National Grid is a network of cables and transformers.
• We use step-up transformers to step power station voltages up to the grid voltage.
• We use step-down transformers to to step the grid voltage down for use in our homes.
• A high grid voltage reduces energy loss and makes the system more efficient.

Since the whole country in connected to the system, power stations can be switched in or out according to demand.

Step-up transformers= transmission at high voltage reduced energy losses in cables, making the system more efficient.

Step-down transformers= too dangerous to supply electricity to consumers at very high voltages- so they are used to reduce the voltage to 230v.

The national grid transmits electricity at a low current.

KEY WORDS: National Grid, step-up transformer, step down transformer

CHECK YOURSELF QUESTIONS:

1) What type of transformer makes voltages smaller?

2) Why is electricity transmitted at very high voltages across the national grid?

3) What type of transformer is found in a local sub-station?

1) State an equivalent unit to the watt.

2) What device transforms sounds energy to electrical energy?

3) What device transforms electrical energy to sound energy?

4) In a 3kW kettle how many joules of electrical energy are transformed from electrical to heat energy each second?

5) An immersion heater converts 36 000 000 J of electrical energy into heat energy when it is switched on for 1 hour. What is the power of the heater in kW?

6) How much does it cost to use a 1200 W vacuum cleaner for 10 mins if electrical energy costs 7p per kW h?

7) Draw a block diagram showing the different parts of the National Grid system and the order in which they are used.

8) Why are voltages reduced to 230V before reaching homes?

Key points-

• Fossil fuels produce greenhouse gases.
• Nuclear fuels produce radioactive waste.
• Renewable energy resources can affect plant and animal life.

Coal, oil, gas and uranium are non-renewable energy resources.

This means that at the rate at which they are used is very much faster than the rate at which they are produced.

If we continue to use them up at the current rate they will soon run out.

Renewable energy sources will not run out.

There are advantages and disadvantages to using each type of energy resource.

Coal- 

• Reliable, bigger reserves than other fossil fuels
• BUT, non-renewable, production of CO2 a (greenhouse gas), & SO2 (causing acid rain)

Oil-

• Reliable
• BUT, non-renewable, production of CO2 a (greenhouse gas), & SO2 (causing acid rain).

Gas-

• Reliable, gas power stations can be started up quickly to deal with sudden demand.
• BUT, non-renewable, production of CO2 a (greenhouse gas).

Nuclear-

• No production of polluting gases, reliable
• BUT, non-renewable, produces nuclear waste which is difficult to dispose of safely, risk of big accident, such as Chernobyl.

Falling water-

• Renewable, free, no production of polluting gases, reliable in wet areas, pumped storage systems allows storage of energy, can be started up quickly to deal with sudden demand.
• BUT, only works in wet and hilly areas, flooding an area affects the local ecology.

Waves-

• Renewable, free, no production of polluting gases.
• BUT, can be a hazard to boats, not reliable

Tides-

• Renewable, free, no production of polluting gases, reliable (always tides twice a day).
• BUT, only a few river estuaries are suitable, building a large barrage affects the local ecology.

Solar-

• Renewable, free, no production of polluting gases, reliable in hot countries in the daytime
• BUT, only suitable for small amounts of electricity, or requires large number of cells, unreliable in less sunny countries.

Geothermal-

• Renewable, free, no production of polluting gases.
• BUT, only economically viable in very few places, drilling through large depth of rock is difficult and expensive. 

KEY WORDS: renewable, non-renewable, polluting.

CHECK YOURSELF QUESTIONS:

1) Are renewable or non-renewable energy sources the most reliable?

2) Which types of power station can be started up quickly to deal with periods of high demand?

3) What type of area would be most suitable for a wind farm? 

1) Explain the difference between renewable and non-renewable energy resources.

2) in which three ways can water be used as an energy resource to generate electricity?

3) How can geothermal energy be used to produce heat and electricity?

4) What are the advantages of using fossil fuels in power stations to produce electricity?

5) What are the disadvantages of using fossil fuels in power stations to produce electricity?

6) Suggest a disadvantage of a nuclear power station.

7) Why is geothermal energy economically unviable in most places?

8) What colour are solar heating panels usually painted?