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Additional Revision Information DocumentTopic Title: Electricity (Physics)

Topic Reference: Paper 2 Revision

Key information: Standard circuit diagram symbols

Electrical charge and currentFor electrical charge to flow through a closed circuit the circuit must include a source of potential difference.

Electric current is a flow of electrical charge. The size of the electric current is the rate of flow of electrical charge. Charge flow, current and time are linked by the equation:

Current, resistance and potential differenceThe current (I) through a component depends on both the resistance (R) of the component and the potential difference (V) across the component. The greater the resistance of the component the smaller the current for a given potential difference (pd) across the component. Questions will be set using the term potential difference. Students will gain credit for the correct use of either

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potential difference or voltage.

Resistors

The resistance of a thermistor decreases as the temperature increases.The applications of thermistors in circuits eg a thermostat is required. The resistance of an LDR decreases as light intensity increases.The application of LDRs in circuits eg switching lights on when it gets dark is required.

Series and parallel circuits

There are two ways of joining electrical components, in series and in parallel. Some circuits include both series and parallel parts. For components connected in series: • there is the same current through each component • the total potential difference of the power supply is shared between the components• the total resistance of two components is the sum of the resistance of each component.

For components connected in parallel:• the potential difference across each component is the same• the total current through the whole circuit is the sum of the currents through the separate components

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• the total resistance of two resistors is less than the resistance of the smallest individual resistor.

Direct and alternating potential differenceMains electricity is an ac supply. In the United Kingdom the domestic electricity supply has a frequency of 50 Hz and is about 230 V.

Mains electricityMost electrical appliances are connected to the mains using three core cable. The insulation covering each wire is colour coded for easy identification:live wire – brown neutral wire – blue earth wire – green and yellow stripes. The live wire carries the alternating potential difference from the supply. The neutral wire completes the circuit. The earth wire is a safety wire to stop the appliance becoming live. The potential difference between the live wire and earth (0 V) is about 230 V. The neutral wire is at, or close to, earth potential (0 V). The earth wire is at 0 V, it only carries a current if there is a fault.

PowerStudents should be able to explain how the power transfer in any circuit device is related to the potential difference across it and the current through it, and to the energy changes over time:

Everyday electrical appliances are designed to bring about energy transfers. The amount of energy an appliance transfers depends on how long the appliance is switched on for and the power of the appliance. Work is done when charge flows in a circuit. The amount of energy transferred by electrical work can be calculated using the equation:

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The National GridThe National Grid is a system of cables and transformers linking power stations to consumers.Electrical power is transferred from power stations to consumers using the National Grid.Step-up transformers are used to increase the potential difference from the power station to the transmission cables then step-down transformers are used to decrease, to a much lower value, the potential difference for domestic use. Students should be able to explain why the National Grid system is an efficient way to transfer energy.

Higher tier only: Students should be able to select and use the equation:potential difference across primary coil x current in primary coil = potential difference across secondary coil x current in secondary coil as given on the equation sheet.

Media LINK

Oak National Academy

https://classroom.thenational.academy/units/electricity-f083Physics only

https://classroom.thenational.academy/units/electricity-ft-0d27(Foundation)

https://classroom.thenational.academy/units/electricity-ht-761e(Higher)

YouTube Clips

https://www.youtube.com/watch?v=EmCGF49Uilshttps://www.youtube.com/watch?v=jSA4WaLSVEA

GCSE Pod https://members.gcsepod.com/shared/podcasts/title/10976https://members.gcsepod.com/shared/podcasts/title/10977https://members.gcsepod.com/shared/podcasts/title/10978https://members.gcsepod.com/shared/podcasts/title/10979

BBC Bitesize

https://www.bbc.co.uk/bitesize/topics/zq8wxnb

Addition Website

https://www.youtube.com/watch?v=62RyyfKZoYg

Q2.Figure 1 shows a three pin plug connected to the cable of a metal toaster.

Figure 1

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https://www.youtube.com/watch?v=62RyyfKZoYg

https://www.bbc.co.uk/bitesize/topics/zq8wxnb

https://members.gcsepod.com/shared/podcasts/title/10979




https://www.youtube.com/watch?v=jSA4WaLSVEA

https://www.youtube.com/watch?v=EmCGF49Uils

https://classroom.thenational.academy/units/electricity-ht-761e

https://classroom.thenational.academy/units/electricity-ft-0d27

https://classroom.thenational.academy/units/electricity-f083

(a) Name wire X.

___________________________________________________________________(1)

(b) What does wire X do?

Tick one box.

It provides extra energy to the toaster when needed.

It completes the circuit in the toaster.

It can prevent an electric shock from the toaster.

It supplies the current to the toaster.

(1)

(c) The toaster is plugged in to the mains electricity supply.

What is the potential difference between the live and neutral wires?

Tick one box.

0 V 120 V 230 V 460 V

(1)

(d) Mains electricity is an alternating supply.

A battery is a direct supply.

Figure 2 shows an alternating supply and a direct supply.

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Figure 2

Give two differences between the alternating supply and the direct supply.

1. _________________________________________________________________

2. _________________________________________________________________(2)

Energy is transferred to homes by the National Grid.

Figure 3 shows the percentage energy losses over the National Grid for different years.

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Figure 3

(e) Describe the changes in percentage energy loss.

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________(2)

(f) Calculate the mean percentage energy loss per year in Figure 3.

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

Mean energy loss per year = ____________________ %(3)

(Total 10 marks)

Q3.An energy input of 1.3 × 1018 J is supplied each year by power stations to the National

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Grid.

Not all of this energy is supplied to consumers. Some of the energy is wasted in the distribution process.

(a) Write the equation which links efficiency, total input energy transfer and useful output energy transfer.

___________________________________________________________________

___________________________________________________________________(1)

(b) The energy supplied each year to consumers is 1.2 × 1018 J

Calculate the efficiency of the distribution process.

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

Efficiency = _______________________(2)

(c) How is electrical power transmitted across the National Grid to make the process as efficient as possible?

Tick one box.

At a high potential difference and a high current

At a high potential difference and a low current

At a low potential difference and a high current

At a low potential difference and a low current

(1)

(d) Write the equation which links energy transferred, power and time.

___________________________________________________________________

___________________________________________________________________(1)

(e) A wind turbine supplies a power output of 8000 kW for 1200 seconds.

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Calculate the energy transferred by the wind turbine in kJ

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

Energy transferred = ___________________________ kJ(3)

(f) Describe the environmental advantages and disadvantages of using wind turbines to generate electricity in the UK.

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________(4)

(Total 12 marks)

Q6.

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(a) A student set up the circuit shown in the diagram. The student uses the circuit to obtain the data needed to plot a current - potential difference graph for a diode.

(i) Draw, in the boxes, the circuit symbol for a diode and the circuit symbol for a variable resistor.

Diode Variable resistor

(2)

(ii) The student made two mistakes when setting up the circuit.

What two mistakes did the student make?

1. ____________________________________________________________

______________________________________________________________

2. ____________________________________________________________

______________________________________________________________(2)

(b) After correcting the circuit, the student obtained a set of data and plotted the graph below.

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Potential difference in volts

(i) At what potential difference did the diode start to conduct an electric current?

___________________________________ V(1)

(ii) Use data from the graph to calculate the resistance of the diode when the potential difference across the diode is 0.3 V.

______________________________________________________________

______________________________________________________________

______________________________________________________________

Resistance = _____________ ohms(3)

(c) The diagram shows the trace produced by an alternating current (a.c.) supply on an oscilloscope.

Each horizontal division on the oscilloscope screen represents a time of 0.01s.

(i) Calculate the frequency of the a.c. supply.

______________________________________________________________

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______________________________________________________________

______________________________________________________________

Frequency = _______________________ hertz(2)

(ii) A diode is now connected in series with the a.c. power supply.

Why does the diode cause the trace on the oscilloscope screen to change?

______________________________________________________________

______________________________________________________________

______________________________________________________________

______________________________________________________________(2)

(Total 12 marks)

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Mark schemes

Q2.(a) earth

1

(b) it can prevent an electric shock from the toaster1

(c) 230 V1

(d) (the potential difference) for the alternating supply changes directionallow current

1

(the potential difference) for the alternating supply changes magnitudeallow currentallow converseallow potential difference of alternating supply is greater

1

(e) there is an overall decreaseallow there is an decrease in percentage energy loss until 2013

1

but there is a (small) increase since 20131

(f) 1.92, 1.72, 1.70, 1.74, 1.771

(1.92 + 1.72 + 1.70 + 1.74 + 1.77)/51

1.77(%)an answer of 1.77(%) scores 3 marks

1[10]

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Q3.

(a)

1

(b) 1

= 0.92or92 (%)

allow an answer that rounds to 0.92or92 (%)

1

ignore unitsan answer of 0.92 or 92 (%) scores 2 marks

(c) at a high potential difference and a low current1

(d)

1

(e) if a conversion of 8000 has been attempted, this mark can be awarded

1

(E =) 1200 × 8000if a conversion of 8000 has been attempted, this mark can be awarded

1

(E =) 9 600 000 (kJ)this answer only

1

an answer of 9 600 000 (kJ) scores 3 marks

(f) any four from:

(environmental advantages)• renewable / sustainable (energy source)• conserves fossil fuels• no release of pollutant gases e.g. sulfur dioxide

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• no release of greenhouse gasesallow does not release carbon dioxide

• does not contribute to global warming

(environmental disadvantages)• noise pollution• visual pollution• bird kill• not always windy so more use of fossil fuel power stations

ignore destruction of habitatmax 3 marks if only refers to advantages or disadvantagesignore references to cost

4[12]

Q6.

(a) (i) symbol for a diode

accept 1

symbol for a variable resistor 1

(ii) voltmeter is in series or voltmeter is not in parallel1

ammeter is in parallel or ammeter is not in seriesaccept an answer in terms of how the circuit should be correctedvoltmeter and ammeter are wrong way around is insufficient

1

(b) (i) 0.2 (V)accept any value between 0.20 and 0.21 inclusive

1

(ii) 37.5allow 1 mark for I = 0.008 or allow 2 marks for correct substitution, ie 0.3 = 0.008 × R or allow 1 mark for a correct substitution using I = 0.8 or I = 0.08or I = 0.009 or allow 2 marks for answers of 0.375 or 3.75 or 33(.3)

3

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(c) (i) 25allow 1 mark for obtaining period = 0.04(s)

2

(ii) diode has large resistance in reverse / one direction1

so stops current flow in that / one directionallow diodes only let current flow one way / directionallow 1 mark for the diode has half-rectified the (a.c. power) supply

1[12]

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dearne academy · web viewthe national grid is a system of cables and transformers linking power...

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