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GCSE Physics Exam Doctor. Mains Electricity. Question 2. Question 3. Question 4. Question 1. Question 5. GCSE Physics Exam Doctor. Mains Electricity. Question 1. - PowerPoint PPT PresentationTRANSCRIPT
GCSE Physics Exam Doctor
Mains Electricity
Question1
Question2
Question3
Question4
Question5
GCSE Physics Exam Doctor
Mains Electricity
Question 1
The diagram shows the wiring inside a plug fitted to a washing machine, though two of the wires are not their correct colours.
a) i) What two errors have been made in wiring the plug?
(2 marks)
ii) Complete the label. (1 mark)
iii) What should be the colour of the live wire?
(1 mark)
b) i) Which part of the washing machine should the earth be connected to ?
(1 mark)
ii) What is the purpose of the fuse?
(1 mark)
If the case becomes live, a large current passes to …………………., causing the ……………………. to melt.
This breaks the ………………….. connection to the mains supply. The fuse and …………………….wire together therefore protect the user from electric shock.
earth fuse live neutral plug
iii) Complete the sentences using the words from the box. Each word may be used once, more than once, or not at all.
(4 marks)
The diagram shows the wiring inside a plug fitted to a washing machine, though two of the wires are not their correct colours.
a) i) What two errors have been made in wiring the plug?
(2 marks)
ii) Complete the label. (1 mark)
The wire to the fuse is all fuzzy. The earth wire is bulging. The outer insulation is too far down.
neutral wire
iii) What should be the colour of the live wire?
(1 mark)
b) i) Which part of the washing machine should the earth be connected to ?
(1 mark)
ii) What is the purpose of the fuse?
(1 mark)
The live wire should be blue
To the green and yellow bit
To limit the current to a safe value.
If the case becomes live, a large current passes to …………………., causing the ……………………. to melt.
This breaks the ………………….. connection to the mains supply. The fuse and …………………….wire together therefore protect the user from electric shock.
earth fuse live neutral plug
iii) Complete the sentences using the words from the box. Each word may be used once, more than once, or not at all.
(4 marks)
the plugfuse eart
hlive
The diagram shows the wiring inside a plug fitted to a washing machine, though two of the wires are not their correct colours.
a) i) What two errors have been made in wiring the plug?
(2 marks)
ii) Complete the label. (1 mark)
Mark scheme
All the bits of the wire, by the fuse, should be inside the insulation.The cord insulation exposing the inner, coloured insulation.
neutral wire
iii) What should be the colour of the live wire?
(1 mark)
b) i) Which part of the washing machine should the earth be connected to ?
(1 mark)
ii) What is the purpose of the fuse?
(1 mark)
Mark scheme
The live wire is brown.
It should be connected to the outer casing of the washing machine.
Cutting off the current if it gets too big.
If the case becomes live, a large current passes to …………………., causing the ……………………. to melt.
This breaks the ………………….. connection to the mains supply. The fuse and …………………….wire together therefore protect the user from electric shock.
earth fuse live neutral plug
iii) Complete the sentences using the words from the box. Each word may be used once, more than once, or not at all.
(4 marks)
Mark scheme
earthfuse
liveearth
The diagram shows the wiring inside a plug fitted to a washing machine, though two of the wires are not their correct colours.
a) i) What two errors have been made in wiring the plug?
(2 marks)
ii) Complete the label. (1 mark)
The wire to the fuse is all fuzzy. The earth wire is bulging. The outer insulation is too far down.
neutral wire
Although this is badly expressed, it is just sufficient to score, as the idea is there that all the bits of the wire should be inside the insulation.
The bulging earth wire is irrelevant, the score is for the cord insulation exposing the inner, coloured insulation.
33
iii) What should be the colour of the live wire?
(1 mark)
b) i) Which part of the washing machine should the earth be connected to ?
(1 mark)
ii) What is the purpose of the fuse?
(1 mark)
The live wire should be blue
To the green and yellow bit
To limit the current to a safe value.
The live wire is brown, the neutral blue.
The question has been misunderstood. The earth wire is the green and yellow one, it should be connected to the outer casing of the washing machine.
Although this looks alright at first sight, it implies that the current reaches a limit and is not allowed to increase anymore, but still flows. An answer including cutting off the current if it gets too big is required.
03
If the case becomes live, a large current passes to …………………., causing the ……………………. to melt.
This breaks the ………………….. connection to the mains supply. The fuse and …………………….wire together therefore protect the user from electric shock.
earth fuse live neutral plug
iii) Complete the sentences using the words from the box. Each word may be used once, more than once, or not at all.
(4 marks)
the plugfuse eart
hlive
A large current passes to earth, causing the fuse to melt. This breaks the live connection to the mains supply. The fuse and earth wire together therefore protect the user from electric shock.
The function of the earth wire is commonly misunderstood. In free-response questions a reply about the “current going through the earth wire instead of through you” is common. The earth wire would be ineffective without the fuse.
14
GCSE Physics Exam Doctor
Mains Electricity
Question 2
a) A current of 0.02 A could give a serious electrical shock to a person. If the resistance of a body is 10 000 ohms, calculate the voltage which will cause this current.
(3 marks)
b) Explain why it is dangerous to operate a light switch with a wet hand.
(2 marks)
c) Some appliances have an earth wire fitted to them to prevent the user receiving an electric shock. i) What is the colour code for the earth wire in a plug? (1 mark)
ii) Appliances with a plastic casing do not have an earth connection. Explain why this connection is not needed.
(2 marks)
d) i) Explain how the earth wire prevents a person receiving an electric shock when the live wire comes into contact with the metal casing of an appliance.
(2 marks)
d) ii) Why must the resistance of the earth wire to an appliance be very low?
(2 marks)
a) A current of 0.02 A could give a serious electrical shock to a person. If the resistance of a body is 10 000 ohms, calculate the voltage which will cause this current.
(3 marks)
b) Explain why it is dangerous to operate a light switch with a wet hand.
(2 marks)
c) Some appliances have an earth wire fitted to them to prevent the user receiving an electric shock. i) What is the colour code for the earth wire in a plug? (1 mark)
V = I x R V = 0.02 x 10 000 = 200
Water conducts electricity and magnifies the shock
Green and yellow
ii) Appliances with a plastic casing do not have an earth connection. Explain why this connection is not needed.
(2 marks)
d) i) Explain how the earth wire prevents a person receiving an electric shock when the live wire comes into contact with the metal casing of an appliance.
(2 marks)
d) ii) Why must the resistance of the earth wire to an appliance be very low?
(2 marks)
You can’t get a shock from plastic. It insulates the current.
The earth wire is a safety device it prevents a shock. The live wire conducts to the casing and causes a shock if you touch it. With an earth wire the current goes to the earth instead of through you.So that all of the current can go down that route instead of through you.
a) A current of 0.02 A could give a serious electrical shock to a person. If the resistance of a body is 10 000 ohms, calculate the voltage which will cause this current.
(3 marks)
b) Explain why it is dangerous to operate a light switch with a wet hand.
(2 marks)
c) Some appliances have an earth wire fitted to them to prevent the user receiving an electric shock. i) What is the colour code for the earth wire in a plug? (1 mark)
Mark scheme
V = I x R V = 0.02 x 10 000 = 200V
Impure water conducts electricity and this may lead to a current in the body.
Green and yellow
ii) Appliances with a plastic casing do not have an earth connection. Explain why this connection is not needed.
(2 marks)
d) i) Explain how the earth wire prevents a person receiving an electric shock when the live wire comes into contact with the metal casing of an appliance.
(2 marks)
d) ii) Why must the resistance of the earth wire to an appliance be very low?
(2 marks)
Mark scheme
Plastic is an insulator, there is no possibility of a current, the casing cannot become live.
The low resistance path to earth allows a large current to flow through the earth wire, which blows the fuse.
The resistance must be low, so that there can be a large current that will blow the fuse.
a) A current of 0.02 A could give a serious electrical shock to a person. If the resistance of a body is 10 000 ohms, calculate the voltage which will cause this current.
(3 marks)
b) Explain why it is dangerous to operate a light switch with a wet hand.
(2 marks)
c) Some appliances have an earth wire fitted to them to prevent the user receiving an electric shock. i) What is the colour code for the earth wire in a plug? (1 mark)
V = I x R V = 0.02 x 10 000 = 200
Water conducts electricity and magnifies the shock
Green and yellow
Scores 1 for using V = I x R
Scores 1 for arriving at 200, but has omitted the unit V
Scores 1 for water conducts, though, in fact, pure water does not conduct, only impure water, but has omitted any references to possibility of a current in the body.
46
ii) Appliances with a plastic casing do not have an earth connection. Explain why this connection is not needed.
(2 marks)
d) i) Explain how the earth wire prevents a person receiving an electric shock when the live wire comes into contact with the metal casing of an appliance.
(2 marks)
d) ii) Why must the resistance of the earth wire to an appliance be very low?
(2 marks)
You can’t get a shock from plastic. It insulates the current.
The earth wire is a safety device it prevents a shock. The live wire conducts to the casing and causes a shock if you touch it. With an earth wire the current goes to the earth instead of through you.So that all of the current can go down that route instead of through you.
Scores 1 for plastic is an insulator, but has not made it clear that there is no possibility of a current or of the casing cannot become live.
An earth wire works only in conjunction with the fuse. This is a very common misunderstanding.
The current does not divert from you to the earth wire. The low resistance path gives a large current in the earth wire, which blows the fuse.
The current is not diverted. The resistance must be low, so that there can be a large current that will blow the fuse. 1
6
GCSE Physics Exam Doctor
Mains Electricity
Question 3
A power station has four generators. Each generator produces 170 MW (1.7 x 108 W). The electricity is generated at 25 000 V.
a) Calculate the current in one generator.
(3 marks)
b) The voltage is increased to 400 000V before the electricity is fed into the national grid. i) Describe how the voltage is increased.
(2 marks)
ii) Explain why the electricity is transmitted at high voltage.
(2 marks)
iii) To what value is the voltage changed for household use in the UK?
(1 mark)
A power station has four generators. Each generator produces 170 MW (1.7 x 108 W). The electricity is generated at 25 000 V.
a) Calculate the current in one generator.
(3 marks)
b) The voltage is increased to 400 000V before the electricity is fed into the national grid. i) Describe how the voltage is increased.
(2 marks)
I = P x V = 170 000 000 x 25 000 = 4 50 000 000 000
The voltage is increased by a transformer
ii) Explain why the electricity is transmitted at high voltage.
(2 marks)
iii) To what value is the voltage changed for household use in the UK?
(1 mark)
It is sent at high voltage to make it more powerful, so it can go further before it runs out.
240V
A power station has four generators. Each generator produces 170 MW (1.7 x 108 W). The electricity is generated at 25 000 V.
a) Calculate the current in one generator.
(3 marks)
b) The voltage is increased to 400 000V before the electricity is fed into the national grid. i) Describe how the voltage is increased.
(2 marks)
Mark scheme
Power = V x I , so I = P/VI = 170 000 000/25 000 = 6800A
“step-up” transformer, more turns on the secondary solenoid.
ii) Explain why the electricity is transmitted at high voltage.
(2 marks)
iii) To what value is the voltage changed for household use in the UK?
(1 mark)
Mark scheme
Understanding of energy loss in the transmission lines, which is greater for a given power (V x I) at high current/low voltage than it is for low current/high voltage.
240V
A power station has four generators. Each generator produces 170 MW (1.7 x 108 W). The electricity is generated at 25 000 V.
a) Calculate the current in one generator.
(3 marks)
b) The voltage is increased to 400 000V before the electricity is fed into the national grid. i) Describe how the voltage is increased.
(2 marks)
I = P x V = 170 000 000 x 25 000 = 4 50 000 000 000
The voltage is increased by a transformer
Power = V x I , so I = P/VI = 170 000 000/25 000 = 6800A
Scores for transformer.
Omits to mention “step-up” transformer, or reference to more turns on the secondary solenoid.
15
ii) Explain why the electricity is transmitted at high voltage.
(2 marks)
iii) To what value is the voltage changed for household use in the UK?
(1 mark)
It is sent at high voltage to make it more powerful, so it can go further before it runs out.
240V
Fails to score. No understanding of energy loss in the transmission lines, which is greater for a given power (V x I) at high current/low voltage than it is for low current/high voltage.
13
GCSE Physics Exam Doctor
Mains Electricity
Question 4
a) The diagram shows a bar magnet and a coil of wire.
i) Explain how the magnet could be used to produce a voltage across the coil of wire.
(1 mark)
ii) State two ways in which this voltage could be increased.
(2 marks)
b) A mains transformer has 10 000 turns of wire on its primary coil and 500 turns of wire on its secondary coil. An alternating voltage of 240V is supplied across the primary coil. i) Write down the equation which could be used to calculate the voltage across the secondary coil.
(1 mark)
ii) Calculate the voltage across the secondary coil.
(2 marks)
c) The diagram shows a transformer connected between the generator in a power station and the power cables which carry electricity around the country.
i) State the function of the transformer.
(1 mark)ii) Explain how this makes the transmission of energy around the country more efficient.
(3 marks)
a) The diagram shows a bar magnet and a coil of wire.
i) Explain how the magnet could be used to produce a voltage across the coil of wire.
(1 mark)
ii) State two ways in which this voltage could be increased.
(2 marks)
b) A mains transformer has 10 000 turns of wire on its primary coil and 500 turns of wire on its secondary coil. An alternating voltage of 240V is supplied across the primary coil. i) Write down the equation which could be used to calculate the voltage across the secondary coil.
(1 mark)
The magnet could be put into the coil
Stronger magnet. Bigger coil.
Np/Vp = Ns/Vs
ii) Calculate the voltage across the secondary coil.
(2 marks)
c) The diagram shows a transformer connected between the generator in a power station and the power cables which carry electricity around the country.
i) State the function of the transformer.
(1 mark)ii) Explain how this makes the transmission of energy around the country more efficient.
(3 marks)
Np/Vp = Ns/Vs = 10 000/240 = 500/Vs = Vs = 1/12 = 0.083
To change the voltage
It sends the electricity better. No energy is lost in the wires
a) The diagram shows a bar magnet and a coil of wire.
i) Explain how the magnet could be used to produce a voltage across the coil of wire.
(1 mark)
ii) State two ways in which this voltage could be increased.
(2 marks)
b) A mains transformer has 10 000 turns of wire on its primary coil and 500 turns of wire on its secondary coil. An alternating voltage of 240V is supplied across the primary coil. i) Write down the equation which could be used to calculate the voltage across the secondary coil.
(1 mark)
Mark scheme
The magnet must be moving, inside the coil.
Use a stronger magnet.
More turns on the coil; move faster
Np/Vp = Ns/Vs Any correct combination of this relationship would score.
ii) Calculate the voltage across the secondary coil.
(2 marks)
c) The diagram shows a transformer connected between the generator in a power station and the power cables which carry electricity around the country.
i) State the function of the transformer.
(1 mark)ii) Explain how this makes the transmission of energy around the country more efficient.
(3 marks)
Mark scheme
Np/Vp = Ns/Vs 10,000/240 = 500/Vs Vs = 500x240/10,000 =12V
To step up the voltage
Less energy wasted even though some is still wasted. Power lost = I2R, so for a given power, less is wasted at high voltage/low current rather than low voltage//high current.
a) The diagram shows a bar magnet and a coil of wire.
i) Explain how the magnet could be used to produce a voltage across the coil of wire.
(1 mark)
ii) State two ways in which this voltage could be increased.
(2 marks)
b) A mains transformer has 10 000 turns of wire on its primary coil and 500 turns of wire on its secondary coil. An alternating voltage of 240V is supplied across the primary coil. i) Write down the equation which could be used to calculate the voltage across the secondary coil.
(1 mark)
The magnet could be put into the coil
Stronger magnet. Bigger coil.
Np/Vp = Ns/Vs
Does not score, because it is not made clear that the magnet must be moving, not placed stationary in the coil.
Scores for stronger magnet.
Fails to score for bigger coil, because it is not clear that it is more turns on the coil, as opposed to larger cross-sectional area. Move faster would also score.
Any correct combination of this relationship would score.
24
ii) Calculate the voltage across the secondary coil.
(2 marks)
c) The diagram shows a transformer connected between the generator in a power station and the power cables which carry electricity around the country.
(1 mark)ii) Explain how this makes the transmission of energy around the country more efficient.
(3 marks)
Np/Vp = Ns/Vs = 10 000/240 = 500/Vs = Vs = 1/12 = 0.083
To change the voltage
It sends the electricity better. No energy is lost in the wires
Scores for substituting in the equation, but the calculation is incorrect.
i) State the function of the transformer.
Could have been more precise as “step-up”, but this is sufficient for 1 mark.
Just scores for no energy loss, though wasted would be better than lost, and some is still wasted. Other marks are for wasted energy as I2R, so for a given power, less is wasted at high voltage/low current rather than low voltage//high current.
36
GCSE Physics Exam Doctor
Mains Electricity
Question 5
a) The table below shows the current in three different electrical appliances when connected to the 240V mains a.c. supply.
4.8toaster
0.4lamp
8.5kettle
Current in AAppliancei) Which appliance has the
greatest electrical resistance? How do the data show this? (2 marks)
ii) The lamp is connected to the mains supply using thin, twin-cored cable, consisting of live and neutral connections. State two reasons why this cable should not be used for connecting the kettle to the mains supply.
(2 marks)
b) i) Calculate the power rating of the kettle when it is operated from the 240V a.c. mains supply.
ii) A holiday-maker takes the kettle abroad where the mains supply is 120V. What is the current in the kettle when it is operated from the 120V supply? You can assume that the resistance of the kettle does not change.
(3 marks)
(1 mark)
iii) The kettle is filled with water. Explain how the time it takes to boil the kettle changes when it is operated from the 120V supply.
(2 marks)
The wire will be too thin. It will catch fire or explode.
a) The table below shows the current in three different electrical appliances when connected to the 240V mains a.c. supply.
4.8toaster
0.4lamp
8.5kettle
Current in AAppliancei) Which appliance has the
greatest electrical resistance? How do the data show this? (2 marks)
ii) The lamp is connected to the mains supply using thin, twin-cored cable, consisting of live and neutral connections. State two reasons why this cable should not be used for connecting the kettle to the mains supply.
(2 marks)
The kettle has the greatest resistance, because the figure is the biggest.
b) i) Calculate the power rating of the kettle when it is operated from the 240V a.c. mains supply.
ii) A holiday-maker takes the kettle abroad where the mains supply is 120V. What is the current in the kettle when it is operated from the 120V supply? You can assume that the resistance of the kettle does not change.
(3 marks)
(1 mark)
iii) The kettle is filled with water. Explain how the time it takes to boil the kettle changes when it is operated from the 120V supply.
(2 marks)
Power = V x I = 240 x 8.5 = 2040
V = I x R I = 120/8.5 = 14.1
It will go much quicker. In fact it might explode.
a) The table below shows the current in three different electrical appliances when connected to the 240V mains a.c. supply.
4.8toaster
0.4lamp
8.5kettle
Current in AAppliancei) Which appliance has the
greatest electrical resistance? How do the data show this? (2 marks)
ii) The lamp is connected to the mains supply using thin, twin-cored cable, consisting of live and neutral connections. State two reasons why this cable should not be used for connecting the kettle to the mains supply.
(2 marks)
Mark scheme
Lamp has the lowest current, therefore the greatest resistance. The higher the current the lower the resistance, for a given voltage.
Too thin.
It should also have an earth wire.
b) i) Calculate the power rating of the kettle when it is operated from the 240V a.c. mains supply.
ii) A holiday-maker takes the kettle abroad where the mains supply is 120V. What is the current in the kettle when it is operated from the 120V supply? You can assume that the resistance of the kettle does not change.
(3 marks)
(1 mark)
iii) The kettle is filled with water. Explain how the time it takes to boil the kettle changes when it is operated from the 120V supply.
(2 marks)
Mark scheme
Power = V x I = 240 x 8.5 = 2040W
V = I x R, if V is halved with R remaining the same, the current will also halve. 4.25A
It will take longer to boil. The voltage is half, therefore the current is half so the energy delivered per second is 1/4
The wire will be too thin. It will catch fire or explode.
a) The table below shows the current in three different electrical appliances when connected to the 240V mains a.c. supply.
4.8toaster
0.4lamp
8.5kettle
Current in AAppliancei) Which appliance has the
greatest electrical resistance? How do the data show this? (2 marks)
ii) The lamp is connected to the mains supply using thin, twin-cored cable, consisting of live and neutral connections. State two reasons why this cable should not be used for connecting the kettle to the mains supply.
(2 marks)
The kettle has the greatest resistance, because the figure is the biggest.The figure given in the table is the current. The higher the
current the lower the resistance, for a given voltage.
Scores for too thin.
Fails to mention that it should also have an earth wire.
14
b) i) Calculate the power rating of the kettle when it is operated from the 240V a.c. mains supply.
ii) A holiday-maker takes the kettle abroad where the mains supply is 120V. What is the current in the kettle when it is operated from the 120V supply? You can assume that the resistance of the kettle does not change.
(3 marks)
(1 mark)
iii) The kettle is filled with water. Explain how the time it takes to boil the kettle changes when it is operated from the 120V supply.
(2 marks)
Power = V x I = 240 x 8.5 = 2040
V = I x R I = 120/8.5 = 14.1
It will go much quicker. In fact it might explode.
Scores for power = V x I
Scores for correct substitution.
Fails to score final mark, unit omitted.
V = I x R, is correct, but 8.5 has been used as R. It is actually the current when on 240V. From V = I x R, if V is halved with R remaining the same, the current will also halve. 4.25A
It will take longer to boil. The voltage is half, therefore the current is half so the energy delivered per second is 1/4 2
6
GCSE Physics Exam Doctor
Mains Electricity
End of questions