Transcript 2 marks - crypt

GCSE Physics Exam Doctor
Mains Electricity
Question Question
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Question Question
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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)
iii) Complete the sentences using the words from the box. Each word may be
used once, more than once, or not at all.
earth
fuse
live
neutral
plug
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.
(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?
neutral wire
The wire to the fuse is all fuzzy. The earth wire is bulging.
The outer insulation is too far down.
(2 marks)
ii) Complete the label.
(1 mark)
iii) What should be the colour of the live wire?
The live wire should be blue
(1 mark)
b) i) Which part of the washing machine should the earth be connected to ?
To the green and yellow bit
(1 mark)
ii) What is the purpose of the fuse?
To limit the current to a safe value.
(1 mark)
iii) Complete the sentences using the words from the box. Each word may be
used once, more than once, or not at all.
earth
fuse
live
neutral
plug
the plug
If the case becomes live, a large current passes to ………………….,
causing the
fuse
…………………….
to melt.
earth
This breaks the …………………..
connection to the mains supply. The fuse and
live
…………………….wire
together therefore protect the user from electric shock.
(4 marks)
Mark scheme
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?
neutral wire
All the bits of the wire, by the fuse, should be inside the insulation.
The cord insulation exposing the inner, coloured insulation.
(2 marks)
ii) Complete the label.
(1 mark)
Mark scheme
iii) What should be the colour of the live wire?
The live wire is brown.
(1 mark)
b) i) Which part of the washing machine should the earth be connected to ?
It should be connected to the outer casing of the washing machine.
(1 mark)
ii) What is the purpose of the fuse?
Cutting off the current if it gets too big.
(1 mark)
Mark scheme
iii) Complete the sentences using the words from the box. Each word may be
used once, more than once, or not at all.
earth
fuse
live
neutral
plug
earth
If the case becomes live, a large current passes to ………………….,
causing the
fuse
…………………….
to melt.
live
This breaks the …………………..
connection to the mains supply. The fuse and
earth
…………………….wire
together therefore protect the user from electric shock.
(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?
neutral wire
The bulging earth wire is
irrelevant, the score is for the
cord insulation exposing the
inner, coloured insulation.
The wire to the fuse is all fuzzy. The earth wire is bulging.
The outer insulation is too far down.
(2 marks)
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.
ii) Complete the label.
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(1 mark)
iii) What should be the colour of the live wire?
The live wire is brown, the neutral blue.
The live wire should be blue
(1 mark)
b) i) Which part of the washing machine should the earth be connected to ?
To the green and yellow bit
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.
(1 mark)
ii) What is the purpose of the fuse?
To limit the current to a safe value.
Although this looks alright at first sight, it implies that the current reaches (1 mark)
a limit and is not allowed to increase anymore, but still flows. An answer
0
including cutting off the current if it gets too big is required.
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iii) Complete the sentences using the words from the box. Each word may be
used once, more than once, or not at all.
earth
fuse
live
neutral
plug
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 plug
If the case becomes live, a large current passes to ………………….,
causing the
fuse
…………………….
to melt.
earth
This breaks the …………………..
connection to the mains supply. The fuse and
live
…………………….wire
together therefore protect the user from electric shock.
(4 marks)
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.
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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.
V=I xR
V = 0.02 x 10 000 = 200
(3 marks)
b) Explain why it is dangerous to operate a light switch with a wet hand.
Water conducts electricity and magnifies the shock
(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? Green and yellow
(1 mark)
ii) Appliances with a plastic casing do not have an earth connection. Explain
why this connection is not needed.
You can’t get a shock from plastic. It insulates the current.
(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.
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 (2 marks)
of through you.
d) ii) Why must the resistance of the earth wire to an appliance be very low?
So that all of the current can go down that route instead of
through you.
(2 marks)
Mark scheme
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.
V = I x R V = 0.02 x 10 000 = 200V
(3 marks)
b) Explain why it is dangerous to operate a light switch with a wet hand.
Impure water conducts electricity and this may lead to a current in the body.
(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? Green and yellow
(1 mark)
Mark scheme
ii) Appliances with a plastic casing do not have an earth connection. Explain
why this connection is not needed.
Plastic is an insulator, there is no possibility of a current, the casing cannot
become live.
(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.
The low resistance path to earth allows a large current to flow through the
earth wire, which blows the fuse.
(2 marks)
d) ii) Why must the resistance of the earth wire to an appliance be very low?
The resistance must be low, so that there can be a large current that
will blow the fuse.
(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.
V=I xR
V = 0.02 x 10 000 = 200
Scores 1 for using V = I x R
Scores 1 for arriving at 200, but has omitted the unit V
(3 marks)
b) Explain why it is dangerous to operate a light switch with a wet hand.
Water conducts electricity and magnifies the shock
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 (2 marks)
current in the body.
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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? Green and yellow
(1 mark)
ii) Appliances with a plastic casing do not have an earth connection. Explain
why this connection is not needed.
You can’t get a shock from plastic. It insulates the current.
Scores 1 for plastic is an insulator, but has not made it clear that there
(2 marks)
is no possibility of a current or of the casing cannot become live.
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.
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.
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 (2 marks)
of through you. An earth wire works only in conjunction with the fuse.
This is a very common misunderstanding.
d) ii) Why must the resistance of the earth wire to an appliance be very low?
So that all of the current can go down that route instead of
through you.
The current is not diverted. The resistance must be low, so that
there can be a large current that will blow the fuse.
(2 marks)
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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.
I = P x V = 170 000 000 x 25 000 = 4 50 000 000 000
(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.
The voltage is increased by a transformer
(2 marks)
ii) Explain why the electricity is transmitted at high voltage.
It is sent at high voltage to make it more powerful, so it can go
further before it runs out.
(2 marks)
iii) To what value is the voltage changed for household use in the UK?
240V
(1 mark)
Mark scheme
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.
Power = V x I , so I = P/V
I = 170 000 000/25 000 = 6800A
(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.
“step-up” transformer, more turns on the secondary solenoid.
(2 marks)
Mark scheme
ii) Explain why the electricity is transmitted at high voltage.
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.
(2 marks)
iii) To what value is the voltage changed for household use in the UK?
240V
(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.
I = P x V = 170 000 000 x 25 000 = 4 50 000 000 000
Power = V x I , so I = P/V
I = 170 000 000/25 000 = 6800A
(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.
Scores for transformer.
The voltage is increased by a transformer
Omits to mention “step-up” transformer, or reference to more turns on the
secondary solenoid.
(2 marks)
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ii) Explain why the electricity is transmitted at high voltage.
It is sent at high voltage to make it more powerful, so it can go
further before it runs out.
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 (2 marks)
voltage than it is for low current/high voltage.
iii) To what value is the voltage changed for household use in the UK?
240V
(1 mark)
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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.
The magnet could be put into the coil
(1 mark)
ii) State two ways in which this voltage could be increased.
Stronger magnet. Bigger coil.
(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.
Np/Vp = Ns/Vs
(1 mark)
ii) Calculate the voltage across the secondary coil.
Np/Vp = Ns/Vs = 10 000/240 = 500/Vs = Vs = 1/12 = 0.083
(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.
To change the voltage
(1 mark)
ii) Explain how this makes the transmission of energy around the country more
efficient.
It sends the electricity better. No energy is lost in the wires
(3 marks)
Mark scheme
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.
The magnet must be moving, inside the coil.
(1 mark)
ii) State two ways in which this voltage could be increased.
Use a stronger magnet.
More turns on the coil; move faster
(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.
Np/Vp = Ns/Vs
Any correct combination of this
(1 mark)
relationship would score.
Mark scheme
ii) Calculate the voltage across the secondary coil.
Np/Vp = Ns/Vs
10,000/240 = 500/Vs Vs = 500x240/10,000
=12V
(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.
To step up the voltage
(1 mark)
ii) Explain how this makes the transmission of energy around the country more
efficient.
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
(3 marks)
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.
The magnet could be put into the coil
Does not score, because it is not made clear that the magnet must be (1 mark)
moving, not placed stationary in the coil.
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ii) State two ways in which this voltage could be increased.
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Scores for stronger magnet.
Stronger magnet. Bigger coil.
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. (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.
Any correct combination of this
Np/Vp = Ns/Vs
relationship would score.
(1 mark)
ii) Calculate the voltage across the secondary coil.
Np/Vp = Ns/Vs = 10 000/240 = 500/Vs = Vs = 1/12 = 0.083
Scores for substituting in the equation, but the calculation is incorrect.
(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.
To change the voltage
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Could have been more precise as “step-up”, but this is sufficient for 1 mark. (1 mark)
ii) Explain how this makes the transmission of energy around the country more
efficient.
It sends the electricity better. No energy is lost in the wires
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 (3 marks)
for a given power, less is wasted at high voltage/low current rather than
low voltage//high current.
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.
i)
Which appliance has the
greatest electrical resistance?
How do the data show this?
Appliance
Current in A
kettle
8.5
lamp
0.4
toaster
4.8
(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.
(3 marks)
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.
(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)
a) The table below shows the current in three different electrical appliances
when connected to the 240V mains a.c. supply.
i)
Which appliance has the
greatest electrical resistance?
How do the data show this?
Appliance
Current in A
kettle
8.5
lamp
0.4
toaster
4.8
The kettle has the greatest resistance, because the figure is
the biggest.
(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.
The wire will be too thin. It will catch fire or explode.
(2 marks)
b) i) Calculate the power rating of the kettle when it is operated from the
240V a.c. mains supply.
Power = V x I = 240 x 8.5 = 2040
(3 marks)
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.
V=I xR
I = 120/8.5 = 14.1
(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.
It will go much quicker. In fact it might explode.
(2 marks)
Mark scheme
a) The table below shows the current in three different electrical appliances
when connected to the 240V mains a.c. supply.
i)
Which appliance has the
greatest electrical resistance?
How do the data show this?
Appliance
Current in A
kettle
8.5
lamp
0.4
toaster
4.8
Lamp has the lowest current, therefore the greatest resistance.
The higher the current the lower the resistance, for a given voltage.
(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.
Too thin.
It should also have an earth wire.
(2 marks)
Mark scheme
b) i) Calculate the power rating of the kettle when it is operated from the
240V a.c. mains supply.
Power = V x I = 240 x 8.5 = 2040W
(3 marks)
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.
V = I x R, if V is halved with R remaining the same, the current will
also halve. 4.25A
(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.
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 marks)
a) The table below shows the current in three different electrical appliances
when connected to the 240V mains a.c. supply.
i)
Which appliance has the
greatest electrical resistance?
How do the data show this?
Appliance
Current in A
kettle
8.5
lamp
0.4
toaster
4.8
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. (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.
Scores for too thin.
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The wire will be too thin. It will catch fire or explode.
Fails to mention that it should also have an earth wire.
(2 marks)
b) i) Calculate the power rating of the kettle when it is operated from the
240V a.c. mains supply. Scores for correct substitution.
Power = V x I = 240 x 8.5 = 2040
Scores for power = V x I
Fails to score final mark, unit omitted.
(3 marks)
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.
V=I xR
I = 120/8.5 = 14.1
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
(1 mark)
remaining the same, the current will also halve. 4.25A
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.
It will go much quicker. In fact it might explode.
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 marks)
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GCSE Physics Exam Doctor
Mains Electricity
End of questions