Transcript 2 marks - crypt

GCSE Physics Exam Doctor
Electricity – Potential Difference
Question Question
2
1
Question
3
Question Question
4
5
Question
6
GCSE Physics Exam Doctor
Electricity – Potential Difference
Question 1
12v battery
variable resistor
A
lamp
fixed resistor R
B
V1
V2 V
a) What is the sum of the readings of voltmeters V1 and V2?
(1 mark)
b) What does your answer assume about the wire which connects the components?
(2 marks)
c) If V2 reads 8v what is the potential difference across the variable resistor?
(1 mark)
d) The fixed resistor R is replaced by one with only half as much resistance.
Predict the reading on V2 and explain why this would happen.
(3 marks)
e) The resistance of the variable resistor is reduced until it is extremely small.
Explain how this affects the reading on voltmeter V2.
(2 marks)
f) If the 12v battery were replaced by a 9v battery, what would be the potential
difference between points A and B?
(1 mark)
12v battery
variable resistor
A
lamp
fixed resistor R
B
V1
V2 V
a) What is the sum of the readings of voltmeters V1 and V2?
12 V
(1 mark)
b) What does your answer assume about the wire which connects the components?
it is made of copper
(2 marks)
c) If V2 reads 8v what is the potential difference across the variable resistor?
6v
(1 mark)
d) The fixed resistor R is replaced by one with only half as much resistance.
Predict the reading on V2 and explain why this would happen.
The reading would rise because it has less resistance
(3 marks)
e) The resistance of the variable resistor is reduced until it is extremely small.
Explain how this affects the reading on voltmeter V2.
The reading would go down because it has too much current in it.
(2 marks)
f) If the 12v battery were replaced by a 9v battery, what would be the potential
difference between points A and B?
9v
(1 mark)
Mark scheme
12v battery
variable resistor
A
lamp
fixed resistor R
B
V1
V2 V
a) What is the sum of the readings of voltmeters V1 and V2?
12V
1MP
(1 mark)
b) What does your answer assume about the wire which connects the components?
the wire has little or no resistance 1MP
the wire has negligible ( zero) potential difference (PD) between its ends 1MP
(2 marks)
Mark scheme
c) If V2 reads 8v what is the potential difference across the variable resistor?
4v
1MP
(1 mark)
d) The fixed resistor R is replaced by one with only half as much resistance.
Predict the reading on V2 and explain why this would happen.
Reading on V2 would fall.
1MP
lower resistance would cause a smaller PD across R
1MP
(3 marks)
total of PD must remain 12v
1MP
e) The resistance of the variable resistor is reduced until it is extremely small.
Explain how this affects the reading on voltmeter V2.
V2 would rise
1MP
because with low resistance the PD across the variable resistor would fall 1MP
(2 marks)
f) If the 12v battery were replaced by a 9v battery, what would be the potential
difference between points A and B?
9v
1MP
(1 mark)
12v battery
variable resistor
A
lamp
fixed resistor R
B
V1
1
3
V2 V
a) What is the sum of the readings of voltmeters V1 and V2?
12 V
(1 mark)
b) What does your answer assume about the wire which connects the components?
it is made of copper The wire may be made of copper but there is no mention
of low resistance or low PD across it so no marks
(2 marks)
c) If V2 reads 8v what is the potential difference across the variable resistor?
No mark the PD is 12 - 8 = 4v
6v
(1 mark)
d) The fixed resistor R is replaced by one with only half as much resistance.
Predict the reading on V2 and explain why this would happen.
The reading would rise because it has less resistance
The reading on V2 would fall. One mark for mentioning less resistance.
(3 marks)
e) The resistance of the variable resistor is reduced until it is extremely small.
Explain how this affects the reading on voltmeter V2.
The reading would go down because it has too much current in it.
Incorrect, the voltmeter reading rises so no mark. The low resistance leads to low
PD across the variable resistor so higher PD across the lamp ( total still = 12v)
(2 marks)
f) If the 12v battery were replaced by a 9v battery, what would be the potential 2
difference between points A and B?
7
9v
(1 mark)
GCSE Physics Exam Doctor
Electricity – Potential Difference
Question 2
The diagram shows an electric blanket connected to the mains electricity. The
switch S allows the blanket to be switched to high or low settings. When the
switch is placed in position A, the current in the circuit is 0.3A
a) i) The resistance of the heating element is 600W. Calculate the voltage across
the heating element in the blanket.
(3 marks)
ii) Calculate the power developed in the blanket when the switch is in position A.
(2 marks)
b) When the switch is moved to position B, will the temperature of the blanket
increase or decrease? Explain your reasoning.
(2 marks)
The diagram shows an electric blanket connected to the mains electricity. The
switch S allows the blanket to be switched to high or low settings. When the
switch is placed in position A, the current in the circuit is 0.3A
a) i) The resistance of the heating element is 600W. Calculate the voltage across
the heating element in the blanket.
V = I x R = 0.3 x 600 = 180V
(3 marks)
ii) Calculate the power developed in the blanket when the switch is in position A.
Power = V x I = 240 x 0.3 = 72W
(2 marks)
b) When the switch is moved to position B, will the temperature of the blanket
increase or decrease? Explain your reasoning.
The blanket will get hotter. The electricity is going
straight to the blanket instead of through the 200W.
(2 marks)
Mark scheme
The diagram shows an electric blanket connected to the mains electricity. The
switch S allows the blanket to be switched to high or low settings. When the
switch is placed in position A, the current in the circuit is 0.3A
a) i) The resistance of the heating element is 600W. Calculate the voltage across
the heating element in the blanket.
V=I xR
V = 0.3 x 600
= 180V
(3 marks)
Mark scheme
ii) Calculate the power developed in the blanket when the switch is in position A.
Power = V x I
= 180V x 0.3A
= 54W
(2 marks)
b) When the switch is moved to position B, will the temperature of the blanket
increase or decrease? Explain your reasoning.
Understanding that the whole 240V would now be across the blanket, giving
greater power, or that the current through the blanket would be higher.
(2 marks)
The diagram shows an electric blanket connected to the mains electricity. The
switch S allows the blanket to be switched to high or low settings. When the
switch is placed in position A, the current in the circuit is 0.3A
a) i) The resistance of the heating element is 600W. Calculate the voltage across
the heating element in the blanket.
V = I x R = 0.3 x 600 = 180V
It would have been equally correct to use the fact that
voltage is in the ratio of the resistances i.e.
(600/800) x 240V = 180V
(3 marks)
3
3
Scores one for Power = V x I
ii) Calculate the power developed in the blanket when the switch is in position A.
Power = V x I = 240 x 0.3 = 72W
240V is not the voltage across the blanket. The figure needed is that from
(2 marks)
part i) 180V. Power = V x I = 180V x 0.3A = 54W
P = I2 R is given at the front of the paper and would yield the correct result
0.3A x 0.3A x 600W = 54W
b) When the switch is moved to position B, will the temperature of the blanket
increase or decrease? Explain your reasoning.
2
The blanket will get hotter. The electricity is going
straight to the blanket instead of through the 20O Ohms.
4
(2 marks)
“Electricity” is too imprecise. Needs to show understanding that the whole 240V
would now be across the blanket, giving greater power, or that the current through
the blanket would be higher, or that energy is not being wasted in the resistor.
GCSE Physics Exam Doctor
Electricity – Potential Difference
Question 3
The circuit shows 2 bulbs connected in series across a 12V supply.
a) i) Lamp A is marked 3V, 0.25A. What does this tell you?
(2 marks)
ii) Lamp B is marked 9V, 0.25A. When the lamps are connected as
shown, what will voltmeter V1 read?
(1 mark)
iii) What will voltmeter V2 read?
(1 mark)
b) i) Calculate the resistance of lamp A.
W
(3 marks)
ii) What is the total resistance in the circuit?
W
(2 marks)
The circuit shows 2 bulbs connected in series across a 12V supply.
a) i) Lamp A is marked 3V, 0.25A. What does this tell you?
It tells me that the lamp uses 3V of electricity and needs a
current of 0.25A
(2 marks)
ii) Lamp B is marked 9V, 0.25A. When the lamps are connected as
shown, what will voltmeter V1 read?
V1 will read 6V.
(1 mark)
iii) What will voltmeter V2 read?
The voltage V2 will also be 6V, since the voltage is split
(1 mark)
between the lamps.
b) i) Calculate the resistance of lamp A.
V = IR
3 = 0.25 R
R = 12 W
(3 marks)
ii) What is the total resistance in the circuit?
9 = 0.25 R
R = 36 W total
= 48 W
(2 marks)
Mark scheme
The circuit shows 2 bulbs connected in series across a 12V supply.
a) i) Lamp A is marked 3V, 0.25A. What does this tell you?
The P.D. across a lamp in series with another lamp depends on the resistance
of
the 2 lamps. The voltage across each will be in the ratio of the resistances. The
(2 marks)
marking on the lamp tells you what voltage it was designed to be
operated on, and if it is operated at that voltage, the current through it will be 0.25A.
Mark scheme
ii) Lamp B is marked 9V, 0.25A. When the lamps are connected as
shown, what will voltmeter V1 read?
The voltage V1 will be 3V .
(1 mark)
iii) What will voltmeter V2 read?
V2 actually reads 9V (12V minus 3V)
(1 mark)
b) i) Calculate the resistance of lamp A.
V = I R R = V/I
= 3/0.25
=12 Ohms
(3 marks)
ii) What is the total resistance in the circuit?
For lamp B R = V/I
= 9/0.25
=36 Ohms
12+36= 48 Ohms
(2 marks)
The circuit shows 2 bulbs connected in series across a 12V supply.
a) i) Lamp A is marked 3V, 0.25A. What does this tell you?
It tells me that the lamp uses 3V of electricity and needs a
current of 0.25A
0
2
This is a common misunderstanding. The P.D. across a lamp in series (2 marks)
with
another lamp depends on the resistance of the 2 lamps. The voltage across each will
be in the ratio of the resistances. The marking on the lamp tells you what voltage it
was designed to be operated on, and if it is operated at that voltage, the current
through it will be 0.25A.
ii) Lamp B is marked 9V, 0.25A. When the lamps are connected as
shown, what will voltmeter V1 read?
V1 will read 6V.
(1 mark)
Another common mistake, to assume that the lamps are identical. These are not.
The voltage V1 will be 3V .
iii) What will voltmeter V2 read?
The voltage V2 will also be 6V, since the voltage is split between
(1 mark)
the lamps. V2 actually reads 9V (12V minus 3V)
This mark could have been awarded as e.c.f if the pupil had said “ V2 will be
12V minus 6V, but “the voltage being split” shows misunderstanding.
b) i) Calculate the resistance of lamp A.
V = IR
3 = 0.25 R
5
7
R = 12 W
(3 marks)
ii) What is the total resistance in the circuit?
9 = 0.25 R
R = 36 W total
= 48 W
(2 marks)
GCSE Physics Exam Doctor
Electricity – Potential Difference
Question 4
The diagram shows two lamps connected in series. Lamp A is marked 2.5V 0.3A.
Lamp B is a normal 60W mains lamp.
a) i) Calculate the resistance of lamp A.
(3 marks)
ii) Mains voltage is 240V. What current would the mains lamp take if it were
operated normally in a mains circuit?
(2 marks)
iii) Calculate the resistance of lamp B.
(2 marks)
b) i) If lamp A were connected in a mains circuit on its own, what would you
expect to happen?
(1 mark)
ii) When lamp A is connected with the mains lamp as shown, what is the
Potential Difference across lamp A?
(2 marks)
The diagram shows two lamps connected in series. Lamp A is marked 2.5V 0.3A.
Lamp B is a normal 60W mains lamp.
a) i) Calculate the resistance of lamp A.
V = I R
2.5 = 0.3 R
R = 2.5/0.3 = 8.33 W
(3 marks)
ii) Mains voltage is 240V. What current would the mains lamp take if it were
operated normally in a mains circuit?
I = 240/60
= 4A
(2 marks)
iii) Calculate the resistance of lamp B.
V = I R
240/ 4
= 60 W
(2 marks)
b) i) If lamp A were connected in a mains circuit on its own, what would you
expect to happen?
I would expect it to explode.
(1 mark)
ii) When lamp A is connected with the mains lamp as shown, what is the
Potential Difference across lamp A?
240V same as across lamp B
(2 marks)
Mark scheme
The diagram shows two lamps connected in series. Lamp A is marked 2.5V 0.3A.
Lamp B is a normal 60W mains lamp.
a) i) Calculate the resistance of lamp A.
V = I R
2.5 = 0.3 R
R = 2.5/0.3 = 8.33 W
(3 marks)
Mark scheme
ii) Mains voltage is 240V. What current would the mains lamp take if it were
operated normally in a mains circuit?
I = 60W/240V = 0.25A
(2 marks)
iii) Calculate the resistance of lamp B.
R = 240V / 0.25A = 960Ω
(2 marks)
b) i) If lamp A were connected in a mains circuit on its own, what would you
expect to happen?
Lamp fails
(1 mark)
ii) When lamp A is connected with the mains lamp as shown, what is the
Potential Difference across lamp A?
The P.Ds are in the ratio of the resistances, so P.D. across lamp A
is (8.33/968.33) x 240V = 2.06V
(2 marks)
The diagram shows two lamps connected in series. Lamp A is marked 2.5V 0.3A.
Lamp B is a normal 60W mains lamp.
a) i) Calculate the resistance of lamp A.
V = I R
2.5 = 0.3 R
R = 2.5/0.3 = 8.33 W
(3 marks)
3
3
ii) Mains voltage is 240V. What current would the mains lamp take if it were
operated normally in a mains circuit?
I = 240/60
= 4A
It is quite common for pupils who are otherwise quite competent at
calculation to get this one upside-down. I = 60W/240V = 0.25A
(2 marks)
iii) Calculate the resistance of lamp B.
V = I R
240/ 4
= 60 W
e.c.f The answer is incorrect, but the calculation is correctly
done. The error has been penalised in part ii).
(2 marks)
R = 240V / 0.25A = 960Ω
b) i) If lamp A were connected in a mains circuit on its own, what would you
expect to happen?
I would expect it to explode.
Although this is a little extreme, it is sufficient for 1 mark.
(1 mark)
ii) When lamp A is connected with the mains lamp as shown, what is the
Potential Difference across lamp A?
240V same as across lamp B
The P.Ds are in the ratio of the resistances, so P.D. across lamp A is
(2 marks)
(8.33/968.33) x 240V = 2.06V
GCSE Physics Exam Doctor
Electricity – Potential Difference
Question 5
The diagram shows incorrect wiring for a car’s headlamp and fog lamp.
a) i) Which lamp/s will be lit if switch H is closed, but switch F open?
(2 marks)
ii) If switch F is closed, while switch H is open, which lamp/s will be lit?
(2 marks)
iii) The lamps are designed to light normally on a 12V supply. In the
conditions of a) ii) the lamps will not be lit normally. Explain why not.
(2 marks)
b) Redraw the diagram so that the lamps are correctly lit and can be turned
on/off independently.
(3 marks)
The diagram shows incorrect wiring for a car’s headlamp and fog lamp.
a) i) Which lamp/s will be lit if switch H is closed, but switch F open?
The headlamp will be on, but the fog lamp off
(2 marks)
ii) If switch F is closed, while switch H is open, which lamp/s will be lit?
Fog lamp will be on, but headlamp off.
(2 marks)
iii) The lamps are designed to light normally on a 12V supply. In the
conditions of a) ii) the lamps will not be lit normally. Explain why not.
Because the headlamp is off
(2 marks)
b) Redraw the diagram so that the lamps are correctly lit and can be turned
on/off independently.
12 V supply
Headlamp
Fog lamp
(3 marks)
Mark scheme
The diagram shows incorrect wiring for a car’s headlamp and fog lamp.
a) i) Which lamp/s will be lit if switch H is closed, but switch F open?
The headlamp will be on, but the fog lamp off
(2 marks)
ii) If switch F is closed, while switch H is open, which lamp/s will be lit?
With F closed, but H open there is a complete circuit for the two lamps in
series. They will both be on.
(2 marks)
Mark scheme
iii) The lamps are designed to light normally on a 12V supply. In the
conditions of a) ii) the lamps will not be lit normally. Explain why not.
The lamps are in series across a 12V supply, so the lamps share the 12V and
(2 marks)
will therefore be dim.
b) Redraw the diagram so that the lamps are correctly lit and can be turned
on/off independently.
(3 marks)
The diagram shows incorrect wiring for a car’s headlamp and fog lamp.
a) i) Which lamp/s will be lit if switch H is closed, but switch F open?
The headlamp will be on, but the fog lamp off
(2 marks)
ii) If switch F is closed, while switch H is open, which lamp/s will be lit?
Fog lamp will be on, but headlamp off.
With F closed, but H open there is a complete circuit for the two lamps in
(2 marks)
series. They will both be on
3
4
iii) The lamps are designed to light normally on a 12V supply. In the
conditions of a) ii) the lamps will not be lit normally. Explain why not.
Because the headlamp is off
The lamps are in series across a 12V supply, so the lamps share the 12V and
(2 marks)
will therefore be dim.
b) Redraw the diagram so that the lamps are correctly lit and can be turned
on/off independently.
12 V supply
Headlamp
Fog lamp
The lamps are correctly in parallel, but with only one switch, they
cannot be switched on/off independently.
1
5
(3 marks)
GCSE Physics Exam Doctor
Electricity – Potential Difference
Question 6
The circuit shows voltmeters used to measure the potential differences across
lamps in a circuit.
i) V1 reads 12V. How much energy is changed from electrical energy to heat and
light energy for each coulomb of charge which passes through the lamp?
(2 marks)
ii) The p.d. across the cell (V) is 18V. What will be the reading on voltmeter V2 ?
(1 mark)
iii) How much energy is converted into electrical energy in the cell for each
coulomb of charge which is drawn from the cell?
(2 marks)
iv) Are the lamps identical? Explain your answer.
(3 marks)
The circuit shows voltmeters used to measure the potential differences across
lamps in a circuit.
i) V1 reads 12V. How much energy is changed from electrical energy to heat and
light energy for each coulomb of charge which passes through the lamp?
12V
(2 marks)
ii) The p.d. across the cell (V) is 18V. What will be the reading on voltmeter V2 ?
6V
(1 mark)
iii) How much energy is converted into electrical energy in the cell for each
coulomb of charge which is drawn from the cell?
18V
(2 marks)
iv) Are the lamps identical? Explain your answer.
Yes the symbol shows they are the same.. But the voltage is
used before the second one can get it.
(3 marks)
Mark scheme
The circuit shows voltmeters used to measure the potential differences across
lamps in a circuit.
i) V1 reads 12V. How much energy is changed from electrical energy to heat and
light energy for each coulomb of charge which passes through the lamp?
12 Joules.
(2 marks)
ii) The p.d. across the cell (V) is 18V. What will be the reading on voltmeter V2 ?
6V
(1 mark)
Mark scheme
iii) How much energy is converted into electrical energy in the cell for each
coulomb of charge which is drawn from the cell?
18 Joules.
(2 marks)
iv) Are the lamps identical? Explain your answer.
No. The lamps have different resistances and the p.d.s across the lamps are in
the ratios of the resistances.
(3 marks)
The circuit shows voltmeters used to measure the potential differences across
lamps in a circuit.
2
3
i) V1 reads 12V. How much energy is changed from electrical energy to heat and
light energy for each coulomb of charge which passes through the lamp?
12V
The unit for energy is Joules. A Volt is the energy transferred (2 marks)
in Joules for each coulomb of charge which passes.
ii) The p.d. across the cell (V) is 18V. What will be the reading on voltmeter V2 ?
6V
(1 mark)
iii) How much energy is converted into electrical energy in the cell for each
coulomb of charge which is drawn from the cell?
18V
The same mistake with units as before, but since it has already been
penalised, both marks awarded.
(2 marks)
iv) Are the lamps identical? Explain your answer.
Yes the symbol shows they are the same.. But the voltage is
used before the second one can get it.
(3 marks)
Shows no understanding of the concept of potential difference in a series circuit.
The lamps have different resistances and the p.d.s across the lamps are in the
ratios of the resistances.
2
5
GCSE Physics Exam Doctor
Electricity – Potential Difference
End of questions