IGCSE Physics
Topic 4 Exam Questions
Resistance
Name: _______
44 marks
Q2.A light meter is used to check the light levels during a cricket match.
Figure 1 shows a cricket umpire using a light meter.
Figure 1
(a)
Some light meters use a light-dependent resistor (LDR).
Figure 2 shows a circuit in a light meter.
Figure 2
(i)
What two measurements are needed to determine the resistance of the LDR?
Measurement 1 ............................................................................
Measurement 2 ............................................................................
(ii)
What will happen to the current in the circuit if the resistance of the LDR increases?
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(b)
(i)
(2)
The table below shows the resistance of the LDR at different light levels.
Light level in lux
0.1
Resistance in ohms
250 000
1
50 000
10
10 000
100
2 000
1000
400
What conclusion can you make from this data?
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(1)
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(ii)
(2)
A game of cricket may be stopped if the light level falls below 100 lux.
What is the potential difference across the LDR when the light level is 100 lux?
Use information from Figure 2 and the table above.
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Potential difference = ........................................ V
Q3.
(1)
(Total 6 marks)
The diagram shows a simple circuit.
(a)
The circuit includes an LDR.
What do the letters LDR stand for?
Draw a ring around your answer.
Light-dependable resistor
light-dependent resistor
light-direct resistor
(1)
(b)
The graph shows how the resistance of an LDR changes with light intensity.
Describe in detail how the resistance of the LDR changes as the light intensity increases
from 0 to 50 lux.
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(c)
(i)
(3)
Complete the following sentence by drawing a ring around the correct line in the box.
decrease
A decrease in the light intensity of light on the LDR will
reading on the ammeter.
not change
the
increase
(1)
ii)
Give a reason for your answer to part (c)(i).
...............................................................................................................
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d)
(1)
An LDR can be used to switch a circuit on and off automatically.
In which one of the following would an LDR be used?
Put a tick () in the box next to your answer.
a circuit to switch on central heating when it gets cold
a circuit to switch on security lighting when it gets dark
a circuit to switch on a water sprinkler when the soil in a greenhouse is dry
(1)
(Total 7 marks)
Q4.A bicycle light uses three light-emitting diodes (LEDs).
By blackplastic [CC BY-SA 2.0], via Flickr
(a)
The bicycle light has three LEDs connected in parallel.
Two 1.5 volt cells are used in series as an electrical power supply.
One switch is used to turn on all three LEDs at the same time.
Complete the circuit diagram to show how all the components are connected.
Use the correct circuit symbols.
(3)
(b)
The diagrams show the electrical characteristics for a filament bulb and an LED.
Filament bulb
(i)
LED
Use the graphs to compare the electrical characteristics of a filament bulb and an
LED.
For each difference you should refer to the characteristics of both the filament bulb
and the LED.
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(ii)
Many car makers are using a set of LEDs in car lights instead of using a filament
bulb.
The table shows information on the characteristics of a set of LEDs and a filament
bulb.
Set of LEDs
Filament bulb
£10
£0.50
50 000 hours
1 000 hours
Power consumption
5W
21W
Power output (light)
3W
3W
Cost
Life span
Car makers are increasing the use of sets of LEDs for cars instead of using filament
bulbs.
(3)
Use the information in the table to give advantages and disadvantages of the
change to using LEDs.
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(4)
(Total 10 marks)
Q1.(a) Figure 1 shows the apparatus used to obtain the data needed to calculate the resistance of
a thermistor at different temperatures.
Figure 1
Power supply
(i)
In the box below, draw the circuit symbol for a thermistor.
(1)
(ii)
Use the data given in Figure 1 to calculate the resistance of the thermistor at 20 °C.
...............................................................................................................
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Resistance = ......................... ohms
(iii)
(2)
Figure 2 shows the axes for a sketch graph.
Complete Figure 2 to show how the resistance of the thermistor will change as the
temperature of the thermistor increases from 20 °C to 100 °C.
Figure 2
Temperature in °C
(iv)
Which one of the following is most likely to include a thermistor?
(1)
Tick (✓) one box.
An automatic circuit to switch a plant watering system on and off.
An automatic circuit to switch an outside light on when it gets dark.
An automatic circuit to switch a heating system on and off.
(1)
(b)
The ammeter used in the circuit has a very low resistance.
Why is it important that ammeters have a very low resistance?
........................................................................................................................
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(c)
(1)
The table below gives the temperature of boiling water using three different temperature
scales.
Temperature
Scale
100
Celsius (°C)
212
Fahrenheit (°F)
80
Réaumur (°Re)
Scientists in different countries use the same temperature scale to measure temperature.
Suggest one advantage of doing this.
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(d)
(1)
A student plans to investigate how the resistance of a light-dependent resistor (LDR)
changes with light intensity.
The student starts with the apparatus shown in Figure 2 but makes three changes to the
apparatus.
One of the changes the student makes is to replace the thermistor with an LDR.
Describe what other changes the student should make to the apparatus.
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(2)
(Total 9 marks)
Q2.(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 ............................................................................................................
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2 ............................................................................................................
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(b)
After correcting the circuit, the student obtained a set of data and plotted the graph below.
Potential difference in volts
(i)
At what potential difference did the diode start to conduct an electric current?
(2)
...................................................................... V
(ii)
(1)
Use data from the graph to calculate the resistance of the diode when the potential
difference across the diode is 0.3 V.
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Resistance = ......................... ohms
(c)
(3)
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
(ii)
(2)
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?
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(2)
(Total 12 marks)
M2.(a)
(i)
current (through the LDR)
in either order
accept amount of amps
1
potential difference / voltage across the LDR
accept amount of volts across the LDR
1
(ii)
decrease
1
(b)
(i)
as the light level increases by a factor of 10 the resistance reduces by a factor
of 5
allow for 1 mark as the light level increases the resistance
decreases
2
(ii)
4.5
1
[6]
M3.
(a)
a light-dependent resistor
1
(b)
any three from:
•
resistance starts at 500 (kilohms)
•
(resistance) falls rapidly as intensity increases from 0
accept resistance falls
accept brightness for intensity
•
(resistance) halves between 10 and 20 lux
•
(resistance) falls slightly between 20 and 50 lux
or
(resistance) almost constant / levels out between 20 and 50 lux
•
•
at 50 lux, resistance = 10 (kilohms)
for full credit the word resistance must be used correctly at least
once
an answer resistance falls as intensity increases gains 2 marks this may be combined with one of the bullet point marks for full
credit
3
(c)
(i)
decrease
1
(ii)
resistance increases
this can score without (c)(i)
1
(d)
A circuit to switch on security lighting when it gets dark.
1
[7]
M4.(a)
3 correct LED symbols in correct direction
max 2 if clear gap / break in circuit
1
parallel circuit with 3 branches
do not award this mark if there is a short circuit
1
switch that can turn all components on / off
1
(b)
(i)
any three from:
allow voltage for p.d. throughout
•
the current can flow both ways through a bulb and only 1 way in an LED
for each mark a difference should include a reference to both LED
and bulb
•
current changes with (all) p.d. for the bulb but current changes for only
some p.d. for LED
•
current changes at varying rate for bulb current changes at constant rate
for LED
•
the diode has a very high resistance in one direction and the resistance
in the bulb varies with current (in both directions)
•
the resistance of a bulb increases with current / p.d. but the LED
resistance decreases after a certain pd.
•
the bulb obeys Ohms’ law for a small range of p.d. the LED does not
3
(ii)
any four from:
Advantages
•
lasts longer
max 3 for advantages
•
lower power consumption
allow lower current
•
same power / light output
if no reference to power consumption / output allow 1mark for
more energy efficient
•
less heat transfer to surroundings
allow less energy is wasted
allow don’t get as hot
•
less waste (from broken bulbs)
•
less cost per hour
Disadvantages
•
higher (initial) cost
•
lower sales / profit from replacement bulbs
•
no increase in power / light output
4
[10]
M1.(a)
(i)
1
(ii)
360
allow 1 mark for correct substitution, ie 9 = 0.025 × R
2
(iii)
sketch graph of correct shape, ie
1
(iv)
An automatic circuit to switch a heating system on and off.
1
(b)
so ammeter reduces / affects current as little as possible
accept so does not reduce / change the current (it is measuring)
accurate reading is insufficient
not change the resistance is insufficient
1
(c)
gives a common understanding
accept is easier to share results
accept can compare results
do not need to be converted is insufficient
prevent errors is insufficient
1
(d)
replace Bunsen (and water) with a lamp
accept any way of changing light level
1
replace thermometer with light sensor
accept any way of measuring a change in light level
datalogger alone is insufficient
1
[9]
M2.(a)
(i)
symbol for a diode
accept
1
symbol for a variable resistor
1
(ii)
voltmeter is in series or voltmeter is not in parallel
1
ammeter is in parallel or ammeter is not in series
accept an answer in terms of how the circuit should be corrected
voltmeter 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)
37.5
allow 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
(c)
(i)
25
allow 1 mark for obtaining period = 0.04(s)
2
(ii)
diode has large resistance in reverse / one direction
1
so stops current flow in that / one direction
allow diodes only let current flow one way / direction
allow 1 mark for the diode has half-rectified the (a.c. power) supply
1
[12]