Write your answer first. Then open the answer and mark it against the mark points.
A Circuits
A1 Model railway lamps
A model railway has two platform lamps, L1 (30 Ω) and L2 (60 Ω), in parallel.
A 25 Ω resistor is in series with the two lamps.
The supply is 9.0 V.
(a) Calculate the total resistance of the circuit. 3 marks
(b) Calculate the current supplied by the battery. 3 marks
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(a)
Rp = (30 × 60) / (30 + 60) = 20 Ω
Rt = 20 + 25
Rt = 45 Ω
1 mark for the parallel calculation (substitution or Rp = 20 Ω).
1 mark for adding the 25 Ω resistor.
1 mark for the answer with unit.
(b)
V = I R
9.0 = I × 45
I = 9.0 / 45
I = 0.20 A
1 mark for substitution.
1 mark for transposition.
1 mark for the answer with unit.
A2 Bike light LED
A bike light has an LED in series with a resistor.
The supply is 9.0 V.
The LED needs 2.0 V across it and a current of 20 mA.
(a) Calculate the resistance of the series resistor. 3 marks
(b) Explain why the LED needs a resistor in series with it. 2 marks
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(a)
VR = 9.0 − 2.0 = 7.0 V
V = I R
7.0 = 0.020 × R
R = 350 Ω
1 mark for the voltage across the resistor.
1 mark for substitution (current in amps).
1 mark for the answer with unit.
(b)
The resistor limits the current through the LED …
… so the LED is not damaged.
1 mark for cause (limits the current).
1 mark for effect (stops the LED being damaged).
B Voltage dividers and sensors
B1 Frost alarm
A frost alarm warns a gardener when the temperature falls to 0 °C.
The sensing circuit is a resistor R above an NTC thermistor. Vin is measured across the thermistor. The supply is 9.0 V.
The table shows the thermistor's resistance.
−10 °C: 26 kΩ
−5 °C: 20 kΩ
0 °C: 15 kΩ
5 °C: 11 kΩ
10 °C: 8.5 kΩ
(a) State the resistance of the thermistor at 0 °C. 1 mark
(b) The alarm is designed to switch on when Vin reaches 5.0 V. Calculate the resistance of R. Use your answer to part (a). 4 marks
Show the answer
(a)
15 kΩ (15 000 Ω)
1 mark for the value with unit.
(b)
VR = 9.0 − 5.0 = 4.0 V
V1 / V2 = R1 / R2
4.0 / 5.0 = R / 15 000
R = (4.0 × 15 000) / 5.0
R = 12 000 Ω (12 kΩ)
1 mark for the voltage across R.
1 mark for substitution.
1 mark for transposition.
1 mark for the answer with unit.
B2 Helmet lights
A cycle helmet has a front LED and a rear LED that switch on when it gets dark.
The sensing circuit is a resistor above an LDR. Vin is measured across the LDR and goes to a transistor switch.
Describe the operation of this circuit as it gets darker. Include the resistance of the LDR, the voltage Vin, the transistor and both outputs. 5 marks
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Answer
The resistance of the LDR increases.
Vin increases.
The transistor switches on.
The front LED lights.
The rear LED lights.
1 mark for each link, in order. Apply follow-through.
A link that goes the wrong way loses that mark only.
C Transistors, relays and diodes
C1 Pond pump
A pond pump switches on when the water gets warm.
A sensing circuit drives a transistor. The transistor switches a relay. The relay switches the pump, which has its own supply.
(a) State the function of the transistor. 1 mark
(b) Describe why a relay is needed to switch the pump. 2 marks
(c) State the function of the base resistor. 1 mark
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(a)
It acts as an (electronic) switch.
1 mark.
(b)
The transistor can only switch a small current …
… the relay lets this small current switch the large current the pump needs (from its own supply).
1 mark for the small current in the transistor / sensing circuit.
1 mark for switching the large current / separate supply of the pump.
(c)
It limits the current into the base of the transistor.
1 mark. Accept: protects the transistor from a large base current.
C2 Protection diode
The circuit shows a transistor switching a relay coil.
A protection diode is connected across the relay coil.
(a) Describe how the diode must be connected. 2 marks
(b) Explain the purpose of the diode. 2 marks
(c) Name the three connections of the transistor. 1 mark
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(a)
Across (in parallel with) the relay coil …
… reverse biased: the bar (cathode) points to the positive supply.
1 mark for position.
1 mark for orientation.
(b)
When the relay switches off, the coil produces a large reverse (back) voltage …
… the diode carries this away, so the transistor is not damaged.
1 mark for cause (back voltage from the coil).
1 mark for effect (protects the transistor).
(c)
Base, collector and emitter.
1 mark for all three.
D Systems
D1 Hand dryer sub-systems
An automatic hand dryer has four sub-systems: a light sensor, a control circuit, a fan motor and a heater.
(a) State which sub-system is the process. 1 mark
(b) Describe how the system boundary is shown on a sub-system diagram. 1 mark
(c) Describe the purpose of the system boundary. 1 mark
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(a)
The control circuit.
1 mark.
(b)
A dashed line drawn round all the sub-systems.
1 mark.
(c)
It separates what is inside the system from the inputs and outputs outside it.
1 mark for containing / separating the system from what is outside.
D2 Hand dryer input
In the hand dryer, a hand blocks light from reaching an LDR.
The LDR is the bottom resistor of a voltage divider. Vin is measured across the LDR.
Describe the operation of the input sub-system as a hand blocks the light. Make reference to the resistance of the LDR and the voltage Vin. 2 marks
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Answer
The resistance of the LDR increases …
… so Vin increases.
1 mark for LDR resistance increasing.
1 mark for Vin increasing.
E Energy
E1 Patio heater
A patio heater runs from a 230 V supply. The current is 8.0 A.
(a) Calculate the power of the heater. 2 marks
(b) In one evening the heater gives out 900 000 J of useful heat. It is 0.75 (75 %) efficient. Calculate the input energy. 3 marks
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(a)
P = V I
P = 230 × 8.0
P = 1840 W
1 mark for substitution.
1 mark for the answer with unit.
(b)
η = Eout / Ein
0.75 = 900 000 / Ein
Ein = 900 000 / 0.75
Ein = 1 200 000 J
1 mark for substitution.
1 mark for transposition.
1 mark for the answer with unit.
E2 Sandwich press
A sandwich press heats two aluminium plates.
mass of the plates: 0.60 kg
specific heat capacity of aluminium: 900 J kg−1 °C−1
heat energy given to the plates: 27 000 J
(a) Calculate the rise in temperature of the plates. 3 marks
(b) Explain how the efficiency of the sandwich press could be improved. 2 marks
Show the answer
(a)
Eh = c m ΔT
27 000 = 900 × 0.60 × ΔT
ΔT = 27 000 / (900 × 0.60)
ΔT = 50 °C
1 mark for substitution.
1 mark for transposition.
1 mark for the answer with unit.
(b)
Insulate the outside of the press / the lid …
… so less heat energy is lost to the surroundings.
1 mark for cause, linked to the sandwich press.
1 mark for effect.
F Contexts
F1 Electric wheelchair
A team is designing a new electric wheelchair.
Describe two tasks the electronic engineer would carry out in this project. 2 marks
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Answer
Design the circuit that controls the motors.
Choose the sensors, such as an obstacle sensor.
Write the program for the microcontroller.
Test the circuit by simulation, then on a prototype.
1 mark for each task, up to 2. It must be an electronics task.
F2 Electric car
A family replaces its petrol car with an electric car.
(a) Describe one positive economic impact of this change for the family. 1 mark
(b) Describe one negative economic impact of this change for the family. 1 mark
Show the answer
(a)
Charging costs less than petrol, so running costs fall.
1 mark.
(b)
The electric car (and a home charger) costs more to buy.
1 mark.
Quiz
Multiple-choice quiz
Pick an answer to see feedback on it. You can then try the others to see why they are wrong. Your score counts first tries only.
A Circuits
1. An ammeter measures the current through one lamp in a parallel circuit. How is it connected?
2. A 40 Ω resistor and a 10 Ω resistor are connected in parallel. What is the total resistance?
3. A 9.0 V supply is connected across a 45 Ω resistor. What is the current?
4. A shop sign has three lamps in parallel. One lamp is removed. What happens?
B Voltage dividers and sensors
5. An LDR is the bottom resistor of a voltage divider. Vin is measured across the LDR. It gets darker. What happens?
6. A voltage divider has R1 = 30 kΩ (top) and R2 = 20 kΩ (bottom) across a 5.0 V supply. What is the voltage across R2?
7. In a frost alarm, a variable resistor VR is above a thermistor. Vin is measured across the thermistor. The alarm must switch on at a higher temperature. What change should be made?
8. Two resistors are in series across a supply. The output is taken from the point between them. What is this arrangement called?
C Transistors, relays and diodes
9. What is the function of the transistor in a sensing circuit?
10. What are the three connections of a transistor?
11. Why is a diode connected across a relay coil?
12. How must the protection diode be connected?
D Systems
13. What is the purpose of the system boundary on a sub-system diagram?
14. In an automatic hand dryer, which part is the process sub-system?
15. What do the arrows between blocks on a sub-system diagram show?
16. In a frost alarm, which is the input sub-system?
E Energy
17. A hairdryer runs from a 230 V supply with a current of 5.0 A. What is its power?
18. A motor is supplied with 40 000 J. It is 0.85 (85 %) efficient. What is the useful output energy?
19. A camping kettle gives 50 160 J of heat to 0.40 kg of water (c = 4180 J kg−1 °C−1). What is the rise in temperature?
20. Which change would improve the efficiency of an electric heater in a garden office?
F Contexts
21. Why do engineers test a circuit using computer simulation before building it?
22. Which of these is an emerging technology?
23. A school fits solar panels on its roof. Which is a negative economic impact for the school?
24. A team designs a fitness tracker. Which task would the electronic engineer carry out?
Past papers
Past-paper questions by area
Each link opens the question paper. The marking instructions are beside it.
