Electric Circuits & Electrical Safety (Cambridge International IGCSE Physics): Exam Questions

A student makes a circuit to switch on a 6.0 V lamp from two different switches X and Y.

Fig. 9.1 shows the circuit.

Switch X is in position P. State the position of switch Y for the lamp to be lit.

Extract:

A student makes a circuit to switch on a 6.0 V lamp from two different switches X and Y.

Fig. 9.1 shows the circuit.

Switch X is in position P.

The current in the lamp is 0.50 A when the potential difference (p.d.) across the lamp is 6.0 V.

Calculate the resistance of the lamp. Include the unit.

resistance = .........................................................

Extract:

A student makes a circuit to switch on a 6.0 V lamp from two different switches X and Y.

Fig. 9.1 shows the circuit.

Switch X is in position P.

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The student connects another 6.0 V lamp in parallel with the first lamp, as shown in Fig. 9.2.

Give two advantages of connecting the lamps in parallel.

A student investigates the electrical resistance of some components.

Fig. 10.1 shows an incomplete diagram of the circuit used by the student.

(i) State the term used for component Y.

(ii) The student uses the circuit to measure the resistance of component Y.

Complete the diagram in Fig. 10.1 by adding electrical symbols to show an ammeter and a voltmeter correctly connected to determine the resistance of component Y.

Fig. 10.2 shows two resistors A and B.

(i) Resistor A and resistor B are connected in series.

State the value of their combined resistance.

............................................ Ω [1]

(ii) Resistor A and resistor B are connected in parallel.

Compare the combined resistance when in parallel with the resistance of resistor A alone.

Fig. 10.1 shows a circuit for determining the resistance of a component.

On Fig. 10.1, label the fixed resistor, by writing the letter R.

Two components in Fig. 10.1 measure electrical quantities.

Identify the quantity that each component measures.

Write each quantity and the unit of each quantity in the correct place in Table 10.1.

Extract:

Fig. 10.1 shows a circuit for determining the resistance of a component.

A student uses the circuit in Fig. 10.1 to determine the resistance of wires made from the same material.

State how the resistance of a wire is related to its length and its diameter.

Fig. 9.1 and Fig. 9.2 each show an electrical circuit. Each circuit has two lamps connected to an electrical supply.

State the term used to describe each electrical circuit. Write the term under each circuit.

Extract:

Fig 9.2 is a series circuit.

State two disadvantages of the circuit in Fig. 9.2.

Extract:

Fig. 9.1 shows an electrical circuit. The circuit has two lamps connected to an electrical supply. Fig 9.1 is a parallel circuit.

Redraw the circuit in Fig. 9.1 with switches that will turn lamps X and Y on and off independently of each other.

Fig. 9.3 shows another circuit.

The lamps can be turned on and off using two different switches S₁ and S₂.

Complete the table stating when the lamps are on or off. The first one has been done for you.

Fig. 10.1 shows the symbol for an electrical component.

State the name of the component shown in Fig. 10.1.

Extract:

Fig. 10.1 shows the symbol for an electrical component.

The resistance of the component shown in Fig. 10.1 varies with temperature. Fig. 10.2 shows a graph of resistance against temperature for the component.

(i) Use the graph to determine the resistance of the component at a temperature of 10 °C.

resistance = ...................................................... Ω [1]

(ii) At another temperature, the resistance of the component is 800 Ω.

Calculate the current in the component when it is connected to a 12.0 V supply.

current = ...................................................... A [3]

Extract:

The component is a thermistor.

The students who built the circuit in part (a) want to change it so that it becomes sensitive to light rather than heat.

State the change the students should make.

Extract:

Remove / replace the thermistor; Add a light dependent resistor / LDR;

Draw the symbol for the replacement component you suggested in part (c).

Fig. 9.1 shows a simple circuit.

(i) The current in the wires of the circuit is a flow of particles. Indicate the name of these particles. Tick one box.

square electrons

square atoms

square protons

(ii) Calculate the combined resistance of the two resistors.

resistance = ...................................................... Ω [1]

(iii) Calculate the potential difference (p.d.) reading that would be shown on the voltmeter.

potential difference (p.d.) = ...................................................... V [3]

Extract:

Fig. 9.1 shows a simple circuit.

The current in the wires of the circuit is a flow of particles.

The circuit is changed.

The two resistors are connected in parallel.

Explain what happens, if anything, to the current reading on the ammeter.

Fig. 9.1 shows a series circuit. Two of the components in the circuit are labelled.

State the name of two other components in the circuit.

1 ................................................................................................................................

2.................................................................................................................................

Fig. 10.1 shows an arrangement for making an electromagnet.

(i) State a material which is suitable for the core of the electromagnet.

(ii) State the name for component X in Fig. 10.1.

(iii) Describe and explain how component X varies the strength of the electromagnet.

Extract:

Fig. 10.1 shows an arrangement for making an electromagnet.

The switch is closed. The reading on the ammeter is 1.5 A.

Calculate the resistance of the circuit.

resistance = .................................................... Ω

A student does an experiment to determine the resistance of a fixed resistor, R.

The student draws an incomplete diagram of the circuit, as shown in Fig. 10.1.

(i) On Fig. 10.1, draw the missing circuit symbols.

(ii) Describe how the student could use the circuit to determine a reliable value for the resistance of R.

Fig. 10.2 shows a 20 Ω resistor connected to a power supply.

A second 20 Ω resistor is connected in series with the first.

Describe how this affects the current in the circuit.

A teacher is investigating the resistance of a lamp.

Fig. 10.1 shows part of the circuit she uses. The circuit is incomplete.

(i) To determine the resistance of the lamp, the teacher adds two meters to her circuit.

On Fig. 10.1, draw circuit symbols to show each meter correctly connected in the circuit.

(ii) When the current in the lamp is 0.25A, the potential difference (p.d.) across the lamp is 4.5 V. Calculate the resistance of the lamp.

resistance = ...................................................... Ω [3]

Extract:

A teacher is investigating the resistance of a lamp.

Fig. 10.1 shows part of the circuit she uses. The circuit is incomplete.

On Fig. 10.1, draw circuit symbols to show each meter correctly connected in the circuit.

When the current in the lamp is 0.25A, the potential difference (p.d.) across the lamp is 4.5 V.

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(i) State the name of component X.

(ii) Describe and explain how the teacher uses component X to investigate the resistance of the lamp.

A teacher uses a power supply in a metal case. The circuit for the power supply includes a fuse.

(i) Draw the electrical symbol for a fuse.

(ii) The metal case of the power supply is earthed. A fault occurs and a live wire touches the metal case.

Explain how earthing the metal case protects the teacher.

Extract:

A teacher uses a power supply in a metal case. The circuit for the power supply includes a fuse.

The metal case of the power supply is earthed. A fault occurs and a live wire touches the metal case.

The power supply circuit includes a transformer. Its input voltage is 240 V. There are 960 turns on the input coil and 64 turns on the output coil.

Calculate the output voltage of the transformer.

output voltage = ..................................................... V

A circuit is made from two lamps, a cell and a switch, as shown in Fig. 10.1.

(i) Draw the circuit symbol for a cell.

(ii) State the term used for the arrangement of lamps in the circuit in Fig. 10.1.

(iii) The switch is closed and the lamps light.

State the name of the charged particles that are flowing through the wires.

Extract:

The lamps are in series (as they are one after another in the same loop in the circuit).

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Fig. 10.2 represents a different type of circuit.

(i) Compare Fig. 10.1 and Fig. 10.2.

State two advantages of the type of circuit shown in Fig. 10.2 with the type of circuit shown in Fig. 10.1.

(ii) The potential difference across the power source in Fig. 10.2 is 3.0 V. The combined resistance of the two lamps is 12 Ω.

Calculate the size of the current in the circuit when the switches are closed.

current = ...................................................... A [3]

Fig. 10.1 shows a simplified circuit diagram for an electric oven. The oven contains a fan driven by a motor.

On Fig. 10.1, circle the symbol representing the heater.

Extract:

Fig. 10.1 shows a simplified circuit diagram for an electric oven. The oven contains a fan driven by a motor.

Fig. 10.1 includes a variable resistor.

Explain the function of the variable resistor in this circuit.

The potential difference across the motor is 250 V. The current in the motor is 2.0 A.

Calculate the resistance of the motor.

resistance = ...................................................... Ω

Extract:

Fig. 10.1 shows a simplified circuit diagram for an electric oven. The oven contains a fan driven by a motor.

[Electrical circuit diagram showing component X, a power supply, a bulb, a switch, a variable resistor, and a motor labelled 'M'.]

State the name of component X and explain how it contributes to the safety of the user.

Fig. 9.1 shows an electric circuit which includes uninsulated resistance wire XY. A teacher shows some students how to complete the circuit by placing the contact C at various positions on the wire XY.

The students move contact C to point P. The resistance of the wire between X and P is 20Ω.

Calculate the total resistance of the resistance wire between X and P and the fixed resistor.

total resistance = ..................................................... Ω

A microwave oven is fitted with a 13 A fuse and an earth wire is connected to the metal case of the microwave oven. A fault occurs and the live wire of the microwave oven touches the metal case.

Explain how the fuse and an earthed metal case protect the appliance and the user

Fig. 5.1 shows a kitchen tap that supplies instant boiling water.

Cold water passes over an electric immersion heater inside the tap.

The metal tap is earthed and there is a fuse in the cable that connects the heater to the mains.

1. Explain how the earth wire protects the user.
2. Explain how the fuse protects the circuit.

The current in the lamp is 0.40 A. The potential difference (p.d.) across the lamp is 6.0 V. Calculate the power dissipated in the lamp.

power = ..................................................... W

Draw on Fig. 9.1 to show a lamp connected in parallel with the lamp in the circuit. Use the correct symbol.

Fig. 9.1

A student investigates the resistance of resistors connected in series.

Fig. 2.1 shows the circuit used.

Fig. 2.1

On Fig. 2.1, draw a voltmeter connected to measure the potential difference across resistor A.

The student closes the switch and records the potential difference $V_{A}$ across resistor A and current $I$ in the circuit. The meter readings are shown in Fig. 2.2.

Fig. 2.2

Record the potential difference $V_{A}$ and current $I$.

Extract:

The student closes the switch and records the potential difference $V_{A}$ across resistor A and current $I$ in the circuit. The meter readings are shown in Fig. 2.2.

Record the potential difference $V_{A}$ and current $I$.

Calculate the resistance $R_{A}$ of resistor A using the equation $R_{A}=\frac{V_{A}}{I}$.

Give your answer to a suitable number of significant figures for this experiment.

Extract:

The student closes the switch and records the potential difference $V_{A}$ across resistor A and current $I$ in the circuit. The meter readings are shown in Fig. 2.2.

The student then connects the voltmeter to measure the potential difference across both resistors A and B. The new potential difference reading is $V_{\mathrm{AB}}=2.6\text{V}$.

Calculate the combined resistance $R_{\mathrm{AB}}$ of the two resistors in series.

Extract:

Calculate the resistance R_A of resistor A:

R_A = V_A / I

R_A = 1.4 / 0.26 = 5.3846...

R_A = 5.4 Ω

Calculate the combined resistance R_AB:

R_AB = V_AB / I

R_AB = 2.6 / 0.26

R_AB = 10 Ω

The resistance of resistor B is known to be $R_{B}=4.5Ω$.

A student suggests that $R_{\mathrm{AB}}$ should be equal to $R_{A}+R_{B}$.

State whether your results support this suggestion. Justify your answer by reference to your values.

(i) The student wants to repeat the experiment with resistor A and resistor B connected in parallel.

Complete the circuit diagram in Fig. 2.3 to show this arrangement.

Fig. 2.3

(ii) The student notices that the resistors become warm during the experiment.

State one change they could make to the procedure to reduce this heating effect.

(iii) State one hazard associated with wires overheating in a circuit.

Extract:

Fig. 7.1 shows a circuit containing a battery of e.m.f. 12 V and three resistors.

A circuit diagram showing a 12-volt battery connected in series with resistor R1 and a parallel combination of resistors R2 and R3. A voltmeter is connected across the parallel combination. R1 has a resistance of 8 ohms, R2 has a resistance of 20 ohms, and R3 has a resistance of 30 ohms.

Fig. 7.1

The voltmeter measures the potential difference (p.d.) across the parallel

combination of resistors.

Show that the combined resistance of R₂ and R₃ is $12Ω$.

Extract:

Fig. 7.1 shows a circuit containing a battery of e.m.f. 12 V and three resistors.

The voltmeter measures the potential difference (p.d.) across the parallel combination of resistors.

R1 has a resistance of 8 ohms, R2 has a resistance of 20 ohms, and R3 has a resistance of 30 ohms.

Calculate the current in resistor R₁.

Extract:

Fig. 7.1 shows a circuit containing a battery of e.m.f. 12 V and three resistors.

The voltmeter measures the potential difference (p.d.) across the parallel combination of resistors.

R1 has a resistance of 8 ohms, R2 has a resistance of 20 ohms, and R3 has a resistance of 30 ohms.

Explain how the reading on the voltmeter can be used to determine the p.d. across resistor R₁.

Fig. 11.1 shows lamps in series. Fig. 11.2 shows lamps in parallel.

The lamps are all identical 6.0 V lamps. In each circuit there are three ammeters A₁, A₂ and A₃.

(i) Compare the readings on ammeters A₁, A₂ and A₃ in Fig. 11.1.

(ii) Compare the readings on ammeters A₁, A₂ and A₃ in Fig. 11.2.

(ii) State two advantages of connecting the 6.0 V lamps in parallel with the 6.0 V battery, compared with connecting the lamps in series with the battery.

Extract:

Fig. 11.1 shows lamps in series. Fig. 11.2 shows lamps in parallel.

The lamps are all identical 6.0 V lamps. In each circuit there are three ammeters A₁, A₂ and A₃.

Each lamp has a resistance of 12 Ω.

(i) Determine the combined resistance of the two lamps connected in series.

resistance = .................................................... Ω [1]

(ii) Compare the resistance of one lamp with the combined resistance of the two lamps in parallel.

Fig. 8.1 shows a circuit.

The lamp has a resistance of 3.0 Ω. Line XY represents a uniform resistance wire of resistance 6.0 Ω.

Calculate the reading on the ammeter.

ammeter reading = ........................................................

Extract:

The length XY of the whole resistance wire is 2.0 m. The whole wire has resistance 6.0 Ω (from part (a)). The lamp has a resistance of 3.0 Ω. There is a 2.0 Ω resistor. The contact is made at Q where the distance XQ is 0.60 m.

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Fig. 8.2 shows the circuit with a different connection to the resistance wire and an added resistor.

The length XY of the whole resistance wire is 2.0 m. The contact is made at Q where the distance XQ is 0.60 m.

Calculate the resistance of the circuit.

resistance = ........................................................

A teacher demonstrates the action of a device. Fig. 10.1 shows the symbol for the device.

State the name of this device.

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Fig. 10.2 shows another device being used in a circuit. The circuit contains a 6.0 V lamp.

(i) The sliding contact of this device is at position A, as shown in Fig. 10.2.

Describe and explain the brightness of the lamp when the sliding contact is in this position.

(ii) The teacher moves the sliding contact from position A to position B. Describe and explain what happens to the brightness of the lamp.

(iii) The teacher moves the sliding contact from position B to position C. Suggest what happens to the lamp.

Fig. 8.1 shows a circuit that contains a battery of electromotive force (e.m.f.) 6.0 V, an ammeter, a 20 Ω resistor and component X.

(i) State the name of component X.

(ii) The potential difference (p.d.) across the 20 Ω resistor is measured with a voltmeter.

On Fig. 8.1, draw the symbol for this voltmeter connected to the circuit.

Extract:

Fig. 8.1 shows a circuit that contains a battery of electromotive force (e.m.f.) 6.0 V, an ammeter, a 20 Ω resistor and component X.

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The p.d. across the 20 Ω resistor is varied from zero to 6.0 V. For each value of p.d. a corresponding current is measured.

On Fig. 8.2, draw a line to indicate how the current measured by the ammeter depends on the p.d. across the 20 Ω resistor.

Extract:

Resistance of the fixed resistor, R = 20 Ω Minimum p.d. = 0 Maximum p.d. = 6.0 V Recall the equation linking voltage, current and resistance: R = V / I Calculate the maximum current: I = V / R = 6.0 / 20 = 0.30 A Straight line drawn from (0,0) to (6.0, 0.30) Remember that for a fixed resistor, the current and voltage are directly proportional.

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A second resistor is connected to the circuit in parallel with the 20 Ω resistor.

(i) State how the combined resistance of the two resistors in parallel compares with the resistance of each of the resistors on its own.

(ii) The p.d. across the two parallel resistors is changed and the current in the battery for each value of the p.d. is measured. A second line could be drawn on Fig. 8.2 to indicate how the current measured by the ammeter depends on the p.d. across the two resistors in parallel.

State how the second line differs from the original line. You are not expected to draw this second line.

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The circuit shown in Fig. 1 includes three ammeters labelled A₁, A₂ and A₃, and one unknown fixed resistor, R.

The voltmeter connected across R reads 24 V.

For the parallel branch, determine

(i) the voltage across the upper 10 Ω resistor.

(ii) the voltage across the lower part of the parallel circuit.

Extract:

The voltage across the upper 10 Ω resistor: In a series arrangement: The total p.d. across the components is equal to the sum of the individual p.d.s across each component. Therefore, the p.d. across the parallel branch = 40 V − 24 V = 16 V. In a parallel arrangement: The p.d. across parallel branches is the same. Therefore, the p.d. across the upper branch = 16 V. The resistance of the upper resistor is 10 Ω.

Calculate the current through the upper 10 Ω resistor.

Extract:

The circuit shown in Fig. 1 includes three ammeters labelled A1, A2 and A3, and one unknown fixed resistor, R. The voltmeter connected across R reads 24 V.

For the parallel branch:

• The voltage across the upper 10 Ω resistor: the p.d. across the parallel branch = 40 V − 24 V = 16 V
• The p.d. across parallel branches is the same, so the p.d. across the upper branch = 16 V and the p.d. across the lower branch = 16 V

Calculate the current through the upper 10 Ω resistor:

• Resistance = 10 Ω
• Potential difference = 16 V
• I = V / R = 16 / 10 = 1.6 A

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Determine the readings on

(i) ammeter A₂

(ii) ammeters A₁ and A₃.

Extract:

The voltmeter connected across R reads 24 V. A1 = A3 = 2.0 A

Calculate the value of R.

Extended Tier Only

Fig. 8.1 is a circuit diagram.

Calculate the combined resistance between Y and Z.

Extract:

Fig. 8.1 is a circuit diagram. Circuit diagram with a 24V battery, three resistors labelled 8Ω, 4Ω, and 6Ω. Resistors connect at points Y and Z. The 8Ω and 4Ω resistors are in series in the top branch, and the 6Ω resistor is in the bottom branch. The branches are in parallel between Y and Z.

Extended Tier Only

Calculate the potential difference (p.d.) across the 8.0 Ω resistor.

p.d. = ..........................................................