Physics

1. 1.

   Two resistors of 6.0 Ω and 3.0 Ω are connected in parallel. Calculate their combined resistance.

   [3 marks]

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
   2. Work through this mathematical step: Uses 1/R = 1/6 + 1/3. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   3. Work through this mathematical step: Obtains 1/R = 1/2. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   4. Work through this mathematical step: States R = 2.0 Ω. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: A parallel combination must have a resistance below the smallest branch resistance.

   Marking points

   • Uses 1/R = 1/6 + 1/3.
   • Obtains 1/R = 1/2.
   • States R = 2.0 Ω.

   Examiner tip: A parallel combination must have a resistance below the smallest branch resistance.

2. 2.

   An ideal transformer has 1200 turns on its primary coil and 80 turns on its secondary coil. The primary voltage is 240 V. Calculate the secondary voltage and state whether it is step-up or step-down.

   [4 marks]

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
   2. Work through this mathematical step: Uses Vp/Vs = Np/Ns or an equivalent transformer equation. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   3. Work through this mathematical step: Substitutes 240/Vs = 1200/80. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   4. Work through this mathematical step: Obtains Vs = 16 V. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   5. Develop this part of the answer: Identifies the transformer as step-down. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Fewer secondary turns means a lower secondary voltage.

   Marking points

   • Uses Vp/Vs = Np/Ns or an equivalent transformer equation.
   • Substitutes 240/Vs = 1200/80.
   • Obtains Vs = 16 V.
   • Identifies the transformer as step-down.

   Examiner tip: Fewer secondary turns means a lower secondary voltage.

3. 3.

   A current of 3.0 A flows through a resistor for 5 minutes. Calculate the charge that flows.

   [3 marks]

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
   2. Work through this mathematical step: Converts time to seconds: 5 × 60 = 300 s. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   3. Work through this mathematical step: Uses charge = current × time. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   4. Work through this mathematical step: Obtains charge = 900 C. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Always convert time to seconds before using Q = It, since current is defined in coulombs per second.

   Marking points

   • Converts time to seconds: 5 × 60 = 300 s.
   • Uses charge = current × time.
   • Obtains charge = 900 C.

   Examiner tip: Always convert time to seconds before using Q = It, since current is defined in coulombs per second.

4. 4.

   A 12 V battery drives a current of 2.0 A through a resistor. Calculate the resistance and the power dissipated.

   [4 marks]

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
   2. Work through this mathematical step: Uses R = V/I. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   3. Work through this mathematical step: Obtains R = 6.0 Ω. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   4. Work through this mathematical step: Uses P = VI. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   5. Work through this mathematical step: Obtains P = 24 W. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Power can be calculated using any of P = VI, P = I²R or P = V²/R — choose whichever uses only the given quantities.

   Marking points

   • Uses R = V/I.
   • Obtains R = 6.0 Ω.
   • Uses P = VI.
   • Obtains P = 24 W.

   Examiner tip: Power can be calculated using any of P = VI, P = I²R or P = V²/R — choose whichever uses only the given quantities.

5. 5.

   State the difference between series and parallel circuits in terms of current at different points in the circuit.

   [2 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Develop this part of the answer: States that in a series circuit, the current is the same at every point in the circuit. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Develop this part of the answer: States that in a parallel circuit, the current splits between branches, so the current differs in each branch (though it recombines to the total at the source). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Current is conserved (Kirchhoff's first law) at every junction — the total current entering a junction always equals the total current leaving it.

   Marking points

   • States that in a series circuit, the current is the same at every point in the circuit.
   • States that in a parallel circuit, the current splits between branches, so the current differs in each branch (though it recombines to the total at the source).

   Examiner tip: Current is conserved (Kirchhoff's first law) at every junction — the total current entering a junction always equals the total current leaving it.

6. 6.

   A wire carries a current through a magnetic field. State the rule used to determine the direction of the force on the wire, and state the three quantities it relates.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Develop this part of the answer: States Fleming's left-hand rule. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Develop this part of the answer: States that it relates the direction of the magnetic field, the direction of the current, and the direction of the resulting force (motion). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Develop this part of the answer: States that the First finger, seCond finger and thuMb represent Field, Current and Motion respectively. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Fleming's left-hand rule is for the motor effect (force on a current-carrying wire); Fleming's right-hand rule is for generator/induction effects — do not mix them up.

   Marking points

   • States Fleming's left-hand rule.
   • States that it relates the direction of the magnetic field, the direction of the current, and the direction of the resulting force (motion).
   • States that the First finger, seCond finger and thuMb represent Field, Current and Motion respectively.

   Examiner tip: Fleming's left-hand rule is for the motor effect (force on a current-carrying wire); Fleming's right-hand rule is for generator/induction effects — do not mix them up.

7. 7.

   Three 4.0 Ω resistors are connected in series with a 24 V battery. Calculate the current flowing and the voltage across each resistor.

   [4 marks]

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
   2. Work through this mathematical step: Calculates total resistance for series resistors: 4.0 + 4.0 + 4.0 = 12.0 Ω. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   3. Work through this mathematical step: Uses I = V/R = 24/12.0. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   4. Work through this mathematical step: Obtains current = 2.0 A. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   5. Work through this mathematical step: Uses V = IR = 2.0 × 4.0 to obtain 8.0 V across each resistor. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: In a series circuit, resistances simply add; the voltage divides between resistors in proportion to their resistance, but the current stays the same throughout.

   Marking points

   • Calculates total resistance for series resistors: 4.0 + 4.0 + 4.0 = 12.0 Ω.
   • Uses I = V/R = 24/12.0.
   • Obtains current = 2.0 A.
   • Uses V = IR = 2.0 × 4.0 to obtain 8.0 V across each resistor.

   Examiner tip: In a series circuit, resistances simply add; the voltage divides between resistors in proportion to their resistance, but the current stays the same throughout.

8. 8.

   Explain why a fuse of appropriate rating is fitted in the live wire of a mains electrical appliance, in terms of safety.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Develop this part of the answer: States that if a fault causes the current to exceed the fuse's rated value, the fuse wire heats up and melts (blows). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Develop this part of the answer: States that this breaks (disconnects) the circuit, stopping the current from flowing. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Develop this part of the answer: States that this prevents excessive current from causing a fire (through overheating of the wiring) or damaging the appliance. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: A fuse must be fitted in the live wire specifically, so that when it blows, the appliance is fully disconnected from the high-potential supply.

   Marking points

   • States that if a fault causes the current to exceed the fuse's rated value, the fuse wire heats up and melts (blows).
   • States that this breaks (disconnects) the circuit, stopping the current from flowing.
   • States that this prevents excessive current from causing a fire (through overheating of the wiring) or damaging the appliance.

   Examiner tip: A fuse must be fitted in the live wire specifically, so that when it blows, the appliance is fully disconnected from the high-potential supply.

9. 9.

   Two 6.0 Ω resistors are connected in parallel, and this combination is connected in series with a 3.0 Ω resistor. Calculate the total resistance of the circuit.

   [4 marks]

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
   2. Work through this mathematical step: Calculates the parallel combination: 1/R = 1/6 + 1/6 = 1/3. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   3. Work through this mathematical step: Obtains the parallel resistance = 3.0 Ω. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   4. Develop this part of the answer: Adds this to the series resistor: 3.0 + 3.0. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   5. Work through this mathematical step: Obtains total resistance = 6.0 Ω. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Work out combined circuits in stages: simplify the parallel section to a single equivalent resistance first, then treat the circuit as a simple series combination.

   Marking points

   • Calculates the parallel combination: 1/R = 1/6 + 1/6 = 1/3.
   • Obtains the parallel resistance = 3.0 Ω.
   • Adds this to the series resistor: 3.0 + 3.0.
   • Obtains total resistance = 6.0 Ω.

   Examiner tip: Work out combined circuits in stages: simplify the parallel section to a single equivalent resistance first, then treat the circuit as a simple series combination.

10. 10.

    Explain how a step-up transformer is used to reduce energy losses during the transmission of electrical power over long distances on the national grid.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Work through this mathematical step: States that a step-up transformer increases the voltage (and correspondingly decreases the current) for a given power being transmitted, since power = voltage × current. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Develop this part of the answer: States that power lost as heat in the transmission cables is given by I²R, so reducing the current greatly reduces this power loss. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Work through this mathematical step: States that since current is squared in the loss formula, even a modest decrease in current produces a much larger decrease in power lost. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    5. Develop this part of the answer: States that a step-down transformer is then used near the point of use to reduce the voltage back to a safe level for consumers. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The key insight is that power loss depends on current squared (I²R), not voltage — this is why high voltage, low current transmission is so much more efficient.

    Marking points

    • States that a step-up transformer increases the voltage (and correspondingly decreases the current) for a given power being transmitted, since power = voltage × current.
    • States that power lost as heat in the transmission cables is given by I²R, so reducing the current greatly reduces this power loss.
    • States that since current is squared in the loss formula, even a modest decrease in current produces a much larger decrease in power lost.
    • States that a step-down transformer is then used near the point of use to reduce the voltage back to a safe level for consumers.

    Examiner tip: The key insight is that power loss depends on current squared (I²R), not voltage — this is why high voltage, low current transmission is so much more efficient.

11. 11.

    A graph of current against potential difference for a resistor kept at constant temperature is a straight line through the origin. State what this shows about the resistance of the resistor, and calculate its resistance if a current of 0.40 A flows when the potential difference is 6.0 V.

    [3 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Develop this part of the answer: States that the resistance is constant (independent of current or potential difference), since current is directly proportional to potential difference. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Work through this mathematical step: Uses R = V/I. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Work through this mathematical step: Obtains R = 6.0/0.40 = 15 Ω. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: A straight line through the origin on an I-V graph is the defining signature of a component obeying Ohm's law at constant temperature.

    Marking points

    • States that the resistance is constant (independent of current or potential difference), since current is directly proportional to potential difference.
    • Uses R = V/I.
    • Obtains R = 6.0/0.40 = 15 Ω.

    Examiner tip: A straight line through the origin on an I-V graph is the defining signature of a component obeying Ohm's law at constant temperature.

12. 12.

    State how the current through a diode depends on the direction of the applied potential difference, and name one application of a diode in a circuit.

    [2 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that a diode allows current to flow easily in one direction (forward bias) but has a very high resistance, so almost no current flows, in the reverse direction. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States a valid application, e.g. as a rectifier to convert alternating current (a.c.) into direct current (d.c.). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Diodes are the basis of rectification: an a.c. supply connected through a diode only allows current through during half of each cycle.

    Marking points

    • States that a diode allows current to flow easily in one direction (forward bias) but has a very high resistance, so almost no current flows, in the reverse direction.
    • States a valid application, e.g. as a rectifier to convert alternating current (a.c.) into direct current (d.c.).

    Examiner tip: Diodes are the basis of rectification: an a.c. supply connected through a diode only allows current through during half of each cycle.

13. 13.

    Describe how the resistance of a filament lamp changes as the current through it increases, and explain why, in terms of temperature.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that the resistance of the filament increases as the current (and potential difference) increases. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that a larger current causes the filament to heat up to a higher temperature. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that this increased temperature increases the resistance of the metal filament, since the vibrating metal ions impede the flow of electrons more. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: This is why a filament lamp's I-V graph curves and flattens rather than staying a straight line — it does not obey Ohm's law once its temperature changes.

    Marking points

    • States that the resistance of the filament increases as the current (and potential difference) increases.
    • States that a larger current causes the filament to heat up to a higher temperature.
    • States that this increased temperature increases the resistance of the metal filament, since the vibrating metal ions impede the flow of electrons more.

    Examiner tip: This is why a filament lamp's I-V graph curves and flattens rather than staying a straight line — it does not obey Ohm's law once its temperature changes.

14. 14.

    A light-dependent resistor (LDR) is used to automatically switch on a street light at night. State how the resistance of an LDR changes as light intensity decreases, and explain how this can be used to switch the light on in darkness.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that the resistance of an LDR increases as light intensity decreases. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that in a potential divider circuit, in darkness the LDR's high resistance causes most of the supply potential difference to appear across it. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that this rising potential difference is used to trigger a switching circuit (e.g. a transistor), which turns the street light on. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: LDRs and thermistors are both used in potential-divider sensing circuits — the key is identifying which component's resistance change dominates the divider's output voltage.

    Marking points

    • States that the resistance of an LDR increases as light intensity decreases.
    • States that in a potential divider circuit, in darkness the LDR's high resistance causes most of the supply potential difference to appear across it.
    • States that this rising potential difference is used to trigger a switching circuit (e.g. a transistor), which turns the street light on.

    Examiner tip: LDRs and thermistors are both used in potential-divider sensing circuits — the key is identifying which component's resistance change dominates the divider's output voltage.

15. 15.

    A thermistor is used as part of a fire alarm circuit. State how the resistance of a thermistor changes as temperature increases, and explain how this allows the circuit to detect a fire.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that the resistance of a thermistor decreases as temperature increases. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that in a potential divider circuit, heat from a fire decreases the thermistor's resistance, changing the potential difference across it (and across the other resistor). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that this changing potential difference is used to trigger a switching circuit, which sounds the fire alarm. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Whether a thermistor circuit switches on heating or cooling depends entirely on where the thermistor is placed relative to the fixed resistor in the potential divider.

    Marking points

    • States that the resistance of a thermistor decreases as temperature increases.
    • States that in a potential divider circuit, heat from a fire decreases the thermistor's resistance, changing the potential difference across it (and across the other resistor).
    • States that this changing potential difference is used to trigger a switching circuit, which sounds the fire alarm.

    Examiner tip: Whether a thermistor circuit switches on heating or cooling depends entirely on where the thermistor is placed relative to the fixed resistor in the potential divider.

16. 16.

    A bar magnet is pushed into a coil of wire connected to a sensitive ammeter, inducing a current. State three factors that would increase the size of the induced current, and state how the direction of the induced current would change if the magnet were withdrawn instead of inserted.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that increasing the speed at which the magnet moves increases the induced current. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that using a stronger magnet increases the induced current. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that increasing the number of turns on the coil increases the induced current. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Develop this part of the answer: States that the induced current reverses direction when the magnet is withdrawn instead of inserted. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The size of an induced current or e.m.f. always depends on the rate of change of the magnetic field through the coil, not on the field's absolute strength alone.

    Marking points

    • States that increasing the speed at which the magnet moves increases the induced current.
    • States that using a stronger magnet increases the induced current.
    • States that increasing the number of turns on the coil increases the induced current.
    • States that the induced current reverses direction when the magnet is withdrawn instead of inserted.

    Examiner tip: The size of an induced current or e.m.f. always depends on the rate of change of the magnetic field through the coil, not on the field's absolute strength alone.

17. 17.

    Describe the basic construction of a simple d.c. electric motor, and state the purpose of the split-ring commutator.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that a rectangular coil of wire is mounted on an axle between the poles of a permanent magnet, connected to a d.c. supply through a split-ring commutator and brushes. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that current flowing through the coil in the magnetic field produces a force on each side of the coil, creating a turning effect (torque) that causes rotation. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that the split-ring commutator reverses the direction of current in the coil every half turn, so the turning effect continues in the same rotational direction rather than reversing. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Without the commutator's reversal every half-turn, the coil would simply oscillate back and forth rather than spinning continuously in one direction.

    Marking points

    • States that a rectangular coil of wire is mounted on an axle between the poles of a permanent magnet, connected to a d.c. supply through a split-ring commutator and brushes.
    • States that current flowing through the coil in the magnetic field produces a force on each side of the coil, creating a turning effect (torque) that causes rotation.
    • States that the split-ring commutator reverses the direction of current in the coil every half turn, so the turning effect continues in the same rotational direction rather than reversing.

    Examiner tip: Without the commutator's reversal every half-turn, the coil would simply oscillate back and forth rather than spinning continuously in one direction.

18. 18.

    State two factors that increase the strength of an electromagnet, and describe one practical application of an electromagnet.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that increasing the current flowing through the coil increases the strength of the electromagnet. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that increasing the number of turns on the coil (or using a soft-iron core) increases the strength of the electromagnet. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: Describes a valid application, e.g. a scrapyard crane using an electromagnet to lift and then release large masses of scrap metal by switching the current on and off. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Unlike a permanent magnet, an electromagnet's strength can be controlled (increased, decreased, or switched off entirely) simply by changing the current — this is why it is useful wherever magnetism needs to be turned on and off.

    Marking points

    • States that increasing the current flowing through the coil increases the strength of the electromagnet.
    • States that increasing the number of turns on the coil (or using a soft-iron core) increases the strength of the electromagnet.
    • Describes a valid application, e.g. a scrapyard crane using an electromagnet to lift and then release large masses of scrap metal by switching the current on and off.

    Examiner tip: Unlike a permanent magnet, an electromagnet's strength can be controlled (increased, decreased, or switched off entirely) simply by changing the current — this is why it is useful wherever magnetism needs to be turned on and off.

19. 19.

    A polythene rod is rubbed with a cloth and becomes negatively charged. Explain, in terms of electron transfer, why the rod becomes negatively charged, and state what type of charge the cloth acquires.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that rubbing transfers electrons from the cloth onto the polythene rod. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that the rod, having gained electrons, becomes negatively charged. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that the cloth, having lost electrons, becomes positively charged, with a charge equal in magnitude to that gained by the rod. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Charging by friction never creates charge — it only transfers electrons from one object to the other, so the total charge on both objects together is conserved.

    Marking points

    • States that rubbing transfers electrons from the cloth onto the polythene rod.
    • States that the rod, having gained electrons, becomes negatively charged.
    • States that the cloth, having lost electrons, becomes positively charged, with a charge equal in magnitude to that gained by the rod.

    Examiner tip: Charging by friction never creates charge — it only transfers electrons from one object to the other, so the total charge on both objects together is conserved.

20. 20.

    A 2.5 kW electric heater is used for 4.0 hours. Calculate the energy transferred in kilowatt-hours, and the cost of using the heater if electricity costs 18 cents per kWh.

    [4 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Work through this mathematical step: Uses energy (kWh) = power (kW) × time (h). Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Work through this mathematical step: Obtains energy = 2.5 × 4.0 = 10 kWh. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Work through this mathematical step: Uses cost = energy (kWh) × price per kWh. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    5. Work through this mathematical step: Obtains cost = 10 × 18 = 180 cents ($1.80). Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The kilowatt-hour is a unit of energy (not power), defined as the energy transferred by a 1 kW appliance running for 1 hour — this is the unit electricity meters and bills actually use.

    Marking points

    • Uses energy (kWh) = power (kW) × time (h).
    • Obtains energy = 2.5 × 4.0 = 10 kWh.
    • Uses cost = energy (kWh) × price per kWh.
    • Obtains cost = 10 × 18 = 180 cents ($1.80).

    Examiner tip: The kilowatt-hour is a unit of energy (not power), defined as the energy transferred by a 1 kW appliance running for 1 hour — this is the unit electricity meters and bills actually use.

21. 21.

    A charge of 5.0 C passes through a component when a potential difference of 12 V is maintained across it. Calculate the electrical energy transferred.

    [3 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Work through this mathematical step: Uses energy transferred = charge × potential difference. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Work through this mathematical step: Substitutes 5.0 × 12. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Work through this mathematical step: Obtains energy transferred = 60 J. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Potential difference is defined as the energy transferred per unit charge — this definition (E = QV) is the basis of this calculation.

    Marking points

    • Uses energy transferred = charge × potential difference.
    • Substitutes 5.0 × 12.
    • Obtains energy transferred = 60 J.

    Examiner tip: Potential difference is defined as the energy transferred per unit charge — this definition (E = QV) is the basis of this calculation.

22. 22.

    Describe the behaviour of a two-input AND logic gate: state the condition needed for its output to be 1 (high), and state the output for all other combinations of inputs A and B.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Work through this mathematical step: States that the output is 1 only when both input A = 1 and input B = 1. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Work through this mathematical step: States that the output is 0 for all three other input combinations: (A=0, B=0), (A=0, B=1) and (A=1, B=0). Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Develop this part of the answer: States that this behaviour, where the output is only high when every input is high, defines the logical AND operation, used where two conditions must both be met before an output is triggered. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Build a truth table systematically by listing all possible input combinations (00, 01, 10, 11) before deciding each output — this avoids missing a case.

    Marking points

    • States that the output is 1 only when both input A = 1 and input B = 1.
    • States that the output is 0 for all three other input combinations: (A=0, B=0), (A=0, B=1) and (A=1, B=0).
    • States that this behaviour, where the output is only high when every input is high, defines the logical AND operation, used where two conditions must both be met before an output is triggered.

    Examiner tip: Build a truth table systematically by listing all possible input combinations (00, 01, 10, 11) before deciding each output — this avoids missing a case.

23. 23.

    Explain why a relay is used to allow a low-current circuit, such as one containing a sensor, to switch a separate high-current circuit, such as one containing a motor.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that a relay contains an electromagnet which, when energised by a small current in the low-current (control) circuit, attracts a switch (armature). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that this closes the switch in the separate high-current circuit, allowing it to operate without the high current passing through the control circuit's components. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that this protects sensitive low-current components (e.g. sensor circuits) from damage by the high current, and electrically isolates the two circuits. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: A relay is essentially a current-operated switch: a small control current switches a completely separate, larger current on or off.

    Marking points

    • States that a relay contains an electromagnet which, when energised by a small current in the low-current (control) circuit, attracts a switch (armature).
    • States that this closes the switch in the separate high-current circuit, allowing it to operate without the high current passing through the control circuit's components.
    • States that this protects sensitive low-current components (e.g. sensor circuits) from damage by the high current, and electrically isolates the two circuits.

    Examiner tip: A relay is essentially a current-operated switch: a small control current switches a completely separate, larger current on or off.

24. 24.

    Describe the shape of the magnetic field pattern around a straight current-carrying wire, and state how the field pattern of a solenoid compares to that of a bar magnet.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that the field around a straight current-carrying wire forms concentric circles centred on the wire, with direction given by the right-hand grip rule. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that the field pattern of a solenoid resembles that of a bar magnet, with distinct north and south poles at its ends. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that outside the solenoid, field lines run from its north pole to its south pole, while inside the solenoid the field is strong and uniform (parallel field lines). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: A solenoid's field strength can be increased using the same factors as any electromagnet: more current, more turns, or a soft-iron core.

    Marking points

    • States that the field around a straight current-carrying wire forms concentric circles centred on the wire, with direction given by the right-hand grip rule.
    • States that the field pattern of a solenoid resembles that of a bar magnet, with distinct north and south poles at its ends.
    • States that outside the solenoid, field lines run from its north pole to its south pole, while inside the solenoid the field is strong and uniform (parallel field lines).

    Examiner tip: A solenoid's field strength can be increased using the same factors as any electromagnet: more current, more turns, or a soft-iron core.

25. 25.

    Three identical 1.5 V cells are connected in series in a circuit. Calculate the total e.m.f. of the battery, and state how the total e.m.f. would differ if the same three cells were instead connected in parallel.

    [3 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Develop this part of the answer: States that for cells connected in series, the total e.m.f. equals the sum of the individual e.m.f.s. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Work through this mathematical step: Obtains total e.m.f. = 1.5 × 3 = 4.5 V. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Develop this part of the answer: States that if the identical cells were instead connected in parallel, the total e.m.f. would remain 1.5 V (the same as a single cell), though the combination could supply current for longer. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Series connection adds e.m.f. but keeps the same current capacity; parallel connection keeps the same e.m.f. but increases the current (and charge) the combination can supply.

    Marking points

    • States that for cells connected in series, the total e.m.f. equals the sum of the individual e.m.f.s.
    • Obtains total e.m.f. = 1.5 × 3 = 4.5 V.
    • States that if the identical cells were instead connected in parallel, the total e.m.f. would remain 1.5 V (the same as a single cell), though the combination could supply current for longer.

    Examiner tip: Series connection adds e.m.f. but keeps the same current capacity; parallel connection keeps the same e.m.f. but increases the current (and charge) the combination can supply.

Original, unofficial practice questions from SubjectScout — not official past-paper questions.