Physics: Higher Level
Electromagnetic induction — HL Theme D
- 1.
A coil of 200 turns has a magnetic flux through it that changes uniformly from 0.0030 Wb to 0.0110 Wb in 0.40 s. Calculate the magnitude of the average EMF induced in the coil.
[4 marks] - 2.
Marking analysis: A learner attempts the following task: “A coil of 200 turns has a magnetic flux through it that changes uniformly from 0.0030 Wb to 0.0110 Wb in 0.40 s. Calculate the magnitude of the average EMF induced in the coil.” Their response addresses only this point: “Calculates the change in flux: ΔΦ = 0.0110 − 0.0030 = 0.0080 Wb.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks] - 3.
State Lenz's law, and use it to explain the direction of the induced current when the north pole of a magnet is pushed toward a coil of wire.
[3 marks] · no calculator - 4.
Marking analysis: A learner attempts the following task: “State Lenz's law, and use it to explain the direction of the induced current when the north pole of a magnet is pushed toward a coil of wire.” Their response addresses only this point: “States Lenz's law: the induced current flows in a direction that opposes the change in flux that produced it.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] · no calculator - 5.
A rectangular coil of area 0.050 m² and 100 turns rotates at a constant angular frequency of 50 rad s⁻¹ in a uniform magnetic field of flux density 0.30 T. Calculate the peak EMF generated.
[4 marks] - 6.
Marking analysis: A learner attempts the following task: “A rectangular coil of area 0.050 m² and 100 turns rotates at a constant angular frequency of 50 rad s⁻¹ in a uniform magnetic field of flux density 0.30 T. Calculate the peak EMF generated.” Their response addresses only this point: “States the peak EMF formula for a rotating coil: EMF₀ = NBAω.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks] - 7.
A step-up transformer has 400 turns on the primary coil and 2000 turns on the secondary coil. The primary current is 8.0 A. Assuming an ideal (100% efficient) transformer, calculate the secondary current.
[4 marks] - 8.
Marking analysis: A learner attempts the following task: “A step-up transformer has 400 turns on the primary coil and 2000 turns on the secondary coil. The primary current is 8.0 A. Assuming an ideal (100% efficient) transformer, calculate the secondary current.” Their response addresses only this point: “States that for an ideal transformer, input power equals output power: VpIp = VsIs.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks] - 9.
Explain why transmitting electrical power over long distances at high voltage and low current, rather than low voltage and high current, reduces power losses in the transmission cables.
[4 marks] · no calculator - 10.
Marking analysis: A learner attempts the following task: “Explain why transmitting electrical power over long distances at high voltage and low current, rather than low voltage and high current, reduces power losses in the transmission cables.” Their response addresses only this point: “States that power loss in a cable is given by P = I²R, depending on current squared, not on voltage.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks] · no calculator - 11.
A metal plate moves through a region of uniform magnetic field and experiences eddy currents that oppose its motion. Explain the origin of these eddy currents and why they cause a braking (retarding) force on the plate.
[4 marks] · no calculator - 12.
Marking analysis: A learner attempts the following task: “A metal plate moves through a region of uniform magnetic field and experiences eddy currents that oppose its motion. Explain the origin of these eddy currents and why they cause a braking (retarding) force on the plate.” Their response addresses only this point: “States that as the plate moves through the field, the magnetic flux through parts of the conducting plate changes.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks] · no calculator - 13.
A straight conducting rod of length 0.60 m moves at 4.0 m s⁻¹ perpendicular to a uniform magnetic field of flux density 0.25 T, with its length also perpendicular to its velocity. Calculate the EMF induced between the ends of the rod.
[3 marks] - 14.
Marking analysis: A learner attempts the following task: “A straight conducting rod of length 0.60 m moves at 4.0 m s⁻¹ perpendicular to a uniform magnetic field of flux density 0.25 T, with its length also perpendicular to its velocity. Calculate the EMF induced between the ends of the rod.” Their response addresses only this point: “Uses the motional EMF equation EMF = BLv for mutually perpendicular B, L and v.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] - 15.
A generator produces an alternating EMF with peak value 340 V. Calculate the root-mean-square (rms) voltage, and explain what this rms value physically represents.
[3 marks] - 16.
Marking analysis: A learner attempts the following task: “A generator produces an alternating EMF with peak value 340 V. Calculate the root-mean-square (rms) voltage, and explain what this rms value physically represents.” Their response addresses only this point: “Uses Vrms = V₀/√2.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] - 17.
A generator's coil rotates at constant angular speed in a magnetic field. State how the induced EMF varies with time, and explain why the EMF is zero when the coil plane is perpendicular to the field.
[3 marks] · no calculator - 18.
Marking analysis: A learner attempts the following task: “A generator's coil rotates at constant angular speed in a magnetic field. State how the induced EMF varies with time, and explain why the EMF is zero when the coil plane is perpendicular to the field.” Their response addresses only this point: “States that the induced EMF varies sinusoidally with time.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] · no calculator - 19.
A transformer with 100% efficiency has a primary voltage of 230 V and a secondary voltage of 11.5 V. Calculate the turns ratio (primary:secondary), and state whether this is a step-up or step-down transformer.
[4 marks] - 20.
Marking analysis: A learner attempts the following task: “A transformer with 100% efficiency has a primary voltage of 230 V and a secondary voltage of 11.5 V. Calculate the turns ratio (primary:secondary), and state whether this is a step-up or step-down transformer.” Their response addresses only this point: “Uses Np/Ns = Vp/Vs.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks] - 21.
Define self-inductance, and calculate the magnitude of the average EMF induced in a coil of self-inductance 0.50 H when the current through it changes from 2.0 A to 6.0 A in 0.20 s.
[4 marks] - 22.
Marking analysis: A learner attempts the following task: “Define self-inductance, and calculate the magnitude of the average EMF induced in a coil of self-inductance 0.50 H when the current through it changes from 2.0 A to 6.0 A in 0.20 s.” Their response addresses only this point: “Defines self-inductance as the property of a circuit (coil) whereby a changing current within it induces an EMF in itself, opposing the change.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks] - 23.
State two reasons why a real transformer is not 100% efficient, and state one design feature used to reduce each.
[4 marks] · no calculator - 24.
Marking analysis: A learner attempts the following task: “State two reasons why a real transformer is not 100% efficient, and state one design feature used to reduce each.” Their response addresses only this point: “States that resistance in the windings causes I²R (resistive) heating losses.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks] · no calculator - 25.
Explain, in terms of mutual inductance, how a transformer transfers electrical energy from its primary coil to its secondary coil without any direct electrical (wired) connection between them.
[3 marks] · no calculator - 26.
Marking analysis: A learner attempts the following task: “Explain, in terms of mutual inductance, how a transformer transfers electrical energy from its primary coil to its secondary coil without any direct electrical (wired) connection between them.” Their response addresses only this point: “States that an alternating current in the primary coil produces a continuously changing magnetic flux.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] · no calculator - 27.
Explain, using Lenz's law, why a spinning electric motor generates a 'back EMF' that opposes the supply voltage driving it, and state what happens to the current drawn by the motor as its speed increases.
[4 marks] · no calculator - 28.
Marking analysis: A learner attempts the following task: “Explain, using Lenz's law, why a spinning electric motor generates a 'back EMF' that opposes the supply voltage driving it, and state what happens to the current drawn by the motor as its speed increases.” Their response addresses only this point: “States that as the motor's coil rotates within the magnetic field, it is itself acting like a generator, so by Faraday's law it induces an EMF.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks] · no calculator - 29.
A coil of 80 turns and area 0.012 m² rotates at constant angular speed 25 rad s⁻¹ in a uniform magnetic field of flux density 0.35 T. Its flux linkage is NBA cos(ωt). (a) Determine the maximum induced emf. (b) Calculate the magnitude of the emf when ωt = 30°. (c) The coil is connected to a 6.0 Ω resistor and internal resistance is negligible. Calculate the instantaneous power in the resistor at this angle. (d) State one change that doubles the maximum emf without changing the field.
[5 marks] - 30.
Marking analysis: A learner attempts the following task: “A coil of 80 turns and area 0.012 m² rotates at constant angular speed 25 rad s⁻¹ in a uniform magnetic field of flux density 0.35 T. Its flux linkage is NBA cos(ωt). (a) Determine the maximum induced emf. (b) Calculate the magnitude of the emf when ωt = 30°. (c) The coil is connected to a 6.0 Ω resistor and internal resistance is negligible. Calculate the instantaneous power in the resistor at this angle. (d) State one change that doubles the maximum emf without changing the field.” Their response addresses only this point: “Uses Faraday's law to obtain ε = NBAω sin(ωt).” Evaluate the response against the complete 5-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[5 marks]