Cambridge International AS & A Level · 20 · structured
Ferrous cores and forces between currents
Ferrous cores and forces between currents · Original GioPhysics question with a detailed, mark-by-mark answer guide.
- Demand
- demanding
- Marks
- 7
- Topics
- 2
- Answer
- Complete
Consider a long solenoid connected to a regulated constant-current supply, and separately two long parallel vertical wires positioned side by side. Neglect the Earth's magnetic field.
- (a)
State and explain qualitatively the effect of inserting a soft-iron core into the solenoid while its current stays fixed.
2 marks - (b)
Both straight wires carry current upward. Use the field produced by one wire and the force on the other to explain whether they attract or repel. Include the field direction at the right-hand wire.
3 marks - (c)
Reverse the current in the right-hand wire only. Predict and explain the change in their forces.
2 marks
Ready to self-mark?Reveal the detailed answer guide
(a)
State and explain qualitatively the effect of inserting a soft-iron core into the solenoid while its current stays fixed.
Identify what the question is testing, organise the response into distinct mark-earning points, and make every conclusion traceable to a physical principle or to the evidence supplied.
- 1
A ferrous core increases the magnetic field for the same coil current.
- 2
The core becomes magnetised in the coil field and its magnetisation reinforces the field; the field change is not being attributed to an increased current.
(b)
Both straight wires carry current upward. Use the field produced by one wire and the force on the other to explain whether they attract or repel. Include the field direction at the right-hand wire.
Identify what the question is testing, organise the response into distinct mark-earning points, and make every conclusion traceable to a physical principle or to the evidence supplied.
- 1
Use the right-hand grip rule: upward current in the left wire gives B into the page at the right-hand wire.
- 2
The right wire's upward current in that field experiences force to the left, towards the left wire.
- 3
The other wire experiences the opposite force, so the pair attracts.
(c)
Reverse the current in the right-hand wire only. Predict and explain the change in their forces.
Identify what the question is testing, organise the response into distinct mark-earning points, and make every conclusion traceable to a physical principle or to the evidence supplied.
- 1
The magnetic field from the unchanged left wire at the right wire still points into the page.
- 2
Reversing the right-wire current reverses its force; the corresponding force on the left wire is also away from the other wire.
Complete marking guidance
Use these worked steps alongside the part-by-part answers. They explain the method and mark allocation; they do not add extra marks.
- (a) A ferrous core increases the magnetic field for the same coil current.
- (a) The core becomes magnetised in the coil field and its magnetisation reinforces the field; the field change is not being attributed to an increased current.
- (b) Use the right-hand grip rule: upward current in the left wire gives B into the page at the right-hand wire.
- (b) The right wire's upward current in that field experiences force to the left, towards the left wire.
- (b) The other wire experiences the opposite force, so the pair attracts.
- (c) The magnetic field from the unchanged left wire at the right wire still points into the page.
- (c) Reversing the right-wire current reverses its force; the corresponding force on the left wire is also away from the other wire.
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