A solenoid is used to lift steel bolts in a factory. Describe how the solenoid produces a magnetic field and explain two ways the engineer could increase the strength of this magnetic field to lift heavier bolts.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A factory uses a solenoid connected to a direct current power supply to sort and lift small steel bolts on a production line. The engineer notices that the solenoid sometimes fails to lift heavier bolts and wants to make the magnetic field stronger.
Model answer (5 marks)
A solenoid is a coil of wire through which a direct current flows. The current creates a magnetic field around each turn of wire; the many turns are arranged so that the individual fields add together, giving a single, uniform field mainly inside the solenoid.
To increase the field strength the engineer can:
1. Increase the current flowing through the coil – a larger current produces a proportionally larger magnetic field.
2. Increase the number of turns of wire – more turns add more magnetic field lines, raising the field strength.
(Optionally, inserting an iron core would also increase the field, but the question asks for two ways.)
To increase the field strength the engineer can:
1. Increase the current flowing through the coil – a larger current produces a proportionally larger magnetic field.
2. Increase the number of turns of wire – more turns add more magnetic field lines, raising the field strength.
(Optionally, inserting an iron core would also increase the field, but the question asks for two ways.)
Examiner tips
- Use the phrase "current flowing through the coil" and "uniform field inside the solenoid" to hit the description marks. Show the two improvements clearly, naming current and number of turns. Mention the iron core only if you have extra marks or if the question allows a third option.
Common mistakes
- Confusing the field outside the solenoid with the field inside; examiners expect the inside field. Saying "more wire" without specifying "more turns". Forgetting to mention that the current must flow through the coil to produce the field.
Mark scheme (5 marks)
- Current flowing through the coiled wire produces a magnetic field (around the wire / inside the solenoid)
- The coiling of the wire causes the individual fields to align / combine to produce a single stronger, uniform magnetic field
- First improvement: increase the current (flowing through the solenoid) — this increases the magnetic field strength
- Second improvement: increase the number of turns (coils / loops) of wire — more turns increases the magnetic field strength
- Adding an iron core (inside the solenoid) increases the magnetic field strength because it is easier for magnetic field lines to pass through iron than air
Key terms in this question
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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