Sketch magnetic fields from electric currents
Sketch field patterns around a wire, circular coil and solenoid, and relate current direction to field and force directions.
A Level; Cambridge International Physics 9702, 2025–2027 syllabus, Topic 20. These original explanations and calculations are study aids; full timed assessment and human subject-teacher review remain outstanding.
Before you begin
- Use current, charge, force, velocity, vectors, area and SI units; distinguish conventional current from electron flow.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the magnetic field pattern around a long straight current-carrying wire?
What you will learn
- Sketch magnetic fields around a straight wire, circular coil and solenoid
- Explain how a ferrous core strengthens a solenoid field
- Explain and determine forces between current-carrying conductors
Current direction determines the field pattern
Around a long straight wire, magnetic-field lines form concentric circles in planes perpendicular to the wire. Use the right-hand grip rule: thumb points with conventional current and curled fingers show the field direction.
A flat circular coil has a field pattern similar to a small bar magnet, with a stronger field through its centre. A long solenoid has nearly parallel, uniform field lines inside and a weaker return field outside; its pattern resembles a bar magnet.
Cores strengthen fields and currents exert forces
A ferrous core increases the magnetic field produced by a solenoid. The core responds to the applied field and concentrates the field through the coil; do not confuse this with increasing the current itself.
Parallel current-carrying conductors exert forces because each wire's current interacts with the magnetic field produced by the other. Currents in the same direction attract; currents in opposite directions repel. Use the field direction around one wire and Fleming's left-hand rule to determine the force on the other.
Pause and explain
What happens between two parallel wires carrying currents in the same direction?
Worked example
Two long parallel wires carry currents in the same direction. State the force between them and explain its origin.
Show the worked solution
- Each current creates a magnetic field around its wire.
- The other wire's current is in that field and experiences a force.
- For parallel currents in the same direction, the force is attractive.
Answer The wires attract because each current experiences magnetic force in the field produced by the other.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe the field inside and outside a long solenoid, and state one way to strengthen its field without changing its current.
Give me a hint
Compare the solenoid pattern with a bar magnet and use the core outcome.
Compare my reasoning
- Inside a long solenoid, field lines are nearly parallel and uniform.
- Outside, the field returns in a weaker loop pattern.
- Inserting a ferrous core increases the field without changing current.
The inside field is nearly uniform and the outside field is weaker; a ferrous core strengthens it.
Look for these in your work
- Inside pattern described
- Outside pattern described
- Core effect stated
Two parallel wires carry currents in opposite directions. Predict their force interaction.
Give me a hint
Apply the same-direction attraction rule and reverse one current.
Compare my reasoning
- Each wire creates a magnetic field around itself.
- The current in the other wire experiences magnetic force.
- Opposite-direction parallel currents repel.
The wires repel.
Look for these in your work
- Mutual field-force origin stated
- Current directions compared
- Force relation correct
A long straight wire carries conventional current upward. Use the right-hand grip rule to describe the magnetic field on the east and west sides of the wire.
Give me a hint
Point your right thumb along current and curl your fingers around the wire.
Compare my reasoning
- Point the right thumb upward with the conventional current.
- The curled fingers show a circular field around the wire.
- On opposite sides, the local field directions are opposite; specify the side orientation consistently in a diagram.
The field circles the wire; its directions on the east and west sides are opposite as given by the right-hand grip rule.
Look for these in your work
- Conventional current used
- Circular pattern applied
- Opposite side directions noted
Common mistakes
- Using electron-flow direction in the right-hand grip rule
- Drawing a solenoid field as circular around its axis inside
- Saying same-direction currents repel
- Attributing wire forces to a direct attraction between electrons
What can you explain now?
What is the magnetic field pattern around a long straight current-carrying wire?
Compare with the explanation
Concentric circles around the wire
The field encircles the current.
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Make it stick
How do a wire's current direction and a solenoid's geometry determine their magnetic-field patterns?
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