A current produces a magnetic field
Whenever a current flows in a conductor it produces a magnetic field around it. Around a straight wire the field lines are concentric circles centred on the wire, closer together (stronger) near the wire. Around a coil/solenoid the turns combine to give a field like a bar magnet (N at one end, S at the other). As for any magnetic field, the strength is shown by how closely spaced the lines are, not their length.
The motor effect
The motor effect is the force on a current-carrying conductor in a magnetic field: the wire's own field interacts with the applied field, pushing the wire. The force is largest when the wire is at right angles to the field and zero when it lies along it. In an electric motor, opposite forces on the two coil sides make it turn. In a loudspeaker, a varying current gives a varying force that pushes the cone in and out to produce sound.
Fleming's rule; how the force varies
Fleming's left-hand rule: thuMb = Motion/force, First finger = Field (north to south), seCond finger = Current (conventional, + to −). How the force varies (statement 6.14): a bigger current or a stronger field gives a bigger force; reversing the current OR the field reverses the force; reversing both leaves it unchanged. Double Award only asks you to state and explain this — is single-science only.
Drawn from real examiner reports.
Field strength = spacing, not length
Field-line diagrams are graded on spacing and direction. Strength is shown where lines are closer together — "longer", "bigger" or "more curved" earns nothing. If you add extra lines, keep them equally spaced (crowded, uneven lines cost the mark). Every line needs an arrow (N to S, or circling a wire), and lines never cross.
Nov 2024 1P Q5a: extra field lines with unequal spacing lost MP1. Jun 2024 1P Q6c: strength linked to line length/curvature instead of spacing scored nothing.
Loudspeaker: link current to vibration
For a loudspeaker, connect the current to the sound. A varying (alternating) current in the coil produces a varying force (the motor effect), which makes the coil and the attached cone vibrate; the vibrating cone makes the air vibrate to produce sound. Candidates lose marks without linking the changing current to the vibration of the cone.
Jun 2023 1P Q12c: candidates failed to link the varying current to the varying force and the vibration of the cone.
Fleming: LEFT hand, right direction
Fleming's rule scores only if the direction is right. Use the left hand (the right hand is for the generator effect). Keep the fingers in order — thuMb–force, First–Field, seCond–Current — and use conventional current (+ to −), not electron flow. A frequent slip is knowing the force is vertical but drawing it up instead of down.
Nov 2024 1P Q5b: candidates knew the force was vertical but drew it upwards rather than downwards (right axis, wrong direction).
Motor effect ≠ "cutting field lines"
The motor effect comes from the wire's field interacting with the applied field. Saying the wire "cuts" field lines describes the generator effect (induction) and scores nothing here. Avoid a vague "there is a force": give the mechanism. In a motor, refer to the force on the individual sides of the coil acting in opposite directions.
Nov 2024 1P Q5c: MP2 was often lost for saying the field lines were "cut"; few referred to the force on the individual wires of the coil.
The wire itself is not a magnet
A current-carrying wire experiences a force because its magnetic field interacts with the applied field — but the wire itself is not a magnet and is not a magnetic material. Do not explain the force by claiming the wire "becomes magnetic" or "is attracted like iron". The effect acts on the current, and disappears the instant the current is switched off.
Jun 2023 1P Q12a: the wire experiences a force but is not itself an electromagnet or a magnetic material.
Set the LEFT hand up for the diagram
Point the first finger along the field (N to S) and the second finger along the conventional current; the thumb then gives the force direction. Re-orient your left hand physically to match the actual diagram rather than guessing the direction.
Explain a motor: two fields interact
To explain a force or motor: current makes a field around the wire, this interacts with the permanent field, and the combined field pushes the wire. For a motor, the two sides of the coil feel opposite forces, so it turns. Never say the wire "cuts field lines".
State how the force changes
A larger current or stronger field gives a bigger force; reversing the current or the field (but not both) reverses it, and reversing both leaves it unchanged. State and explain the change — do not just assert it.
Double Award is qualitative — no F = BIL
Double Award treats the motor effect qualitatively: you state and explain what happens to the size or direction of the force, and never calculate a value. is single-science only and out of scope, so a question here will not need a numeric answer.
Double Award treats the motor effect qualitatively — there is no calculation. What you must know are the following relationships and rules.
Fleming's left-hand rule (directions of force, field and current):
| Left hand | Represents |
|---|---|
| Thumb | Thrust — the force / motion of the wire |
| First finger | Field — magnetic field, north to south |
| seCond finger | Current — the conventional current (+ to −) |
How the force changes (statement 6.14):
Double Award is qualitative here: you state and explain what happens to the force, you do not calculate a value ( is single-science only).
What is produced around a wire when a current flows through it, and what shape does it take around a straight wire?
A straight wire carrying a current sits at right angles to a magnetic field, and experiences a force. The magnetic field is kept the same and the current is then increased.
State and explain what happens to the size of the force on the wire.
