Motor effect: F = BIL
A current-carrying conductor in a magnetic field experiences a force (the motor effect): , with in N, = magnetic flux density (T), = current (A), = length in the field (m). The three factors multiply — double any one and doubles. It is greatest when the conductor is perpendicular to the field, zero when parallel, and always acts at right angles to BOTH current and field. Example: T, A, m gives N.
Fleming's left-hand rule (FBI)
Fleming's left-hand rule gives the force direction: on the LEFT hand, held mutually at right angles — thumb = Force (motion), index = magnetic Field (N to S), middle = conventional Current (+ to −). The mnemonic FBI runs thumb→index→middle. It applies to conventional (positive) current; for electrons the force is reversed. Use the left hand for motors; the right hand is for generators.
d.c. motor and the commutator
A d.c. motor has a rectangular coil on an axle between magnet poles; current makes opposite sides feel opposite forces (Fleming's LHR), giving a turning effect. The split-ring commutator reverses the current through the coil every half-turn, so the forces keep turning the coil the same way — continuous rotation. Without it the coil would just oscillate. Increase the turning effect with more current, a stronger field, or more turns.
Drawn from real examiner reports.
Motor effect needs current AND field
Writing "a wire in a magnetic field experiences a force" scores zero — a wire with no current feels nothing. State both conditions: "a current-carrying conductor placed in a magnetic field experiences a force". For to apply directly, the conductor must be perpendicular to the field.
June 2024 Paper 2P Q6(b) — many candidates could state the rule but not apply it precisely; the same imprecision appears on definition questions across the paper.
Use the LEFT hand, not the right
The motor effect uses Fleming's left-hand rule. The right hand is for the generator effect (induction). Using the right hand flips the force by 180°, giving exactly the wrong direction. Say which hand you are using in your working.
June 2024 Paper 2P Q6(b)(i): many candidates gave an incorrect force direction despite being reminded to use the left-hand rule, suggesting systematic hand/finger confusion.
Use conventional current, not electrons
The middle finger must point along conventional current (from + to − outside the cell), not electron flow (− to +). Using the electron-flow direction reverses the predicted force by 180°. Mark the conventional-current arrow on the diagram before applying the rule.
F = BIL: length in metres
In the length must be in metres. Substituting a length in centimetres (e.g. 15 cm instead of 0.15 m) makes the force 100 times too large. Convert cm to m first, then substitute, and give the force in newtons.
Commutator reverses, not switches off
"The commutator switches off the current" or "breaks the circuit" is wrong — it reverses the direction of the current in the coil every half-turn. That reversal keeps the forces turning the coil the same way; switching off would just let it stop. Mark-scheme wording: "reverses the current every half-turn".
Commutator vs slip rings
A split-ring commutator (d.c. motor) reverses the current every half-turn. Slip rings (a.c. generator) are unbroken and do NOT reverse the current — the a.c. output alternates on its own. Don't say the commutator keeps the current flowing; that is a slip ring.
Annotate before applying the LHR
For "state the direction of the force": (1) mark field B (N to S); (2) mark conventional current I (+ to −); (3) hold the LEFT hand, index along B, middle along I; (4) read the thumb as F. State the direction in clear words (up/down/into the page).
F = BIL calculation scaffold
Write , then substitute with in metres, in amps and in tesla, and give the answer in newtons. For a rearrangement, isolate the unknown first — e.g. — keeping the denominator as one product .
State direction in spatial words
Give the force direction with a clear spatial reference — "upward", "into the page", "to the left" — not vague words like "forward" or "away". The examiner needs an unambiguous direction to award the mark.
Name which factor increases the force
For "how would you increase the force", name a specific factor from : a stronger field (larger ), a bigger current (), or more length/turns (). "Increase the force" without saying how earns nothing.
where = force on the conductor (N), = magnetic flux density (T), = current (A), = length of conductor in the field (m).
Conditions: the conductor must carry a current AND be inside a magnetic field. The formula gives the maximum force when the conductor is perpendicular to the field. Force is zero when the conductor is parallel to the field.
Proportionality: , , — doubling any one factor doubles the force.
Define the motor effect.
A straight wire of length 0.12 m is placed perpendicular to a uniform magnetic field of flux density 0.25 T. The wire carries a current of 3.0 A.
Calculate the force acting on the wire. Give your answer in newtons.
[2 marks]
