Fleming's left-hand rule (HT) — AQA GCSE Physics

SPEC 4.7.2.2 • MAGNETISM & ELECTROMAGNETISM

🔵 HIGHER TIER ONLY

Fleming's Left-Hand Rule

A current-carrying wire in a magnetic field feels a force. The motor effect. Fleming's left hand gives the direction; F = BIl gives the size.

What This Lesson Covers HT

Three things, each tested every year on Paper 2.

The Motor Effect

A current-carrying wire in a magnetic field experiences a force.

F=BIl

Size of the Force

Equation on the equation sheet. Units: N, T, A, m.

Direction of Force

Fleming's left-hand rule. Three fingers, three quantities, mutually perpendicular.

Why It Matters

This is the principle behind every electric motor (next lesson), every loudspeaker, and is the synoptic counterpart to the generator effect in 9.01. Get this lesson and the next two are easy.

The Motor Effect HT

Put a current-carrying wire inside a magnetic field — and something dramatic happens.

Motor Effect — Spec Wording

When a conductor carrying a current is placed in a magnetic field, the magnet producing the field and the conductor exert a force on each other. This is called the motor effect.

Newton's Third Law

The wire feels a force from the magnet — and the magnet feels an equal and opposite force from the wire. Two interacting fields; both objects feel the consequence.

Magnetic Flux Density (B) HT

Before we get to the equation, we need a name for the strength of a magnetic field.

Magnetic Flux Density

A measure of the strength of a magnetic field. Symbol: B. Unit: tesla (T).

A Sense of Scale

One tesla is a very strong field. Real-world examples:

Earth's field: ~ 0.00005 T (~ 50 μT)

A fridge magnet: ~ 0.005 T (~ 5 mT)

A strong bar magnet: ~ 0.1 T

An MRI scanner: 1 – 3 T

A neutron-star magnetar: ~ 10¹⁰ T (cosmic territory)

Memory Hook

Tesla is rarely confused — but make sure you read the question carefully. Sometimes B is given in milliteslas (mT) or microteslas (μT) — convert before substituting.

The Equation: F = B I l HT

For a conductor at right angles to a magnetic field carrying a current, the size of the force is given by:

Spec Equation

F = B × I × l

📎 On Equation Sheet

SymbolQuantityUnit

FForce on the conductornewtons, N

BMagnetic flux densitytesla, T

ICurrent in the conductoramperes, A

lLength of conductor inside the fieldmetres, m

Read The Length Carefully

"l" is the length of the wire inside the magnetic field, not the full length of the wire. If the wire is 1.0 m long but only 0.40 m of it sits between the magnet's poles, use l = 0.40 m.

Geometry: When is the Force Maximum? HT

F = BIl assumes one specific geometry. Get the geometry wrong and the force goes to zero.

MAX Force

Current PERPENDICULAR (at 90°) to the field

F = BIl applies in full. This is the geometry assumed by the equation, and the geometry of nearly every exam question.

ZERO Force

Current PARALLEL to the field

No force at all on the wire. The wire and field don't "interact" in the motor-effect sense when they're aligned.

What About Angles Between?

Between 0° and 90°, the force lies between zero and maximum. At GCSE you don't need to calculate this — but you should know the force decreases as the angle to perpendicular increases. Check every exam diagram: is the wire definitely at right angles to the field?

Fleming's Left-Hand Rule HT

The equation tells us the size of the force. To find the direction, use your left hand.

The Rule

Hold your left hand so the thumb, first finger, and second finger are all at right angles to each other. Then:

First Finger = direction of the Field (N → S)

seCond Finger = direction of the Current (conventional, + to −)

thuMb = direction of the Motion (force on the wire)

Mnemonic

First-finger = Field · seCond-finger = Current · thuMb = Motion. The capital letters spell it out: FBI from top to bottom (Force, B-field, current I).

Reverse an Input → Reverse the Force HT

Examiners love asking what happens when you flip the current — or flip the magnets. The logic is built into Fleming's rule.

What You ChangeEffect On Force Direction

Reverse the current onlyForce reverses (e.g. was upward → now downward)

Reverse the field only (swap N and S poles)Force reverses

Reverse both current and fieldForce is unchanged (two reversals cancel out)

Why This Matters For Motors

This rule is exactly what makes a DC electric motor work: the current in the coil is reversed every half-turn (by a split-ring commutator), so the force direction stays consistent and the coil keeps rotating the same way. You'll meet this fully in 7.04.

Worked Example — F = BIl HT

A copper wire of length 0.50 m carries a current of 4.0 A. The wire lies perpendicular to a uniform magnetic field of flux density 0.30 T. Calculate the force on the wire.

E F = B × I × lS F = 0.30 × 4.0 × 0.50

U Force in newtons (N)

A F = 0.60 N

ESUA Reminder

Equation → Substitute → Unit → Answer. Show all four steps even on calculation questions. Examiners reward each step separately — if you skip straight to "0.60 N", you risk losing the working marks.

Examiner Alert — Which Hand? HT

The single most common error in this entire topic. Fleming's left hand and Fleming's right hand are two different rules for two different effects.

Wrong (loses ALL marks)

Using the right hand for the motor effect.

This gives the opposite direction of force. Every direction in your answer will be wrong.

Right

Always the LEFT hand for the motor effect.

Current in → force out. Left hand. Every time.

How To Pick The Right Hand — Pattern Match

Forget mnemonics — just identify what the question is doing:

If the question gives you a current and asks for a force or motion → motor effect → LEFT hand

If the question gives you motion (a wire moving through a field) and asks for an induced current or pd → generator effect → RIGHT hand

The cause is the input; the effect is the output. Motor: current is input. Generator: motion is input.

Check Yourself Before You Write

Before writing any direction in the exam, ask: does the question start with a current and ask for a force? If yes → motor effect → LEFT hand. (Generator effect is covered in 7.05.)

Factors That Affect the Force HT

Spec wording: "Students should be able to recall the factors that affect the size of the force on the conductor." Four factors come straight out of F = BIl, plus one geometric factor.

FactorEffect on ForceFrom the Equation

Magnetic flux density BLarger B → larger forceF is proportional to B

Current ILarger I → larger forceF is proportional to I

Length l in the fieldLonger wire in the field → larger forceF is proportional to l

Angle between wire and fieldMaximum at 90° · zero at 0°Implicit — F = BIl assumes 90°

Direction Factor (Separate)

Reversing the current or reversing the field reverses the force direction (but does not change its size). The size factors above only change how much force, not which way.

Practice Question 1

A copper rod has a total length of 0.60 m. A section of the rod, 0.40 m long, lies between the poles of a permanent magnet that produces a magnetic field of flux density 0.25 T. The rod is perpendicular to the field and carries a current of 6.0 A.

Calculate the force on the rod.3 marks

E

F = B × I × l

S

Use l = 0.40 m (length inside the field), not 0.60 m.

F = 0.25 × 6.0 × 0.40

U

Force in newtons (N)

A

F = 0.60 N

Key trap: only the part of the rod inside the magnetic field counts. Reading "a section, 0.40 m long, lies between the poles" tells you l = 0.40 m, not 0.60 m.

Practice Question 2

A horizontal copper rod is held loosely between the poles of a permanent magnet. The N pole of the magnet is on the left and the S pole is on the right. A current flows through the rod, directed away from the viewer (into the page).

Use Fleming's left-hand rule to explain in which direction the rod will move.4 marks

P

Identify the directions: the field runs from N to S, so from left to right; the current is going into the page (away from the viewer).

E

Apply Fleming's left-hand rule with the three fingers at right angles: point the first finger to the right (field direction) and the second finger into the page (current direction).

E

With the first finger pointing right and the second finger pointing into the page, the thumb of the left hand points downwards.

C

The thumb gives the direction of motion, so the rod will move downwards.

Compare with the lesson example: in slide 3, with the same field but current out of the page, the force was upwards. Reversing the current direction reverses the force — exactly as predicted in slide 8.

Common Errors to Avoid HT

Three errors examiners flag every year on this topic.

Students Often WriteCorrect VersionWhy It Matters

Use the right hand for the motor effectLEFT hand for the motor effect (current → force)Right hand gives the opposite direction — all direction marks lost

"Force is maximum when current is parallel to field"Force is maximum when current is perpendicular to field; zero when parallelBackwards geometry — reverses what the equation assumes

Using the full wire length for lUse only the length of wire inside the magnetic fieldCommon trap on numerical questions — wrong l → wrong F

Checkpoint HT

You should now be able to do all of the following without prompting.

You Can Now

State the motor effect — a current-carrying conductor in a magnetic field experiences a force

Define magnetic flux density (B) — the strength of the field, measured in tesla (T)

State and apply F = B I l with correct units, using only the length of wire inside the field

State when the force is maximum and when it is zero — max at 90°, zero at 0°

Apply Fleming's left-hand rule — First-Field, seCond-Current, thuMb-Motion

Predict how reversing current or field reverses the force

List the factors affecting the size of the force — B, I, l, angle

Distinguish motor effect (LEFT hand) from generator effect (RIGHT hand)

Coming Next — 7.04 (HT)

Electric motors. The motor effect put to work — a coil of wire rotates in a magnetic field, with a split-ring commutator reversing the current every half-turn. You'll see exactly why Fleming's left-hand rule gives the rotation direction.