Fleming's left hand rule and the motor effect

Fleming's left hand rule tells you which way a current-carrying wire is pushed when it sits in a magnetic field. Hold your left hand with the thumb, first finger and second finger all at right angles to each other. Point your First finger along the magnetic Field (north to south) and your seCond finger along the Current (positive to negative). Your thuMb then points in the direction of the force, and so the Motion.

The motor effect

When a wire carrying a current is placed in a magnetic field, the wire and the magnet exert a force on each other. This is called the motor effect. The current makes its own magnetic field around the wire, and that field interacts with the magnet's field, pushing the wire sideways.

The force is biggest when the wire is at right angles to the field, and there is no force at all when the wire runs parallel to the field. Fleming's left hand rule shows how the three directions fit together, for the case where the wire is at right angles to the field.

How to use Fleming's left hand rule

1. Hold out your left hand, with your thumb pointing up, your first finger pointing forwards and your second finger bent to point sideways, so all three are at right angles.
2. Turn your hand until your first finger points along the field, from N to S.
3. Keep that, and rotate your hand about your first finger until your second finger points along the current.
4. Read the force off your thumb.

The memory aid is in the capital letters: First finger, Field; seCond finger, Current; thuMb, Motion. Some people use FBI instead: thumb for Force, first finger for B (the symbol for magnetic field), second finger for I (the symbol for current).

Worked examples: finding the direction

In diagrams, a dot (⊙) means the current is coming out of the page towards you, and a cross (⊗) means it is going into the page, away from you. Think of the point and the tail feathers of an arrow.

Example 1: field to the right, current into the page

A magnetic field points from left to right across the page. A wire carries current into the page. Which way is the force?

First finger points right (field). Second finger points into the page (current). Your thumb points down the page.

Example 2: field down the page, current to the right

The north pole is at the top and the south pole at the bottom, so the field points down the page. The current flows from left to right. First finger down, second finger right: the thumb points into the page.

Example 3: an electron beam

A beam of electrons moves to the right through a magnetic field pointing into the page. Which way is the beam pushed?

Electrons are negative, so the conventional current is in the opposite direction to their motion: to the left. First finger into the page, second finger to the left: the thumb points down the page. Forgetting to reverse the direction for electrons is a classic exam trap.

Try one yourself before moving on.

How big is the force? F = BIl

The direction comes from Fleming's left hand rule. The size of the force comes from this equation, for a wire at right angles to the field:

So the force gets bigger with a stronger magnet, a bigger current or a longer wire in the field. Reversing either the current or the field reverses the force. Reversing both leaves it the same.

Example 4: calculating the force

A 0.05 m length of wire with a current of 3 A lies across a 0.2 T magnetic field, at 90° to it. Find the force on the wire.

Example 5: finding the flux density

A 0.12 m length of wire carries 4 A at right angles to a magnetic field and feels a force of 0.024 N. Find the flux density.

At an angle (A-level)

If the wire is at an angle θ to the field, only the part of the current across the field counts, and the equation becomes F = BIl sin θ. At 90°, sin θ = 1 and you are back to F = BIl. At 0°, parallel to the field, sin θ = 0 and there is no force. A 0.1 m wire carrying 2 A at 30° to a 0.5 T field feels a force of 0.5 × 2 × 0.1 × sin 30° = 0.05 N.

How an electric motor uses the rule

A simple DC motor is a coil of wire between the poles of a magnet. Current flows along one side of the coil in one direction and back along the other side in the opposite direction. By Fleming's left hand rule, the two sides are pushed in opposite directions, one up and one down, so the coil turns.

After half a turn, the sides would be pushed back the other way and the coil would stop. A split ring commutator prevents that: it swaps the connections every half turn, reversing the current in the coil so the forces keep turning it the same way. To make a motor spin faster, increase the current, use a stronger magnet or add more turns to the coil. The Field Lines Simulator shows the magnetic field around magnets, which helps you picture the field direction before you use the rule.

Fleming's left hand rule and right hand rule

Fleming also gave a right hand rule, for generators rather than motors. When a wire is moved through a magnetic field, a current is induced in it, and the right hand rule gives that current's direction, with the same finger meanings: thumb for the motion, first finger for the field and second finger for the induced current. To keep them apart, remember that the left hand is for the motor effect and the right hand for the generator effect. AQA's GCSE specification asks you to use the left hand rule for the motor effect; the right hand rule is not required there, though the generator effect is.

Common mistakes

• Using the right hand. The motor effect needs the left hand.
• Getting the field direction backwards. Magnetic field lines go from north to south outside a magnet.
• Using electron flow for the current. Use conventional current, + to −. For a beam of electrons, the current is opposite to their motion.
• Mixing up the fingers. First finger is Field, seCond is Current, thuMb is Motion.
• Applying the rule to a wire parallel to the field. There is no force then.
• Length outside the field. In F = BIl, l is only the length of wire inside the field.

Fleming's left hand rule at GCSE and A-level

In AQA's GCSE Physics specification, you need to show that Fleming's left hand rule represents the relative directions of the force, the current and the magnetic field, and recall the factors that affect the size of the force. F = BIl is on AQA's equations sheet, checked in November 2026, marked as Higher tier. At A-level, the rule returns for the force on a current, the force on a moving charge, F = BQv, and the circular paths of charged particles in magnetic fields.

Practise in a short timed paper

GCSE Physics Higher and A-level Physics: Year 2 each give you a short timed paper every day, with 8 questions worth 25 to 35 marks across the course, including magnetism and electromagnetism. The Electricity course covers current, charge and fields from the start.

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