GCSE Physics revision

Fleming's left-hand rule (HT only)

The motor effect

AQA 4.7.2.2

Your specification

AQA student objectives

Learning pathway

ALL 🎯

Show that Fleming's left-hand rule represents the relative orientation of the force, the current in the conductor and the magnetic field.

MOST 🎯🎯

Apply the specified scientific knowledge of fleming's left-hand rule to a relevant example.

SOME 🎯🎯🎯

Analyse an unfamiliar example of fleming's left-hand rule using the specified scientific ideas.

Revision summary

Read on screen, then print for Cornell-style active revision.

The motor effect is the phenomenon where a current-carrying wire placed in an external magnetic field experiences a force.
The wire's own magnetic field (produced by the current) interacts with the external magnetic field, effectively pushing the wire out of the field.
The force is greatest when the wire is at exactly 90° to the magnetic field, and zero when the wire runs parallel to the field.

1. What is the Motor Effect?AQA

The motor effect is the phenomenon where a current-carrying wire placed in an external magnetic field experiences a force.
The wire's own magnetic field (produced by the current) interacts with the external magnetic field, effectively pushing the wire out of the field.
The force is greatest when the wire is at exactly 90° to the magnetic field, and zero when the wire runs parallel to the field.

Fleming's left-hand rule is used to determine the direction of the force on a current carrying wire in a magnetic field.
Hold your left hand with your thumb pointing upwards, your first (index) finger pointing forwards, and your second (middle) finger pointing to the side.
Your first finger represents the direction of the magnetic field (from north to south pole), your second finger represents the direction of the current, and your thumb indicates the direction of the force.
Practising the hand position several times is key — you can tilt your whole arm or body to help align the fingers correctly.

2. Fleming's Left-Hand RuleAQA

Fleming's left-hand rule is used to determine the direction of the force on a current carrying wire in a magnetic field.
Hold your left hand with your thumb pointing upwards, your first (index) finger pointing forwards, and your second (middle) finger pointing to the side.
Your first finger represents the direction of the magnetic field (from north to south pole), your second finger represents the direction of the current, and your thumb indicates the direction of the force.
Practising the hand position several times is key — you can tilt your whole arm or body to help align the fingers correctly.

To apply Fleming's left-hand rule correctly, you must first identify the direction of the magnetic field, which runs from the north pole to the south pole.
You must also identify the direction of conventional current flow in the wire before determining the direction of the force.
Changing either the direction of the current or the direction of the magnetic field will reverse the direction of the force on the wire.

3. Direction of the Magnetic Field and CurrentAQA

To apply Fleming's left-hand rule correctly, you must first identify the direction of the magnetic field, which runs from the north pole to the south pole.
You must also identify the direction of conventional current flow in the wire before determining the direction of the force.
Changing either the direction of the current or the direction of the magnetic field will reverse the direction of the force on the wire.

A common demonstration of the motor effect uses a horseshoe magnet with three metal rails, where two outer rails are fixed and a central rail is free to roll.
When the power source is switched on, the current in the rails generates a magnetic field that interacts with the horseshoe magnet's field, producing a force on the free rail.
Using Fleming's left-hand rule — with the field pointing downward (north to south) and the current flowing towards you — the resulting force acts to the right, causing the rail to roll.

4. The Motor Effect in a Horseshoe MagnetAQA

A common demonstration of the motor effect uses a horseshoe magnet with three metal rails, where two outer rails are fixed and a central rail is free to roll.
When the power source is switched on, the current in the rails generates a magnetic field that interacts with the horseshoe magnet's field, producing a force on the free rail.
Using Fleming's left-hand rule — with the field pointing downward (north to south) and the current flowing towards you — the resulting force acts to the right, causing the rail to roll.

The force on a current-carrying wire at right angles to a magnetic field is calculated using the equation F = BIL, where F is force in Newtons (N), B is magnetic flux density in Teslas (T), I is current in Amperes (A), and L is the length of the wire in metres (m).
Magnetic flux density (B) is the scientific term for magnetic field strength and is measured in Teslas (T).
This equation is provided in the GCSE exam, so you do not need to memorise it, but you must be able to apply it correctly.

5. The Equation for Force on a WireAQA

The force on a current-carrying wire at right angles to a magnetic field is calculated using the equation F = BIL, where F is force in Newtons (N), B is magnetic flux density in Teslas (T), I is current in Amperes (A), and L is the length of the wire in metres (m).
Magnetic flux density (B) is the scientific term for magnetic field strength and is measured in Teslas (T).
This equation is provided in the GCSE exam, so you do not need to memorise it, but you must be able to apply it correctly.

A 10 cm wire carrying a 5 A current is placed at 90° in a magnetic field with a magnetic flux density of 0.4 T.
The length must first be converted into metres: 10 cm ÷ 100 = 0.1 m, ensuring all values are in the correct SI units before substituting into the equation.
Substituting into F = BIL: F = 0.4 × 5 × 0.1 = 0.2 N, so the wire experiences a force of 0.2 Newtons.

6. Worked Example: Calculating the ForceAQA

A 10 cm wire carrying a 5 A current is placed at 90° in a magnetic field with a magnetic flux density of 0.4 T.
The length must first be converted into metres: 10 cm ÷ 100 = 0.1 m, ensuring all values are in the correct SI units before substituting into the equation.
Substituting into F = BIL: F = 0.4 × 5 × 0.1 = 0.2 N, so the wire experiences a force of 0.2 Newtons.

The wire must be at 90° to the magnetic field to experience the maximum force as given by F = BIL.
If the wire is at an angle less than 90° to the field, the force experienced will be reduced.
If the wire runs parallel to the magnetic field (0°), it experiences no force at all.