State Fleming's left-hand rule.

With the first finger pointing along the field, the second finger along the conventional current, and the thumb at right angles to both, the thumb gives the direction of the force.

Why is the force always at right angles to both the current and the field?

The three directions are mutually perpendicular, which is exactly what Fleming's left-hand rule expresses.

What is the origin of the force on the wire?

The field of the current interacts with the applied field, so the combined field is stronger on one side of the wire than the other and the wire is pushed towards the weaker side.

CCEA A-Level Physics 1210

Physical Quantities, Scalars and Vectors

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Magnetic Fields

The force on a current-carrying conductor

3 things to know
Unit A2 2

Overview

Fleming's left-hand rule, origin of the force on the wire and experiment to show the force on a current-carrying conductor.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

State the condition for a current-carrying wire in a magnetic field to experience a force.

Key term

When is the force on a current-carrying wire in a magnetic field zero?

Key term

Which hand is used for the motor effect, and which for induction?

Questions from this subtopic

Each one with the model answer underneath.

1
State the condition for a current-carrying wire in a magnetic field to experience a force.
Model answer
The wire must have a component at an angle to the field, and the force is greatest when the wire is at right angles to the field.
2
When is the force on a current-carrying wire in a magnetic field zero?
Model answer
When the wire lies parallel to the field, so there is no component of the current perpendicular to it.
3
State Fleming's left-hand rule.
Model answer
With the first finger pointing along the field, the second finger along the conventional current, and the thumb at right angles to both, the thumb gives the direction of the force.
4
Which hand is used for the motor effect, and which for induction?
Model answer
The left hand for the force on a current in a field, and the right hand for the direction of an induced current.
5
Why is the force always at right angles to both the current and the field?
Model answer
The three directions are mutually perpendicular, which is exactly what Fleming's left-hand rule expresses.
6
What is the origin of the force on the wire?
Model answer
The field of the current interacts with the applied field, so the combined field is stronger on one side of the wire than the other and the wire is pushed towards the weaker side.

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