AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
For a conductor at right angles to a magnetic field and carrying a current:
Full specification wording
Fleming's left-hand rule (HT only): For a conductor at right angles to a magnetic field and carrying a current:
When a current-carrying conductor sits at right angles to a magnetic field, the fields interact to produce a force on the conductor.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 4 of 10
Study notes
From the checked teaching notes for this specification point.
The idea
When a current-carrying conductor sits at right angles to a magnetic field, the fields interact to produce a force on the conductor. The force is perpendicular to both the current and the magnetic field. The magnitude of this force is calculated using the equation F = B I l, where F is the force in newtons (N), B is the magnetic flux density in tesla (T), I is the current in amperes (A), and l is the length in metres (m). For example, a 2.0 m wire carrying 3.0 A in a 0.5 T field experiences a force of 3.0 N. Fleming's left-hand rule predicts the force direction: point the first finger along the field, the second finger along the current, and the thumb shows the force. If the conductor is parallel to the field, no force acts.
What earns the marks
- States that the force acts at right angles to both the magnetic field and the current direction.
- Identifies the equation F = B I l to calculate the force on a conductor at right angles to a magnetic field.
- Defines the terms in the equation: F is force in newtons, B is magnetic flux density in tesla, I is current in amperes, and l is length in metres.
- Describes the left-hand rule: first finger = magnetic field (N to S), second finger = current, thumb = force/motion.
- Recognises that if the conductor is parallel to the field, the force is zero.
Common mistakes and how to put them right
Omitting the equation F = B I l when asked to calculate the force.
Put it right: Is to recall and apply the formula with correct units.
Using the right hand instead of the left hand.
Put it right: The left hand is used for the motor effect, while the right hand is used for the generator effect.
Pointing the first finger along the current and the second finger along the field.
Put it right: The first finger is the field and the second finger is the current.
Exam tips
- Always check that length is in metres and current is in amperes before substituting into F = B I l.
- Sketch a simple diagram with the field, current and force arrows labelled at right angles to each other to secure method marks.
- When asked to describe the rule, name each finger and what it represents in a clear sequence.
Try the lesson
The start of the Fleming's left-hand rule (HT only): For a conductor at right angles to a magnetic field and carrying a current: lesson, made from checked teaching notes for this specification point.
Have a go first
A conductor is placed at right angles to a magnetic field and carries a current. Which equation correctly links the force on the conductor to the magnetic flux density, current and length?
Listen
When a current-carrying conductor sits at right angles to a magnetic field, the fields interact to produce a force on the conductor. The force is perpendicular to both the current and the magnetic field. The magnitude of this force is calculated using the equation F = B I l, where F is the force in newtons (N), B is the magnetic flux density in tesla (T), I is the current in amperes (A), and l is the length in metres (m). For example, a 2.0 m wire carrying 3.0 A in a 0.5 T field experiences a force of 3.0 N. Fleming's left-hand rule predicts the force direction: point the first finger along the field, the second finger along the current, and the thumb shows the force. If the conductor is parallel to the field, no force acts. A strong answer earns marks for these points. States that the force acts at right angles to both the magnetic field and the current direction. Identifies the equation F = B I l to calculate the force on a conductor at right angles to a magnetic field. Defines the terms in the equation: F is force in newtons, B is magnetic flux density in tesla, I is current in amperes, and l is length in metres. Describes the left-hand rule: first finger = magnetic field (N to S), second finger = current, thumb = force/motion. A common slip: Omitting the equation F = B I l when asked to calculate the force: the error prevents finding the magnitude. Instead, is to recall and apply the formula with correct units. Exam tip: Always check that length is in metres and current is in amperes before substituting into F = B I l.
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