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PhysicsInternational GCSEEdexcel & Cambridge (CIE)Years 10–11

IGCSE Physics Magnetism & Electromagnetism Notes

A current creates a magnetic field, and a current in a field feels a force. Learn the motor effect and Fleming's left-hand rule to explain motors.

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Key concepts

Magnetic field
A region where a magnetic material or current feels a force; field lines run from north to south.
Electromagnet
A current-carrying coil (solenoid) with a soft iron core; the field can be switched on and off.
Motor effect
A current-carrying wire in a magnetic field experiences a force, found using Fleming's left-hand rule.
  • The strength of an electromagnet increases with current, number of turns and an iron core.
  • Fleming's left-hand rule: thumb = force, first finger = field, second finger = current.
  • Reversing the current or the field reverses the direction of the force.

Key formulas

Force on a conductor

F = B × I × L

field strength × current × length (Edexcel).

Required practicals

  • Investigate how the force on a current-carrying wire varies with current.
  • Plot the magnetic field around a bar magnet using a plotting compass.

Worked example

State the direction of the force on a wire when current flows up and the field points from left to right.

  1. 1Point the first finger along the field (left to right).
  2. 2Point the second finger along the current (upwards).
  3. 3The thumb then gives the force direction.
  4. 4Read off the thumb's direction (out of or into the page).

Answer: The force acts perpendicular to both, given by the thumb (out of the page).

Common mistakes

✗ Mixing up the fingers in Fleming's left-hand rule.

✓ ThuMb = Motion/force, First = Field, seCond = Current.

✗ Forgetting an iron core strengthens an electromagnet.

✓ A soft iron core greatly increases the field strength.

✗ Thinking the force is parallel to the current.

✓ The force is perpendicular to both the current and the field.

Quick check

Try these, then reveal the answers.

What three things increase an electromagnet's strength?Reveal answer

More current, more turns and an iron core.

In Fleming's left-hand rule, what does the thumb show?Reveal answer

The direction of the force (motion).

What happens to the force if the current is reversed?Reveal answer

The force reverses direction.

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Exam tips

  • Set up Fleming's left-hand rule carefully — practise so it is quick in the exam.
  • State that the force is perpendicular to both field and current.
  • Link electromagnet strength to current, turns and core material.

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Frequently asked questions

What is the motor effect?+

The force experienced by a current-carrying conductor placed in a magnetic field.

How do you find the direction of the force?+

Use Fleming's left-hand rule: first finger field, second finger current, thumb force.

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