Electromagnetism & The Motor EffectEdexcel GCSE Physics: Revision notes
Section 1
How does a current create a magnetic field?
A current flowing through a long straight conductor (wire) creates a magnetic field around it.
- The field forms concentric circles around the wire
- The direction of the field depends on the direction of the current — reversing the current reverses the field direction
- The strength of the field depends on: the size of the current (larger current = stronger field), and the distance from the conductor (further away = weaker field)
This can be shown experimentally by placing plotting compasses or iron filings around a current-carrying wire.
Section 2
What is a solenoid and how does it produce a strong field?
A solenoid is a coil of wire carrying a current. Inside the solenoid, the magnetic fields produced by each individual turn (coil) of the wire add together, producing a very strong, almost uniform magnetic field along its centre.
Outside the solenoid, the fields from the individual coils cancel out more, giving a much weaker field there. The overall field pattern of a solenoid resembles that of a bar magnet, with clear north and south poles at its ends.
Think of a solenoid as many single-wire fields stacking up like layers of paint — inside the coil they combine into one strong, even field.
Section 3
What is the motor effect? (Higher tier)
When a current-carrying conductor is placed in a magnetic field, it experiences a force — this is called the motor effect. By Newton's third law, an equal and opposite force also acts on the magnet producing the field.
This force occurs because the magnetic field created by the current interacts with the external magnetic field it is placed in — magnetic forces are the result of interactions between magnetic fields.
Fleming's left-hand rule is used to find the relative directions of the force, current and magnetic field, when they are mutually perpendicular:
- Thumb = Thrust (force)
- First finger = Field
- seCond finger = Current
Students often use the right hand instead of the left for Fleming's rule — the motor effect specifically uses the LEFT-hand rule (the right-hand rule is used elsewhere, for generators/induction).
Section 4
How is the force on a conductor calculated? (Higher tier)
F = B × I × l
- F = force (N)
- B = magnetic flux density (T)
- I = current (A)
- l = length of the conductor in the field (m)
This equation only applies when the current and magnetic field are perpendicular to each other.
A 0.5 m wire carrying a current of 3 A in a field of flux density 0.2 T experiences a force F = 0.2 × 3 × 0.5 = 0.3 N.
Section 5
How is the motor effect used in an electric motor? (Higher tier, Physics only)
An electric motor uses the motor effect to cause continuous rotation. A current-carrying coil is placed in a magnetic field; the force on each side of the coil (in opposite directions, due to the current flowing in opposite directions on each side) creates a turning effect (a pair of moments), causing the coil to rotate. This rotation is used to turn machinery, fans, and other devices.
Must Know
- A current in a long straight conductor creates a circular magnetic field around it
- Field strength increases with current and decreases with distance from the conductor
- A solenoid's fields add together inside (strong, uniform) and cancel outside (weak)
- Motor effect: a current-carrying conductor in a magnetic field experiences a force (higher tier)
- Fleming's LEFT-hand rule gives the relative directions of force, field and current (higher tier)
- F = B × I × l calculates the force on a current-carrying conductor (higher tier)
- In an electric motor, forces on opposite sides of a current-carrying coil cause continuous rotation (higher tier, Physics only)
That's the notes covered.
Carry on to the next subtopic.