Electromagnets, the motor effect and Fleming's left-hand rule
Electromagnets
A current-carrying conductor produces a magnetic field around it. When the conductor is wound into a solenoid (coil of wire), the magnetic field is concentrated inside and resembles the field of a bar magnet.
Factors that increase the strength of an electromagnet:
- Increase the current
- Increase the number of turns (coils)
- Add a soft iron core (greatly amplifies the field — soft iron is easily magnetised and demagnetised)
Advantages over permanent magnets: can be switched on/off; strength is controllable.
Applications: electric bells, relays, scrapyard cranes, MRI machines.
The motor effect
A current-carrying conductor placed in a magnetic field experiences a force. This is the motor effect.
The direction of the force is given by Fleming's left-hand rule (FLHR):
- First finger → Field (direction of magnetic field, N to S)
- Second finger → Current (conventional current direction, + to −)
- Thumb → Motion (direction of force on the conductor)
All three are at right angles to each other.
Force on a current-carrying conductor
$$F = BIL$$
Where:
- F = force (N)
- B = magnetic flux density (T — tesla)
- I = current A
- L = length of conductor in the field (m)
Example: A wire of length 0.5 m carrying a current of 2 A is placed in a magnetic field of flux density 0.3 T. Calculate the force. F = BIL = 0.3 × 2 × 0.5 = 0.3 N
Increasing the force:
- Increase B (stronger magnet)
- Increase I (larger current)
- Increase L (longer conductor in field)
The electric motor
A simple DC motor uses the motor effect:
- Current flows through a coil in a magnetic field.
- Forces on opposite sides of the coil act in opposite directions → the coil rotates.
- A split-ring commutator reverses the current direction every half-turn → the coil continues rotating in the same direction.
- Carbon brushes make electrical contact with the spinning commutator.
Increasing motor speed:
- Increase current
- Increase magnetic field strength
- Increase number of turns on the coil
Relay
A relay is an electrically operated switch:
- Small current in the control circuit → electromagnet magnetised.
- Electromagnet attracts an iron armature → closes contacts in the high-power circuit.
- Allows a small current to control a large current (e.g. car ignition, circuit breakers).
⚠Common mistakes
- FLHR is for conventional current (positive to negative), not electron flow. If the question gives electron flow, reverse the second finger.
- FLHR vs FRHR: Left-hand rule = motor effect (force on conductor). Right-hand rule = generator effect (induced current direction).
- The force is zero when the conductor is parallel to the field — the wire must have a component perpendicular to B for a force to exist.
- Soft iron ≠ steel: soft iron is used in electromagnet cores because it demagnetises easily when current is removed. Steel retains magnetism (used for permanent magnets).
AI-generated · claude-opus-4-7 · v3-edexcel-combined-science