The magnetic effect of a current and the motor
Electricity makes magnetism
- A wire carrying a current creates a magnetic field around itself.
- Put that wire in another magnet's field and it feels a force — the idea behind every electric motor.
- This is where electricity and magnetism meet.
The magnetic effect of a current
- A current in a straight wire makes circular field lines around it; reverse the current and the field reverses.
- A solenoid (a long coil) makes a field just like a bar magnet, with a N and a S pole.
- Make it stronger by: more current, more turns, or a soft-iron core.
- Used in relays and loudspeakers.
The magnetic field of a solenoid (a long coil carrying current) looks like the field of:
A solenoid produces a bar-magnet-like field, with a north pole at one end and a south at the other.
Which change makes an electromagnet stronger?
More current, more turns, and a soft-iron core all make the field stronger.
The motor effect
- A current-carrying wire in a magnetic field feels a force — the motor effect.
- Reverse the current, or reverse the field, and the force reverses.
- A bigger current or stronger field gives a bigger force; the force is greatest when the wire is at right angles to the field.
- Find the direction with the left-hand rule: thumb = force, first finger = field (N→S), second finger = current.
In the motor effect, reversing the current direction:
Reversing either the current or the field reverses the force. Reversing both leaves it unchanged.
In Fleming's left-hand rule, the thumb points in the direction of the:
Thumb = force, first finger = field, second finger = current — all at right angles.
The force on a current-carrying wire is greatest when the wire is at right angles to the magnetic field.
The force is largest when the wire is perpendicular to the field, and zero when the wire lies along the field.
The d.c. motor
- A current-carrying coil in a field feels a turning effect: the forces on its two sides act in opposite directions.
- A split-ring commutator swaps the current direction every half-turn, so the coil keeps spinning the same way.
- The turning effect is bigger with more turns, a bigger current, or a stronger field.
What is the job of the split-ring commutator in a d.c. motor?
Without the commutator the coil would stop after half a turn. Swapping the current each half-turn keeps it rotating the same way.
You've got it
- a current makes a magnetic field: circles round a wire, bar-magnet field for a solenoid
- stronger electromagnet: more current, more turns, soft-iron core
- motor effect: a wire carrying current in a field feels a force (left-hand rule)
- a d.c. motor spins; the split-ring commutator reverses the current each half-turn