Force on a Current-Carrying ConductorCambridge IGCSE Physics: Revision notes
Section 1
The motor effect
When a current-carrying wire is placed inside an external magnetic field, the wire experiences a force. This is called the motor effect, and it arises because the wire's own magnetic field (from the current) interacts with the external field.
This can be demonstrated experimentally: place a wire between the poles of a strong magnet, pass a current through it, and observe the wire move. Reversing either the current direction or the magnetic field direction reverses the direction of the force; reversing both leaves the force direction unchanged.
Reverse either the current or the field to reverse the force — but reverse both together and the force direction stays the same.
Section 2
Relative directions: force, field and current
The directions of the force, the magnetic field and the current are all mutually at right angles to one another. Given any two of these directions, the third can be worked out.
This relationship also applies to a single moving charged particle (not just a wire): a beam of charged particles moving through a magnetic field experiences a force perpendicular to both its velocity and the field, causing it to curve.
A beam of electrons fired through a magnetic field curves into a circular path because the force is always perpendicular to its direction of travel.
Section 3
Turning effect on a current-carrying coil
If a coil of wire carrying a current is placed in a magnetic field, the forces on the two sides of the coil (which carry current in opposite directions relative to the field) act in opposite directions, creating a turning effect on the coil.
This turning effect is increased by increasing:
- The number of turns on the coil
- The size of the current
- The strength of the magnetic field
Section 4
The electric motor
An electric motor uses the turning effect on a current-carrying coil in a magnetic field to produce continuous rotation. Key parts:
- The coil is mounted so it can rotate freely between the poles of a magnet
- A split-ring commutator reverses the direction of the current in the coil every half-turn
- Brushes maintain electrical contact between the fixed circuit and the rotating commutator
Without the commutator reversing the current at the right moment, the coil would simply oscillate back and forth rather than spinning continuously — reversing the current every half-turn keeps the turning force acting in the same rotational direction.
Don't confuse the split-ring commutator (used in a d.c. motor to keep spinning one way) with slip rings (used in an a.c. generator, which do not reverse the connection).
Must Know
- A current-carrying conductor in a magnetic field experiences a force — the motor effect
- Reversing the current or the field reverses the force direction; reversing both leaves it unchanged
- Force, field and current directions are all mutually at right angles
- A current-carrying coil in a magnetic field experiences a turning effect, increased by more turns, more current, or a stronger field
- An electric motor uses this turning effect, with a split-ring commutator reversing the current every half-turn to keep the coil spinning in one direction
- Brushes maintain contact between the fixed circuit and the rotating commutator
That's the notes covered.
Carry on to the next subtopic.