📖 In-Depth Theory
The Motor Effect
When a CURRENT-CARRYING CONDUCTOR is placed in a MAGNETIC FIELD, it experiences a FORCE.
This is called the MOTOR EFFECT.
The force occurs because the magnetic field of the current interacts with the external magnetic field — the two fields combine to create a stronger field on one side and weaker on the other, pushing the conductor towards the weaker side.
FACTORS AFFECTING THE SIZE OF THE FORCE:
MAGNITUDE OF CURRENT — larger current → larger force.
MAGNETIC FLUX DENSITY (B) — stronger field → larger force.
LENGTH of conductor in the field — longer conductor → larger force.
EQUATION:
F = B × I × l
F = force (N)
B = magnetic flux density (tesla, T)
I = current (A)
l = length of conductor in the field (m)
The force is MAXIMUM when the conductor is PERPENDICULAR to the field.
If the conductor is PARALLEL to the field — no force.
The force direction can be REVERSED by:
Reversing the current direction, OR
Reversing the magnetic field direction.
Fleming's Left-Hand Rule
FLEMING'S LEFT-HAND RULE predicts the direction of force on a current-carrying conductor in a magnetic field.
Hold the left hand with three fingers mutually perpendicular:
FIRST FINGER (index) → direction of MAGNETIC FIELD (N to S)
SECOND FINGER (middle) → direction of CONVENTIONAL CURRENT (+ to −)
THUMB → direction of FORCE (MOTION) on the conductor
Memory: FBI — First finger = Field, seCond finger (B ignored? no) —
Simpler: F(orce) = thuMb, B(field) = First finger, I(current) = seCond finger.
All three must be at right angles to each other.
EXAMPLE:
Horizontal wire carrying current to the right, in a magnetic field pointing upward:
Field = upward (first finger up)
Current = right (second finger right)
Force = out of the page (thumb points out) — conductor is pushed towards you.
RIGHT-HAND RULE is used for GENERATORS (opposite — motion given, find induced current direction).
Applications of the Motor Effect
The motor effect is the basis of ELECTRIC MOTORS and other electromagnetic devices.
LOUDSPEAKERS:
A coil of wire (voice coil) is attached to a paper cone and sits in a magnetic field.
Alternating current through the coil → alternating force → cone vibrates → produces sound.
Frequency of current = frequency of sound produced.
DC MOTORS (covered in next subtopic):
A coil in a magnetic field — motor effect creates a turning force (torque).
METER MOVEMENTS (galvanometers):
Current through a coil in a magnetic field → coil rotates → needle deflects.
The deflection is proportional to the current — used as a current measuring device.
MAGLEV TRAINS:
Electromagnets in the track interact with superconducting magnets in the train.
Motor effect provides both levitation and propulsion.
PARTICLE ACCELERATORS:
Charged particles moving through magnetic fields experience forces (motor effect for moving charges).
This curves the path of particles in cyclotrons and synchrotrons.
⚠️ Common Mistake
Fleming's LEFT-hand rule is for MOTORS (force on current in a field). Fleming's RIGHT-hand rule is for GENERATORS (induced current from motion in a field). Students often use the wrong hand. Remember: Left = motor effect (current → force). Right = generator effect (motion → current).