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).
📐 Variables
FForce on conductor (F) is measured in newtons (N)
BMagnetic flux density (B) is measured in tesla (T)
ICurrent (I) is measured in amperes (A)
lLength of conductor in field (l) is measured in metres (m)
📐 Key Equations
F = B × I × l
📌 Key Note
Motor effect: force on current-carrying conductor in magnetic field. F = BIl. Fleming's LEFT-hand rule: First finger = Field, seCond = Current, thuMb = Motion/Force. Force maximised when conductor ⊥ field. Reverse current or field → reverse force. Applications: loudspeakers, motors, meters.
🎯 Matching Activity — Fleming's Left-Hand Rule
Match each finger to what it represents in Fleming's Left-Hand Rule. — drag the symbols on the right to match the component names on the left.
First finger (index)
Drop here
Second finger (middle)
Drop here
Thumb
Drop here
F = BIl
Drop here
Direction of conventional CURRENT — points from + to −
Direction of FORCE (motion) on the conductor
Force increases with stronger field, larger current, longer conductor
Direction of MAGNETIC FIELD — points from N to S
⚽ FIFA Worked Examples
Motor Effect Force
A wire of length 0.05 m carries a current of 4 A in a magnetic field of flux density 2 T (perpendicular to the wire). Calculate the force on the wire.
F
F = B × I × l
I
B = 2 T, I = 4 A, l = 0.05 m
F
F = 2 × 4 × 0.05 = 2 × 0.2
A
F = 0.4 N
🎯 Test Yourself
Question 1 of 2
1. A horizontal wire carries current to the right in a downward magnetic field. Using Fleming's Left-Hand Rule, in which direction is the force on the wire?
2. How can the direction of force on a current-carrying conductor in a magnetic field be reversed?
⭐ How Well Do You Understand This Topic?
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