When a current carrying conductor is placed in the magnetic field, conductor experiences a force due to the magnetic field. While when a conductor moved in a magnetic field, an emf is induced across the conductor according to Faraday’s law of electromagnetic induction.
J. A. Fleming introduced two important rules to determine the direction of motion or force in case of motor and the direction of induced current in case of a generator. These rules are called as Fleming’s left-hand rule (used for motors) and Fleming’s right-hand rule (used for generators) respectively.
Fleming’s Left-Hand Rule
The conductor experiences a force whenever a current carrying conductor is placed in a magnetic field. The direction of the force is perpendicular to both the magnetic field and the direction of the current.
Fleming’s left-hand rule says that if the thumb, forefinger and middle finger of the left hand are stretched and perpendicular to each other as shown in the figure below. Then the forefinger will represent the magnetic field direction, middle finger represents the current direction, then the thumb represents the force direction. Fleming’s left-hand rule is used in motors for finding the direction of rotation of the motor.
Trick 1: Assume the thumb with thrust, forefinger with a magnetic field and center-finger with current i.e. First letters will make you remember the fingers with their directions.
1. Thumb represents the direction of Thrust on the conductor (force on the conductor).
2. Forefinger represents the magnetic Field direction.
3. Center finger (middle finger) represents the Current direction.
Trick 2: Assume Fleming’s left-hand rule with the short form FBI.
where F represents Force, B represents magnetic flux density and I represents Current. Attribute these letters F to the thumb, B to the first finger and I to the middle finger.