Fleming’s Left Hand Rule & Fleming’s Right Hand Rule

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.

When a current-carrying conductor is placed in the magnetic field, the 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.

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, the 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.

Fleming's Left-Hand Rule
Fig. 1 Fleming’s Left Hand Rule

How To Remember Fleming’s Left Hand Rule?

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.

Fleming’s Right Hand Rule

When a conductor is moved in an electromagnetic field, an emf is induced across the conductor according to Faraday’s law of electromagnetic induction. If this conductor is given a closed path, then an induced emf causes a current to flow in the conductor.
Fleming’s right hand rule says that if the thumb, forefinger and middle finger of the right hand are stretched and perpendicular to each other as shown in the figure below. Then the thumb represents the conductor movement direction, fore-finger represents the magnetic field direction, then the middle finger represents the induced current direction. Fleming’s right-hand rule is applicable for electrical generators.
Fleming's right-hand rule
Fig. 2 Fleming’s right hand rule

How To Remember Fleming’s Right Hand Rule?

As we have given tricks to remember Fleming’s Left Hand Rule. Similarly, these tricks can be applied to Fleming’s Right Hand Rule.

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.

(a). Thumb represents the direction of Thrust on the conductor (force on the conductor).
(b). Forefinger represents the magnetic Field direction.
(c). Center finger (middle finger) represents the Current direction.

Trick 2: Assume Fleming’s right 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.

Frequently Asked Questions

Q1. How do I know when to use Fleming’s right hand rule and when to use Fleming’s Left Hand Rule?

Answer. When we need to find the force direction acting on a current-carrying conductor placed in a magnetic field then we use Fleming’s left hand rule. This is applicable to motors. When we need to find the emf polarity developed in a conductor moving in a magnetic field then we use Fleming’s right hand rule. This is applicable to generators.

Q2. Is Fleming’s left hand rule only used in DC motors and not AC motors?

Answer. Fleming’s left hand rule is applicable to both DC motors and AC motors.

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