Fleming's left hand rule and right hand rule

If a current-carrying conductor is placed in a magnetic field, it experiences a force due to that field. Conversely, when a conductor moves through a magnetic field, an electromotive force (emf) is induced across it. This phenomenon is described by (Faraday's law of electromagnetic induction).

The first phenomenon — where a current carrying conductor inside a magnetic field experiences force/movement — is the basic principle behind electric motors. The second phenomenon —where movement of a wire in a magnetic field produces current — is the basic principle behind electric generators.

To easily remember the direction of motion or current in these situations, John Ambrose Fleming introduced two helpful rules:

• Fleming's left hand rule: used for motors (to find the direction of force or motion),
• Fleming's right hand rule: used for generators (to find the direction of induced current).

Fleming's left hand rule

Whenever a current carrying conductor is placed in a magnetic field, the conductor experiences a force which is perpendicular to both the magnetic field and the direction of current. According to Fleming's left hand rule, if the thumb, fore-finger and middle finger of the left hand are stretched to be mutually perpendicular as shown in the illustration at left -

• Forefinger points in the direction of the magnetic field.
• Middle finger shows the direction of the current.
• Thumb indicates the direction of the resulting force (or motion).

This rule is primarily used to determine motion in electric motors.

How to remember Fleming's left hand rule?

Method 1: Use the initials

Relate the thumb with thrust, fore finger with field and center-finger with current as explained below.

• The Thumb represents the direction of Thrust on the conductor (force on the conductor).
• The Fore finger represents the direction of the magnetic Field.
• The Center finger (middle finger) the direction of the Current.

Method 2: Use the acronym FBI Relate the Fleming's left-hand rule with FBI (wait! NOT the agency). Here, F for Force, B is the symbol of magnetic flux density and I is the symbol of Current. Attribute these letters F,B,I to the thumb, first finger and middle finger respectively.

Fleming's right hand rule

[Also read: Maxwell's right-hand grip rule]

Fleming's right hand rule is applicable for electrical generators. As per Faraday's law of electromagnetic induction, whenever a conductor is forcefully moved in an electromagnetic field, an emf gets induced across the conductor. If the conductor is provided a closed path, then the induced emf causes a current to flow. According to the Fleming's right hand rule, the thumb, fore finger and middle finger of the right hand are stretched to be perpendicular to each other as shown in the illustration at right -

• Thumb indicates the direction of the conductor’s motion.
• Forefinger shows the direction of the magnetic field.
• Middle finger points in the direction of the induced current.

How to remember Fleming's right hand rule?

Use the same mnemonic techniques as for the left-hand rule—just use your right hand instead:

• F = Force (Motion)
• B = Magnetic Field
• I = Induced Current