All electrical machines work on the principle of electromagnetic induction. According to this principle, if there is relative motion between a conductor and a magnetic field, then an EMF is induced in the conductor. On the other hand, if a current carrying conductor is placed in a magnetic field, the conductor experiences a force. For practical and analytical purposes, it is important to determine the direction of induced EMF and force acting on the conductor. Fleming’s hand rules are used for that.

An English electrical engineer and physicist John Ambrose Fleming stated two rules in late 19th century to determine the direction of induced EMF and force acting on a current carrying conductor placed in a magnetic field. These rules popularly known as Fleming’s Left Hand Rule and Fleming’s Right Hand Rule.

Basically, both left hand rule and right hand rule show a relationship between magnetic field, force and current.

Fleming’s left hand rule is used to determine the direction of force acting on a current carrying conductor when it placed in a magnetic field, hence it is mainly applicable in electric motors. Whereas, Fleming’s right hand rule is used to determine the direction of induced EMF in a conductor moving relative to a magnetic field, thus it is mainly applicable in electric generators.

Fleming’s Left Hand Rule

Fleming’s left hand rule is particularly suitable to find the direction of force on a current carrying conductor in a magnetic field and it may be stated as under −

Stretch out the forefinger, middle finger and thumb of your left hand so that they are at right angles (perpendicular) to one another as shown in figure 1. If the forefinger points in the direction of magnetic field, middle finger in the direction of current in the conductor, then the thumb will point in the direction of force on the conductor.

In practice, Fleming’s left hand rule is applied to determine the direction of motion of conductor in electric motors.

Fleming’s Right Hand Rule

Fleming’s right hand rule is particularly suitable to determine the direction of induced EMF and hence electric current in a conductor when there is a relative motion between the conductor and magnetic field. Fleming’s left hand rule may be stated as under −

Stretch out the forefinger, middle finger and thumb of your right hand so that they are at right angles (perpendicular) to one another as shown in figure 2. If the forefinger points in the direction of magnetic field, thumb in the direction of motion of the conductor, then the middle finger will point in the direction of induced EMF or current.

In practice, Fleming’s right hand rule is used to determine the direction of induced EMF and current in the electric generators.

Comparison of Fleming’s Left Hand Rule and Right Hand Rule

The following table gives a brief comparison of Fleming’s left hand and right hand rules −