Whenever a current-carrying conductor comes under a magnetic field, there will be a force acting on the conductor. The direction of this force can be found using Fleming's Left-Hand Rule (also known as 'Flemings left-hand rule for motors').
Similarly, if a conductor is forcefully brought under a magnetic field, there will be an induced current in that conductor. The direction of this force can be found using Fleming's Right Hand Rule.
In both Fleming's left and right-hand rules, there is a relation between the magnetic field, the current and force.
These rules do not determine the magnitude but instead show the direction of any of the three parameters (magnetic field, current, force) when the direction of the other two parameters is known.
Fleming's Left-Hand Rule is mainly applicable to electric motors and Fleming's Right-Hand Rule is mainly applicable to electric generators.
Fleming's Left-Hand Rule
It is found that whenever a current-carrying conductor is placed inside a magnetic field, a force acts on the conductor, in a direction perpendicular to both the directions of the current and the magnetic field.
Hold out your left hand with the forefinger, second finger and thumb at the right angle to one another. If the forefinger represents the direction of the field and the second finger represents that of the current, then thumb gives the direction of the force.
While current flows through a conductor, one magnetic field is induced around it. The magnetic field can be imagined by considering the numbers of closed magnetic lines of force around the conductor.
Fig.no.1: Illustration of magnetic field around a current-carrying conductor
Now if a horizontal magnetic field is applied externally to the conductor, these two magnetic fields i.e. field around the conductor due to the current through it and the externally applied field will interact with each other. We observe in the picture that the magnetic lines of force of the external magnetic field are from N to S pole that is from left to right.
Fig.no.2: Right hand rule magnetic force
The magnetic lines of force of the external magnetic field and magnetic lines of force due to the current in the conductor are in the same direction above the conductor, and they are in the opposite direction below the conductor. Hence there will be larger numbers of co-directional magnetic lines of force above the conductor than that of below the conductor.
Consequently, there will be a larger concentration of magnetic lines of force in a small space above the conductor. As magnetic lines of force are no longer straight lines, they are under tension like stretched rubber bands.
As a result, there will be a force that will tend to move the conductor from the more concentrated magnetic field to a less concentrated magnetic field, which is from the present position to downwards. Now if you observe the direction of the current, force and magnetic field in the above explanation, you will find that the directions are according to the Fleming left-hand rule.
Fig.no.3: Flemings Left hand rule
Fleming Right Hand Rule
As per Faraday's law of electromagnetic induction, whenever a conductor moves inside a magnetic field, there will be an induced current in it. If this conductor gets forcefully moved inside the magnetic field, there will be a relation between the direction of applied force, magnetic field, and the current. This relation between these three directions is determined by Fleming's right-hand rule.
This rule states 'Hold out the right hand with the first finger, second finger and thumb at the right angle to each other. If forefinger represents the direction of the line of force, the thumb points in the direction of motion or applied force, then second finger points in the direction of the induced current'.
Fig.no.4: Flemings Right hand rule
Who Invented The Left and Right Hand Thumb Rules?
The left and right-hand thumb rules were founded by John Ambrose Fleming in the late 19th century.
Fig.no.5: John Ambrose Fleming
John discovered both of these rules and named them after himself. The rules are now well known as Fleming's left and right-hand rule.