Question

What is Left hand rule? Explain it with a diagram.

Answer 1

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. In the figure, it is shown that a portion of a conductor of length L placed vertically in a uniform horizontal magnetic field strength H, produced by two magnetic poles N and S. If i is the current flowing through this conductor, the magnitude of the force acts on the conductor is,

F = BiL

Hold out your left hand with the forefinger, second finger and thumb at a right angle to one another. If the forefinger represents the direction of the field and the second finger 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. This can be imagined by considering numbers of closed magnetic lines of force around the conductor. The direction of magnetic lines of force can be determined by Maxwell's corkscrew rule or right-hand grip rule. As per these rules, the direction of the magnetic lines of force (or flux lines) is clockwise if the current is flowing away from the viewer, that is if the direction of current through the conductor is inward from the reference plane as shown in the figure.

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 external magnetic field are from N to S pole that is from left to right. The magnetic lines of force of 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 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 which will tend to move the conductor from more concentrated magnetic field to less concentrated magnetic field, that is from the present position to downwards. Now if you observe the direction of current, force and magnetic field in the above explanation, you will find that the directions are according to the Fleming left-hand rule.