Fleming’s Left Hand Rule

This rule connects three things :- 1) a magnetic field direction, 2) a current flow direction, 3) a Force direction on a conductor. The Fleming’s Left Hand Rule is about getting the right directions. In a situation where all these 3 things exist, the starting point is that you take your left Hand and then point precisely towards the direction of the magnetic field.

1. keep only your forefinger pointing in the direction of magnetic field.

2. turn your middle finger to point in the direction of current flow.

Now, the two directions are mutually perpendicular, at 90 degrees. Open out your thumb as shown, it will show the RESULT, the OUTPUT, which is the direction of movement of the conductor (through which the current is flowing).

Now, the thumb is again at right angles to the other two fingers. This is same as a X,Y,Z axes situation. We can use vector quantities for magnetic field, current, Force.

So, based on inputs of magnetic field and current directions, the Fleming’s Left hand Rule shows the Output direction of the Force. This force will create motion if a conductor is free to move. That’s the working principle behind a DC Motor works. Even if a conductor like a metal rod is just hanging out there freely inside a magnetic field, and we pass a current though the conductor, the conductor will surely move , in line with Fleming’s Left Hand Rule.

A magnetic field direction is always from North Pole towards South Pole. A DC current is always taken as flowing from a positive terminal to negative terminal of a battery. In cases of Alternating current, the Fleming’s Left Hand Rule can be applied based on which direction the current is flowing at any given time.

The sequence of actions here must be clear. If we move the conductor first and then try to measure the current flow direction, that would not be Fleming’s Left Hand Rule. For generators, where the conductor is moved first as an INPUT, the Fleming’s Right Hand Rule is used to get the direction of current flow.