Fleming's Left Hand Rule

Did you know that a current-carrying wire in a magnetic field will experience force? This force is called the electromotive force, or emf. This property forms the basis for all modern electric motors and generators. So we know that there is a force that is induced on the current-carrying conductor, but how do we know its direction? If only there was an easy method that could be applied in almost all conditions. Wait! Fleming's left-hand rule does just that. John Ambrose Fleming in the late 19th century discovered an important method using which the direction of this electromotive force can be determined. Fleming devised a simple method of finding the direction of force, current, and magnetic field, and to make things better, all you need is your left hand!

Definition of Fleming's left-hand rule

Fleming's left-hand rule shows the direction of the thrust on a conductor carrying a current in a magnetic field, Wikimedia Commons.

An easy way to remember this is by using the acronym FBI, where the letters stand for the properties defined above. The letters represent each of the fingers starting from the thumb and ending at the middle finger. Now let us work on a few simple example problems where we can apply Fleming's left-hand rule.

Examples of Fleming's left-hand rule

The figure above shows the working of an electric motor. We see that the current enters the loop from the right arm. Point the middle finger in the direction of the current in the right arm of the coil. The index finger points to the south pole as it represents the direction of the magnetic field (north to south). Pointing the middle finger in the direction of the current will give you the direction of the force on both arms of the coil as shown in the picture. In conclusion, the coil experiences two forces equal in magnitude but opposite in direction, which are responsible for driving the electric motor.

Difference between the left-hand and right-hand rule

In the right-hand rule, the thumb shows the direction of motion of the conductor, Wikimedia Commons.

Fleming also has a right-hand rule, which is used in the context of electromagnetic induction.

Here, the thumb indicates the motion of the conductor instead of the force on the conductor. These two directions are each other's opposite, so it's important to remember this major difference with the function of the thumb in Fleming's left-hand rule. The index and middle finger both have the same function as in the left-hand rule. We see that this right-hand rule tells us the same as Fleming's left-hand rule. We can also see this by positioning the index fingers of our hands together, and then placing the middle fingers together: our thumbs point in opposite directions, displaying the difference in their function between the two hand rules. It is important t know the difference between these two rules.

Importance of Fleming's left-hand rule

Fleming's left-hand rule is important because it provides a simple and accurate way of finding the direction of the force experienced by a current-carrying conductor in a magnetic field, given that you know the direction of both the current and the magnetic field. We saw the importance of this rule in the use case of an electric motor, where we could easily identify the direction in which the force acts on the arms of an electric motor. Considering that the workings of the (indeed very important) electromotor are well-understood in part due to Fleming's left-hand rule, we can conclude that Fleming's left-hand rule is, although indirectly, important in everyday life as well.