Question

What should be the angle between thumb (main) finger, forefinger and central finger in Fleming’s left hand rule? What does each finger indicate?

Answer 1

Hint: When a current carrying conductor is placed in an external magnetic field, force acts in that conductor, so to study the direction of force we use Fleming’s left hand rule. In Fleming’s left hand rule thumb, fingers and forefinger are used to show the direction of the force, direction of current flow and direction of magnetic field using the formula, $\overset{\to }{\mathop{F}}\,=\overset{\to }{\mathop{I}}\,l\times \overset{\to }{\mathop{B}}\,$

Formula used: $\overset{\to }{\mathop{F}}\,=\overset{\to }{\mathop{I}}\,l\times \overset{\to }{\mathop{B}}\,$

Complete step-by-step solution -
Before understanding the Fleming’s left hand rule we will understand relation between current carrying wire, force and magnetic field which can be given as,
$\overset{\to }{\mathop{F}}\,=\overset{\to }{\mathop{I}}\,l\times \overset{\to }{\mathop{B}}\,$
Where, F is magnetic force, I is current and B is magnetic field and their direction is shown by the vector. Now, the cross sign between current and Magnetic field denotes that magnetic field is perpendicular to the direction of flow of current. So, the force acting on it will also be perpendicular to them.
As directions are perpendicular to each other we can say that angles between them will be ${{90}^{\circ }}$, as shown in figure,

Now, to find the direction of magnetic field and force we will place our thumb in direction of current as per the Fleming’s left hand rule and then based on that we will place our hand. By doing these we will get to know that the forefinger will be in the direction of Magnetic field and the middle finger will point in the direction of Force acting. It can also be seen in figure below,

Note: Fleming’s left hand rule is used to determine flow of current mostly in electric motors. In an electric motor when a current is passed through a conductor, it generates a cylindrical magnetic field around the conductor. Now, if any magnetic field producing object is brought near to this conductor, then interaction takes place between magnetic field and electric field. Due to this force acts on the conductor and to determine the direction of force Fleming’s left hand rule is used. So, this is one of the applications of Fleming’s left hand rule.