Question

What is the principle of electric motors?

Answer 1

Hint: The working principle of electric motor is based on that when a charge is moving under the influence of a magnetic field there acts a force on the charge. The measure of this force is given by Lorentz’s force. The flow of charge from one point to another is called current.

Complete answer:
Principle involved: An Electric motor is a rotating device that converts electrical energy to mechanical energy. A current carrying conductor placed in a magnetic field experiences a force, the measure of the force is given by the Lorentz’s force and the direction of which is given by Fleming’s left-hand rule.

The Lorentz’s force says that when a charge is moving under the influence of a magnetic field the force due to the magnetic field on the charge is given by, \[\vec F = q\vec v \times \vec B\] where, \[q\] is the charge of each particle (mainly electron),\[\vec v\] is the velocity of the charge and \[\vec B\] is the applied magnetic field now we know that the current is nothing but the charge passing per time through the conductor.

So, charge passing in one second is equal to \[Q = qs\] where \[s\] is the displacement of each charge in one second. So that is nothing but the current.So, current through the conductor will be, \[I = qv\]. Hence, the measure of force on a current carrying conductor will be, \[\vec F = \vec I \times \vec B\]. So, this force is perpendicular to the direction of the current and the magnetic field. This direction of force is easily detected by Fleming’s left-hand rule.

Construction: An electric motor consists of following parts:
-A power supply
-Magnets
-Rectangular coil
-Split rings (as commutator)
-Brushes

An electric motor consists of a rectangular coil $ABCD$ made of an insulated copper wire. The coil consists of a large number of turns and it is wound on a soft iron core. The coil is placed between the two poles of a magnetic field such as the arms $AB$ and $CD$ are perpendicular to the direction of the magnetic field.

The ends of the coil are connected to the two half rings ${R_1}$ and ${R_2}$ called split rings. The inner sides of the split rings are insulated and are attached to an axle. The external conducting edges of ${R_1}$ and ${R_2}$ touch two conducting stationary carbon brushes ${B_1}$ and ${B_2}$ respectively.


Working: Current in the coil $ABCD$ enters from the source battery through conducting brush ${B_2}$ and flows back to the battery through brush ${B_1}$. The current in arm $AB$ of the coil flows from $B$ to $A$ and in arm $CD$ it flows from $D$ to $C$, i.e., opposite to the direction of current through arm AB. i.e., the current flows along the path $DCBA$.

On applying Fleming’s left-hand rule, we find that the force acting on arm $AB$ pushes it upwards while the force acting on arm $CD$ pushes it downwards. Hence, the coil and the axle, mounted free to turn about an axis, starts rotating. As the coil makes half rotation, ${R_2}$ makes contact with the brush ${B_1}$ and ${R_1}$ with brush ${B_2}$.

Thus, the current in the coil reverses a flow along the path $ABCD$. As a result, the directions of force acting on the two arms $AB$ and $CD$ are also reversed. The arm $AB$ of the coil that was earlier pushed down is now pushed up. Due to this, the direction of rotation of the coil remains the same. The reversing of the current is repeated after every half rotation, giving rise to a continuous rotation of the coil and the axle.

Note: The term electric motor and electric generator sounds similar but they are totally different. The electric motor generates mechanical power using electricity whereas the electric generator uses mechanical power to generate electricity. The principle of electric motor is based on the Lorentz force whereas the principle of electric generator is based on Lenz’s law. Electric motor is used as an important component in various electrical devices such as electric fans, washing machines, mixers, computers, etc.