ICSE Class 10 Answered

Sir ! Will you please clear my doubt in the working of electric motor and electric generator

Asked by abhinchaware07 | 12 Dec, 2019, 06:30: AM

Expert Answer

When a current carrying conductor is placed in magneteic field so that direction of current and direction of magnetic induction

are perpendicular to each other, then current carrying conductor experiences force, which is called as electromagnetic force.

This phenomenon is explained by Flemings Left hand rule as shown in above figure.

According to this rule, stretch the thumb, forefinger and middle finger of your left hand such that they are mutually perpendicular

as shown in figure. If the first finger points in the direction of magnetic field and the second finger in the direction of current,

then the thumb will point in the direction of motion or the force acting on the conductor.

This phenomenon of getting the electromagnetic force using current carrying conductor and magnetic field is used

in electric motor to get mechanical energy from electrical energy.

An electric motor, as shown in above Figure, consists of a rectangular coil ABCD of insulated copper wire.

The coil is placed between the two poles of a magnetic field such that the arm AB and CD are perpendicular

to the direction of the magnetic field. The ends of the coil are connected to the two halves P and Q of a split ring.

The inner sides of these halves are insulated and attached to an axle. The external conducting edges of P and Q

touch two conducting stationary brushes X and Y, respectively, as shown in the Figure.

Current in the coil ABCD enters from the source battery through conducting brush X and flows back to the

battery through brush Y. Notice that the current in arm AB of the coil flows from A to B. In arm CD it flows from C to D,

that is, opposite to the direction of current through arm AB. On applying Fleming’s left hand rule for the direction of force

on a current-carrying conductor in a magnetic field . We find that the force acting on arm AB pushes it downwards

while the force acting on arm CD pushes it upwards. Thus the coil and the axle O, mounted free to turn about an axis,

rotate anti-clockwise. At half rotation, Q makes contact with the brush X and P with brush Y. Therefore the current in the

coil gets reversed and flows along the path DCBA.

A device that reverses the direction of flow of current through a circuit is called a commutator. In electric motors,

the split ring acts as a commutator. The reversal of current also reverses the direction of force acting on the two arms

AB and CD. Thus the arm AB of the coil that was earlier pushed down is now pushed up and the arm CD previously

pushed up is now pushed down. Therefore the coil and the axle rotate half a turn more in the same direction.

The reversing of the current is repeated at each half rotation, giving rise to a continuous rotation of the coil and to the axle.

-------------------------------------

Electric generators are working on the principle of electromagnetic induction. Electromagnetic induction is the process,

by which a changing magnetic field in a conductor induces a current in another conductor.We can induce current in a coil

either by moving it in a magnetic field or by changing the magnetic field around it. It is convenient in most situations to move

the coil in a magnetic field. The induced current is found to be the highest when the direction of motion of the coil is

at right angles to the magnetic field.

we can use Fleming's right-hand rule to know the direction of the induced current as shown in figure .
Stretch the thumb, forefinger and middle finger of right hand so that they are perpendicular to each other, as shown in
Figure. If the forefinger indicates the direction of the magnetic field and the thumb shows the direction of motion
of conductor, then the middle finger will show the direction of induced current.

This simple rule is called Fleming’s right-hand rule.

An electric generator, as shown in Figure, consists of a rotating rectangular coil ABCD placed between the two poles

of a permanent magnet. The two ends of this coil are connected to the two rings R1 and R2.

The inner side of these rings are made insulated. The two conducting stationary brushes B1 and B2 are kept pressed

separately on the rings R1 and R2, respectively. The two rings R1 and R2 are internally attached to an axle.

The axle may be mechanically rotated from outside to rotate the coil inside the magnetic field.

Outer ends of the two brushes are connected to the galvanometer to show the flow of current in the given external circuit.
When the axle attached to the two rings is rotated such that the arm AB moves up (and the arm CD moves down)

in the magnetic field produced by the permanent magnet. Let us say the coil ABCD is rotated clockwise in the arrangement

shown in Figure. By applying Fleming’s right-hand rule, the induced currents are set up in these arms along the directions

AB and CD. Thus an induced current flows in the direction ABCD. If there are larger numbers of turns in the coil,

the current generated in each turn adds up to give a large current through the coil. This means that the current in the

external circuit flows from B2 to B1. After half a rotation, arm CD starts moving up and AB moving down.
As a result, the directions of the induced currents in both the arms change, giving rise to the net induced current

in the direction DCBA. The current in the external circuit now flows from B1 to B2. Thus after every half rotation the polarity

of the current in the respective arms changes. Such a current, which changes direction after equal intervals of time,

is called an alternating current (abbreviated as AC). This device is called an AC generator.

Answered by Thiyagarajan K | 12 Dec, 2019, 08:32: AM