Lesson: Magnetic Effects of Electric Current

Which of the following correctly describes the magnetic field near a long straight wire?
(a) The field consists of straight lines perpendicular to the wire
(b) The field consists of straight lines parallel to the wire
(c) The field consists of radial lines originating from the wire
(d) The field consists of concentric circles centred on the wire

d

The phenomenon of electromagnetic induction is:
(a) the process of charging a body
(b) the process of generating magnetic field due to a current passing through a

coil
(c) producing induced current in a coil due to relative motion between a magnet

and the coil
(d) the process of rotating a coil of an electric motor

c

The device used for producing electric current is called a:
(a) generator
(b) galvanometer
(c) ammeter
(d) motor

a

The essential difference between an AC generator and a DC generator is that
(a) AC generator has an electromagnet while a DC generator has permanent

magnet.
(b) DC generator will generate a higher voltage.
(c) AC generator will generate a higher voltage.
(d) AC generator has slip rings while the DC generator has a commutator.

d

At the time of short circuit, the current in the circuit:
(a) reduces substantially
(b) does not change
(c) increases heavily
(d) vary continuously

c

State whether the following statements are true or false.
(a) An electric motor converts mechanical energy into electrical energy.
(b) An electric generator works on the principle of electromagnetic induction.

(c) The field at the centre of a long circular coil carrying current will be parallel

straight lines.

(d) A wire with a green insulation is usually the live wire of an electric supply.

(a)False

(b)True

(c) True

(d) False

List three sources of magnetic fields.

Three sources of magnetic fields are:

·         permanent magnets,

·         electromagnets,

·         current-carrying conductors.

How does a solenoid behave like a magnet? Can you determine the north and south poles of a current-carrying solenoid with the help of a bar magnet? Explain.

A solenoid is a made up of insulated copper wire coiled to form circular loops. Magnetic field lines are produced around the solenoid when the current flows through it.

The magnetic field produced by a solenoid is given below. The solenoid acts like a bar magnet.

The end of the solenoid connected to the negative terminal of the battery behaves as a north pole and the other end behaves as a south pole.

When is the force experienced by a current-carrying conductor placed in a magnetic field largest?

When the direction of current in a conductor is perpendicular to the direction of a magnetic field, the force experienced by the current-carrying conductor is the largest.

Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of magnetic field?

The direction of the magnetic field can be found by applying Fleming's left hand rule. In the given case, the direction of the magnetic field inside the chamber is downward.

Draw a labelled diagram of an electric motor. Explain its principle and working. What is the function of a split ring in an electric motor?

Principle: The principle of an electric motor is based on the magnetic effect of electric current.  A motor works on the principle that when current is passed in a rectangular coil placed in a magnetic field; it experiences a force and rotates. The direction of rotation of the coil is given by the Fleming’s left-hand rule.

Working: When a current is allowed to flow through the coil MNST by closing the switch, the coil starts rotating anti-clockwise. This happens because a downward force acts on length MN and at the same time, an upward force acts on the length ST. As a result, the coil rotates anti-clockwise.

After half rotation, current in the length MN flows from M to N and the magnetic field acts from left to right, normal to length MN. Therefore, according to Fleming's left hand rule, a downward force acts on the length MN. Similarly, current in the length ST flows from S to T and the magnetic field acts from left to right, normal to the flow of current. Therefore, an upward force acts on the length ST. These two forces cause the coil to rotate anti-clockwise.
To avoid this flip-flop movement, after half a rotation, a split-ring commutator is used. The half-ring D comes in contact with brush A and the half-ring C comes in contact with brush B. Hence, the direction of current in the coil MNST gets reversed.
The current flows through the coil in the direction TSNM. The change in the direction of current-flow through the coil MNST repeats after every half rotation. As a result, the coil rotates continuously in the same direction.

Name some devices in which electric motors are used?

Some devices in which electric motors are used are:

·         Car fan,

·         Battery rickshaw,

·         Electric Car,

·         Electric Scooter.

A coil of insulated copper wire is connected to a galvanometer. What will happen if a bar magnet is (i) pushed into the coil, (ii) withdrawn from inside the coil, (iii) held stationary inside the coil?

(i) The needle of the galvanometer deflects in a particular direction.

(ii) The needle of the galvanometer deflections in the opposite direction.
(iii) The needle of the galvanometer does not deflect at all.

Two circular coils A and B are placed closed to each other. If the current in the coil A is changed, will some current be induced in the coil B? Give reason.

Two circular coils A and B are placed close to each other. When there is change in current in coil A, there is a change in the magnetic field associated with it. As a result, the magnetic field around coil B also changes. This change in magnetic field lines around coil B induces an electric current in it. This phenomenon is known as electromagnetic induction.

State the rule to determine the direction of a (i) magnetic field produced around a straight conductor-carrying current, (ii) force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it, and (iii) current induced in a coil due to its rotation in a magnetic field.

(i) Right hand thumb rule
(ii) Fleming's left hand rule
(iii) Fleming's right hand rule

Explain the underlying principle and working of an electric generator by drawing a labeled diagram. What is the function of brushes?

Principle:

An electric generator works on the principle of electromagnetic induction. As per this principle, an induced current is set up in the coil whenever the coil is rotated between the poles of a magnet. The direction of the induced current is given by Fleming’s right hand rule.

Construction of an AC generator:

a)    An electric generator consists of a rotating rectangular coil, say ABCD placed between the two poles of a permanent magnet.

b)   The ends of the coil are connected to the two rings say $R_1 \text{and }{R}_2$, insulated from each other.

c)    There are two conducting stationary brushes say $B_1 \text{and }{B}_2$. These are kept pressed separately on the rings $R_1 \text{and }{R}_2$, respectively.

d)   $R_1 \text{and }{R}_2$ are internally attached to an axle. 

e)    The axle is mechanically rotated from outside to rotate the coil inside the magnetic field.

f)    The outer ends of the two brushes are connected to the galvanometer.

g)   The galvanometer shows the flow of current in the given external circuit.

Working of an AC generator:

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 induces electric current along the directions AB and CD. Thus, an induced current flows in the direction ABCD.

The current in the external circuit flows from $B_2 \text{to }{B}_1$.

After half a rotation, arm CD starts moving up and AB moving down. These results induced currents in the direction DCBA.  The current in the external circuit now flows from $B_1 \text{to }{B}_2$.

There is a change in direction of the induced current after every half rotation in equal interval of time. The current produced is called AC current.

Converting AC current to DC current.

DC current does not change its direction after a fixed interval of time. To convert AC current to DC current, we can use a split-ring commutator in place of slip-rings. This will ensure no change in the direction of the induced current.

Function of Brushes:

Brushes are connected to the galvanometer on one side and to the slip-rings on the other. The brushes facilitate the transfer of current from the coil ABCD to the external circuit.

When does an electric short circuit occur?

When a live wire comes in contact with a neutral wire, this contact is known as a short circuiting.

An electric short circuit occurs when:

a)   Electrical wires or the insulation of the wires used in the circuit are damaged.

b)   Faulty appliances are in use.

What is the function of an earth wire? Why is it necessary to earth metallic appliances?

An earth wire is a metal wire that connects the metal case of an electrical appliance to the earth (zero potential). It facilitates transfer of leaked current, if any, to the ground. Thus, the earth wire prevents any electric shock to the user.