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
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
(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
The device used for producing electric current is called 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
(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.
At the time of short circuit, the current in the circuit:
(a) reduces substantially
(b) does not change
(c) increases heavily
(d) vary continuously
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
The field at the centre of a long circular coil carrying current will be
A wire with a green insulation is usually the live wire of an electric supply.
List three sources of magnetic fields.
Three sources of magnetic fields are:
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
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
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.
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.
some devices in which electric motors are used?
Some devices in which electric motors are used are:
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
(ii) The needle of the galvanometer deflections in the
(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?
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.
of an AC generator:
An electric generator consists of a
rotating rectangular coil, say ABCD placed between the two poles of a permanent
The ends of the coil are connected to the
two rings say ,
insulated from each other.
There are two conducting stationary
brushes say .
These are kept pressed separately on the rings ,
internally attached to an axle.
The axle is mechanically rotated from
outside to rotate the coil inside the magnetic field.
The outer ends of the two brushes are
connected to the galvanometer.
The galvanometer shows the flow of current
in the given external circuit.
Working of an AC generator:
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.
current in the external circuit flows from .
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 .
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.
AC current to DC current.
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
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
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:
Electrical wires or the insulation
of the wires used in the circuit are damaged.
Faulty appliances are in
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.