CBSE Class X

Question 11:

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?

Answer:

An electric motor converts electrical energy into mechanical energy.

It works on the principle of the magnetic effect of current. A current-carrying coil

rotates in a magnetic field. The following figure shows a simple electric motor.

 

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 length ST.

As a result, the coil rotates anti-clockwise.

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.

After half a rotation, the position of MN and ST interchange. The half-ring D comes

in contact with brush A and 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 reversal of current

through the coil MNST repeats after each half rotation. As a result, the coil rotates

unidirectional.

The split rings help to reverse the direction of current in the circuit. These are

called the commutator.

Question 10:

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?

Answer:

The direction of magnetic field is given by Fleming’s left hand rule. Magnetic field

inside the chamber will be perpendicular to the direction of current (opposite to

the direction of electron) and direction of deflection/force i.e., either upward or

downward. The direction of current is from the front wall to the back wall because

negatively charged electrons are moving from back wall to the front wall. The

direction of magnetic force is rightward.

Hence, using Fleming’s left hand rule, it can be concluded that the direction of

magnetic field inside the chamber is downward.

Question 9:

When is the force experienced by a current-carrying conductor placed in a

magnetic field largest?

Answer:

The force experienced by a current-currying conductor is the maximum when the

direction of current is perpendicular to the direction of the magnetic field.

Question 8:

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.

Answer:

A solenoid is a long coil of circular loops of insulated copper wire. Magnetic field

lines are produced around the solenoid when a current is allowed to flow through

it. The magnetic field produced by it is similar to the magnetic field of a bar

magnet. The field lines produced in a current-carrying solenoid is shown in the

following figure.

 

In the above figure, when the north pole of a bar magnet is brought near the end

connected to the negative terminal of the battery, the solenoid repels the bar

magnet. Since like poles repel each other, the end connected to the negative

terminal of the battery behaves as the north pole of the solenoid and the other

end behaves as a south pole. Hence, one end of the solenoid behaves as a north

pole and the other end behaves as a south pole.

Question 7:

List three sources of magnetic fields.

Answer:

Three sources of magnetic fields are as follows:

(a) Current-carrying conductors

(b) Permanent magnets

(c) Electromagne

Question 6:

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.

Answer:

(a) False

An electric motor converts electrical energy into mechanical energy.

(b) True

A generator is an electric device that generates electricity by rotating a coil in

a magnetic field. It works on the principle of electromagnetic induction.

(c) True

A long circular coil is a long solenoid. The magnetic field lines inside the solenoid

are parallel lines.

(d) False

Live wire has red insulation cover, whereas earth wire has green insulation colour

in the domestic circuits.

Question 5:

At the time of short circuit, the current in the circuit

(a) reduces substantially

(b) does not change

(c) increases heavily

(d) vary continuously

Answer:

(c) When two naked wires of an electric circuit touch each other, the amount of

current that is flowing in the circuit increases abruptly. This causes short-circuit.

Question 4:

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.

Answer:

(d) An AC generator has two rings called slip rings. A DC generator has two

half rings called commutator. This is the main difference between both the

types of generators.

Question 3:

The device used for producing electric current is called a

(a) generator

(b) galvanometer

(c) ammeter

(d) motor

Answer:

(a) An electric generator produces electric current. It converts mechanical energy

into electricity.

Question 2:

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

Answer:

(c) When a straight coil and a magnet are moved relative to each other, a current

is induced in the coil. This phenomenon is known as electromagnetic induction.

Question 1:

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

Answer:

(d) The magnetic field lines, produced around a straight current-carrying conductor,

are concentric circles. Their centres lie on the wire.

Question 3:

What precaution should be taken to avoid the overloading of domestic electric

circuits?

Answer:

The precautions that should be taken to avoid the overloading of domestic circuits

are as follows:

(a) Too many appliances should not be connected to a single socket.

(b) Too many appliances should not be used at the same time.

(c) Faulty appliances should not be connected in the circuit.

(d) Fuse should be connected in the circuit.

Question 2:

An electric oven of 2 kW is operated in a domestic electric circuit (220 V) that

has a current rating of 5 A. What result do you expect? Explain.

Answer:

Current drawn by the electric oven can be obtained by the expression,

P = VI

I = P/V

Where,

Current = I

Power of the oven, P = 2 kW = 2000 W

Voltage supplied, V = 220 V

  

Hence, the current drawn by the electric oven is 9.09 A, which exceeds the safe

limit of the circuit. Fuse element of the electric fuse will melt and break the circuit.

Question 1:

Name two safety measures commonly used in electric circuits and appliances.

Answer:

Two safety measures commonly used in electric circuits and appliances are as

follows:

(i) Each circuit must be connected with an electric fuse. This prevents the flow

of excessive current through the circuit. When the current passing through the

wire exceeds the maximum limit of the fuse element, the fuse melts to stop

the flow of current through that circuit, hence protecting the appliances

connected to the circuit.

(ii) Earthing is a must to prevent electric shocks. Any leakage of current in an

electric appliance is transferred to the ground and people using the appliance

do not get the shock.

Question 4:

Choose the correct option.

A rectangular coil of copper wires is rotated in a magnetic field. The direction

of the induced current changes once in each

(a) two revolutions (b) one revolution

(c) half revolution (d) one-fourth revolution

Answer:

(c) When a rectangular coil of copper is rotated in a magnetic field, the direction

of the induced current in the coil changes once in each half revolution. As a result,

the direction of current in the coil remains the same.

Question 3:

Which sources produce alternating current?

Answer:

AC generators, power plants, etc., produce alternating current

Question 2:

Name some sources of direct current.

Answer:

Some sources of direct current are cell, DC generator, etc.

Question 1:

State the principle of an electric generator.

Answer:

An electric generator works on the principle of electromagnetic induction. It

generates electricity by rotating a coil in a magnetic field.

Question 1:

Explain different ways to induce current in a coil.

Answer:

The different ways to induce current in a coil are as follows:

(a) If a coil is moved rapidly between the two poles of a horse-shoe magnet,

then an electric current is induced in the coil.

(b) If a magnet is moved relative to a coil, then an electric current is induced

in the coil.

Question 3:

What is the role of the split ring in an electric motor?

Answer:

The split ring in the electric motor acts as a commutator. The commutator

reverses the direction of current flowing through the coil after each half

rotation of the coil. Due to this reversal of the current, the coil continues

to rotate in the same direction.