Question 1:
What happens when a current-carrying conductor is placed in a magnetic field ?
Solution :
When a current-carrying conductor is placed in a magnetic field, a mechanical
force is exerted on the conductor which can make the conductor move.
Question 2:
When is the force experienced by a current-carrying conductor placed in a
magnetic field largest ?
Solution :
The force experienced by a current-carrying conductor placed in a magnetic
field is the largest when the current carrying conductor is at right angles to
the magnetic field.
Question 3:
In a statement of Fleming’s left-hand rule, what do the following represent ?
(a) direction of centre finger.
(b) direction of forefinger.
(c) direction of thumb.
Solution :
(a) Current.
(b) Magnetic field.
(c) Force acting on the conductor.
Question 4:
Name one device which works on the magnetic effect of current.
Solution :
Electric bell works on the magnetic effect of current. It uses an electromagnet
to produce sound.
Question 5:
Name the device which converts electrical energy into mechanical energy.
Solution :
Electrical motor.
Question 6:
A motor converts one form of energy into another. Name the two forms.
Solution :
Electrical energy to mechanical energy.
Question 7:
State whether the following statement is true or false :
An electric motor converts mechanical energy into electrical energy.
Solution :
False
An electric motor converts electrical energy into mechanical energy.
Question 8:
For Fleming’s left-hand rule, write down the three things that are 90° to each
other, and next to each one write down the finger or thumb that represents it.
Solution :
(a) Current – direction of center finger.
(b) Magnetic field – direction of fore finger.
(c) Force or Motion – direction of thumb.
Question 9:
Name the device which is used to reverse the direction of current in the coil
of a motor.
Solution :
A commutator reverses the direction of current in the coil of a motor.
Question 10:
What is the other name of the split ring used in an electric motor ?
Solution :
Commutator.
Question 11:
What is the function of a commutator in an electric motor ?
Solution :
The function of commutator rings is to reverse the direction of current flowing
through the coil every time the coil just passes the vertical position during a
revolution.
Question 12:
Of what substance are the brushes of an electric motor made ?
Solution :
Carbon.
Question 13:
Of what substance is the core of the coil of an electric motor made ?
Solution :
The core of the coil of an electric motor made of soft iron.
Question 14:
In an electric motor, which of the following remains fixed and which rotates
with the coil ?
Commutator ; Brush
Solution :
Brush remains fixed. Commutator rotates with the coil.
Question 15:
What is the role of the split ring in an
electric motor ?
Solution :
The function of split rings is to reverse the direction of current flowing
through the coil every time the coil just passes the vertical position during a
revolution.
Question 16:
Fill in the following blanks with suitable words :
(a) Fleming’s Rule for the motor effect uses the…………………..
(b) A motor contains a kind of switch called a……………………. which reverses the
current every half…………………..
Solution :
(a) left
(b) Commutator; rotation.
Question 17:
(a) A current-carrying conductor is placed perpendicularly in a magnetic field. Name the rule which can be
used to find the direction of force acting on the conductor.
(b) State two ways to increase the force on a current-carrying conductor in a magnetic field.
(c) Name one device whose working depends on the force exerted on a current-carrying coil placed in a magnetic field.
Solution :
(a) Fleming’s left hand rule.
(b) By increasing the current flowing in the conductor; by increasing the strength of magnetic field.
(c) Electric motor.
Question 18:
State Fleming’s left-hand rule. Explain it with the help of labelled diagrams.
Solution :
Fleming’s left hand rule: Hold the forefinger, the centre finger and the thumb of your left hand to right angles to one other. Adjust your hand in such a way that the forefinger points in the direction of magnetic field and the and the centre finger points in the direction of current, than the direction in which thumb points, gives the direction of force acting on the conductor.
Question 19:
What is the principle of an electric motor ? Name some of the devices in which electric motors are used.
Solution :
Question 20:
(a) In a d.c. motor, why must the current to the coil be reversed twice during
each rotation ?
(b) What device reverses the current ?
Solution :
(a) The current to the coil must be reversed twice during each rotation so that
the coil keeps rotating continuously in the same direction.
(b) Commutator.
Question 21:
(a) State what would happen to the direction of rotation of a motor if :
(i) the current were reversed
(ii) the magnetic field were reversed
(iii) both current and magnetic field were reversed simultaneously.
(b) In what ways can a motor be made more powerful ?
Solution :
(a)
(i) Direction of rotation would be reversed
(ii) Direction of rotation would be reversed
(iii) Direction of the rotation would remain unchanged
(b) Motor can be made more powerful by winding the coil on a soft iron core of
by increasing the number of turns of the coil.
Question 22:
(a) What is an electric motor ? With the help of a labelled diagram, describe the working of a simple electric motor.
(b) What are the special features of commercial electric motors ?
Solution :
(a) Electric motor is a device used for converting electrical energy into mechanical energy.
Working of an electric motor:
Initially, the coil ABCD is in the horizontal position. On pressing the switch, current enters the coil through carbon brush P and commutator half ring X. The current flows in the direction ABCD and leaves via ring Y and brush Q. The direction of magnetic field is from N pole to S pole of the magnet. According to Fleming’s left-hand rule, the force on sides AB and CD is in the downward and upward directions respectively. This makes the coil ABCD move in the anticlockwise direction.
When the coil reaches vertical position, then the brushes P and Q will touch the gap between the two commutator rings and current is cut off. But the coil does not stop rotating as it has already gained momentum. When the coil goes beyond the vertical position, the side CD comes on the left side and side AB comes to the right side, and the two commutator rings change contact from one brush to the other. This reverses the direction of current in the coil, which in turn reverses the direction of forces acting on the sides AB and CD of the coil. The side CD is pushed down and side AB is pushed up. Thus, the coil rotates anticlockwise by another half rotation.
The reversing of current in the coil is repeated after every half rotation due to which the coil (and its shaft) continues to rotate as long as current from the battery is passed through it. The rotating shaft of electric motor can drive a large number of machines which are connected to it.
(b) In commercial electric motors:
i. the coil is wound on a soft iron core. This increases the strength of magnetic field, which makes the motor more powerful.
ii. the coil contains a large number of turns of insulated copper wire.
iii. a powerful electromagnet is used in place of permanent magnet.
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