"AKC SCIENCE CLASSES"
CLASS 10 TH (CBSE AND MP BOARD)
CHAPTER 13
MAGNETIC EFFECTS OF ELECTRIC CURRENT
EXERCISE QUESTIONS AND ANSWERS
Q.01:- 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.
Ans:- (d)
Q.02:- 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.
Ans:- (c)
Q.03:- The device used for producing electric current is called a
(a) generator
(b) galvanometer
(c) ammeter
(d) motor
Ans:- (a)
Q.04:- 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.
Ans:- (d)
Q.05:- At the time of short circuit, the current in the circuit
(a) reduces substantially.
(b) does not change.
(c) increases heavily.
(d) vary continuously.
Ans:- (c)
Q.06:- State whether the following statement 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.
Ans:- (a) False. It converts electrical energy into mechanical energy.
(b) True.
(c) True. The field is almost uniform at the centre of the coil.
(d) False. The wire with green insulation is usually the live wire.
Q.07:- List three methods of producing magnetic fields.
Ans:- The three methods of producing magnetic fields are :-
- Permanent magnets,
- Electro-magnets and
- Earth's magnetic field.
Ans:- The patterns of the magnetic fields of both the solenoids and a bar magnet are exactly similar. Like a bar magnet, one end of the solenoid has N-polarity and the other end has S-polarity.
We bring the N-pole of the bar magnet near one end of the solenoid: If there is an attraction, then that end of the solenoid has south polarity and the other has north polarity. If there is a repulsion, then that end of the solenoid has north polarity and the other end has south polarity.
Q.09:- When is the force experienced by a current-carrying conductor placed in a magnetic field largest?
Ans:- When the conductor carries current in a direction perpendicular to the direction of the magnetic field, the force experienced by the conductor is largest.
Q.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?
Ans:- According to Fleming's left hand rule, the magnetic field acts in the vertically downward direction. Note that the direction of current will be opposite to that of the electron beam.
Q.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?
Ans:- Labelled diagram of an electric motor :-
The Working of an electric motor :- When the current is allowed to pass through the coil by a battery then according to Fleming's left hand rule two equal forces in opposite directions are produced on AB and CD, the sides of coil ABCD, which rotates the coil in clockwise direction. With coil the slip rings also rotate. Slip rings control the direction of current and keep it in one direction only. So the motor rotates in one rotation, that is clockwise direction.
The function (role) of split ring in an electric motor :- The function (role) of split ring in a motor is as commutator i.e., it changes the direction of current flowing in coil from B to A and D to C respectively.
Q.12:- Name some devices in which electric motors are used.
Ans:- Electric motor is used in appliances like electric fans, washing machines, mixers, grinder, blenders, computers, MP3 players, etc.
Q.13:- 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) withdraw from inside the coil,
(ⅲ) held stationary inside the coil?
Ans:- (ⅰ) An electric current is induced in the coil and the galvanometer shows a deflection.
(ⅱ) An electric An electric current is induced in the coil but in opposite direction. The galvanometer shows a deflection in reverse direction.
(ⅲ) No current is induced in the coil. The galvanometer shows no deflection.
Q.14:- 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.
Ans:- Yes. When the current in coil A is changed, some current is induced in the coil B. Due to the change in current in coil A, the magnetic field lines linked with coil A and with coil B get changed. This sets up induced current in coil B.
Q.15:- 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 magnetic field.
Ans:- (ⅰ) Right hand thumb rule :- If the current carrying conductor is held in the right hand such that the thumb points in the direction of the current, then the direction of the curl of the fingers will given the direction of the magnetic field.
(ⅱ) Fleming's left hand rule :- Stretch the forefinger, the central finger and the thumb of the left hand mutually perpendicular to each other. If the forefinger points in the direction of the magnetic field, the central finger in the direction of current, then the thumb points in the direction of force in the conductor.
(ⅲ) Fleming's right hand rule :- Stretch the thumb, forefinger and the central finger of the right hand mutually perpendicular to each other. If the forefinger points in the directions of magnetic field, thumb in the direction of motion of the conductor, then the central finger points in the direction of current induced in the conductor.
Q.16:- Explain the underlying principle and working of an electric generator by drawing a labelled diagram.What is the function of brushes?
Ans:- An electric generator converts mechanical energy into electrical energy. Alternating current generators or AC generators produce alternating current.
Construction :- Alternating current generator consists of a rectangular coil ABCD called armature. The armature coil consists of large number of turns of insulated copper wire wound over a soft iron core. The coil is kept between the poles of a strong magnet. In the figure shown R₁ and R₂ are two slip rings connected to the two ends of the coil and convey the current produced to outside circuit. B₁ and B₂ are two carbon brushes remain in sliding contact with slip rings.
Working :- The coil of the generator is rotated with the help of an axle. When coil rotates, it cuts the magnetic lines of force (magnetic field) near the north (N) and south (S) poles of the magnet. By electromagnetic induction, a current is induced in the coil. The direction of this current is given by fleming's right hand rule. As this coil turns clockwise, arm AB moves up and arm CD goes down. The direction of current is from A to B in AB and C to D in CD. The induced current is taken out through slip rings and carbon brushes. After half this rotation, AB and CD interchange their position. Now CD moves upwards and AB downward.
The direction of induced current is form D to C in CD and B to A in AB, during this half rotation. The slip rings R₁ and R₂ also rotate along with the coil ABCD. Thus, their polarities keep changing after every half rotation. Thus, the current changes its direction twice in one complete revolution of the coil. Such a current which changes its polarity after fixed intervals is called alternating current.
Function of Brushes :- The brushes B₁ and B₂ remain fixed in their positions and maintain sliding contacts with the rotatable slip rings R₁ and R₂ respectively. It is through these brushes that the current induced in the armature coil is fed to the external circuit by means of line wires.
Q.17:- When does an electric short circuit occur?
Ans:- As a result of live wire touching the neutral wire, the resistance offered to the flow of current becomes almost zero and this is called short-circuiting. In this situation, a large current flows through the circuit, causes a spark or damage to the appliance.
Q.18:- What is the function of an earth wire? Why is it necessary to earth metallic appliances?
Ans:- The earth wire connects the metallic body of the high powered appliance to the earth. It is a safety measure which ensures any leakage of current of the metallic body of the appliance keeps its potential equal to that of the earth (zero volt) and the user may not get a severe electric shock.
