Page no – 224

Solution 1

A compass needle is a small bar magnet. When it is brought near a bar magnet, its magnetic field lines interact with that of the bar magnet. Hence, a compass needle shows a deflection when brought near a bar magnet.

Concept Comprehension: Magnetic fields interact with each other. Interaction of magnetic field of earth and that of the compass needle is used to find directions.

Page no – 228

Solution 1

Magnetic field lines of a bar magnet emerge from the north pole and terminate at the south pole. Inside the magnet, the field lines emerge from the south pole and terminate at the north pole, as shown in the given figure.

Concept Comprehension:–  Note that magnetic field lines never intersect each other.

Solution 2

The properties of magnetic lines of force are as follows.

1. Outside a magnet, magnetic field lines are directed from North pole to South pole.

2. The direction of field lines inside the magnet is from the South pole to the North pole.

3. Magnetic lines do not intersect with each other.

4. Magnetic lines of force are crowded near the poles of a magnet but they are widely separated at other places.

Solution 3

If two field lines of a magnet intersect, then at the point of intersection, there would be two directions of magnetic field. This is not possible. Hence, two field lines do not intersect each other.

Concept Comprehension:–  Note that magnetic field lines never intersect each other.

Page no – 229

Solution 1

Applying right hand thumb rule to the loop:For right side of the circular loop, the direction of magnetic field lines will be as if they are emerging from the table outside the loop and merging in the table inside the loop.

Similarly, for left side of the circular loop, the direction of magnetic field lines will be as if they are emerging from the table outside the loop and merging in the table inside the loop, as shown in the given figure.

Solution 2

Concept Comprehension: The uniform magnetic field is represented by parallel and equidistant magnetic field lines.

Page no – 230

Solution 1

(d) is the same at all points
Concept Comprehension:-  The magnetic field inside a long, straight, current-carrying solenoid is uniform. It is the same at all points inside the solenoid.

Page no – 231

Solution 1

(c) velocity and

(d) momentum

Concept Comprehension:-  When a proton enters in a region of magnetic field, it experiences a magnetic force. As a result of the force, the path of the proton becomes circular. So, its velocity and hence momentum change.

Page no – 232

Solution 1

Concept Comprehension:-A force is experienced by a current-carrying conductor placed in a magnetic field. The magnitude of force increases with the increase in the amount of current, strength of the magnetic field, and the length of the conductor. Hence, the magnetic force exerted on rod AB and its deflection will increase i

(i) current in rod AB is increased

(ii) a stronger horse-shoe magnet is used

(iii) length of rod AB is increased

Solution 2

(d) upwardThe direction of the magnetic field can be determined by the Fleming’s left hand rule.

Concept Comprehension:- According to Fleming’s left hand rule, if we arrange the thumb, the central finger, and the forefinger of the left hand at right angles to each other and if the forefinger points in the direction of magnetic field, the central finger points in the direction of current, then the thumb points in the direction of motion of the conductor.Since the direction of positively charged alpha particle is towards west, the direction of current will be the same i.e., towards west. Again, the direction of magnetic force is towards north. Hence, according to Fleming’s left hand rule, the direction of magnetic field will be upwards.

Page no – 233

Solution 1

According to Fleming’s left hand rule, if we arrange the thumb, the central finger, and the forefinger of the left hand at right angles to each other and if the forefinger points in the direction of magnetic field, the central finger points in the direction of current, then the thumb points in the direction of motion or the force acting on the conductor.

Solution 2

The working principle of an electric motor is based on the magnetic effect of current. A current-carrying loop experiences a force and rotates when placed in a magnetic field. The direction of rotation of the loop is given by the Fleming’s left-hand rule.

Solution 3

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.

Page no – 236

Solution 1

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

1. Induce current on a coil by moving the coil in a magnetic field.

2. Induce current on a coil by changing the  magnetic field across it.

3. If a coil is moved rapidly between the two poles of a horse-shoe magnet, then an electric current is induced in the coil. 

Page no – 237

Solution 1

Concept Comprehension:- An electric generator works on the principle of electromagnetic induction. It generates electricity by rotating a coil in a magnetic field.

Solution 2

Concept Comprehension:- Some sources of direct current are cell, DC generator, etc.

Solution 3

Concept Comprehension:- AC generators, power plants, etc., produce alternating current.

Solution 4(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.
Concept Comprehension:- The commutator reverses the direction of current flowing through the coil after each half rotation of the coil.

Page no – 238

Solution 1

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.

Concept Comprehension:- Safety measures like fuse and earthing are very necessary in case of electrical circuits and appliances as an electric shock may prove to be deadly.

Solution 2

Concept Comprehension:-Current drawn by the electric oven can be obtained by the expression,

P = VI

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.

Solution 3

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.

Concept Comprehension:- Safety measures like fuse and earthing are very necessary in case of electrical circuits and appliances as an electric shock may prove to be deadly.

Page no – 240

Solution 1

(d) The field consists of concentric circles centred on the wire.

Concept Comprehension:- The magnetic field lines, produced around a straight current-carrying conductor, are concentric circles. Their centres lie on the wire.

Solution 2

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

Concept Comprehension:- When a coil and a magnet are moved relative to each other, a current is induced in the coil. This phenomenon is known as electromagnetic induction. 

Solution 3(a) generator

Concept Comprehension:- An electric generator produces electric current. It converts mechanical energy into electricity. 

Solution 4

(d) AC generator has slip rings while the DC generator has a commutator

Concept Comprehension:- 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. 

Solution 5

(c) increases heavily

Concept Comprehension:- 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. 

Solution 6

(a) False
Concept Comprehension:- An electric motor converts electrical energy into mechanical energy.
(b) True
Concept Comprehension:- 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
Concept Comprehension:- A long circular coil is a long solenoid. The magnetic field lines inside the solenoid are parallel lines.
(d) False
Concept Comprehension:- Live wire has red insulation cover, whereas earth wire has green insulation colour in the domestic circuits. 

Solution 7

Two methods of producing magnetic field are as follows:

(a) By using current-carrying conductors

(b) By using permanent magnets

(c) By electromagnets

Page no – 241

Solution 1

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

Solution 2

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.

Solution 3

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.

Solution 4

Concept Comprehension:- 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 clockwise. This happens because an inward force acts on length MN and at the same time, an outward force acts on length ST. As a result, the coil rotates 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, an inward 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 outward force acts on the length ST. These two forces cause the coil to rotate 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.

Solution 5

Some devices in which electric motors are used are as follows:

(a) Water pumps
(b) Electric fans
(c) Electric mixers
(d) Washing machines

Concept Comprehension:- An electric motor converts electrical energy into mechanical energy.

Solution 6

Concept Comprehension:- A current induces in a coil if a bar magnet is moved relative to it. This is the principle of electromagnetic induction.

(i) When a bar magnet is pushed into a coil of insulated copper wire, a current is induced momentarily in the coil. As a result, the needle of the galvanometer deflects momentarily in a particular direction.

(ii) When the bar magnet is withdrawn from inside the coil of the insulated copper wire, a current is again induced momentarily in the coil in the opposite direction. As a result, the needle of the galvanometer deflects momentarily in the opposite direction.

(iii) When a bar magnet is held stationary inside the coil, no current will be induced in the coil. Hence, galvanometer will show no deflection. 

Solution 7

Two circular coils A and B are placed close to each other. When the current in coil A is changed, the magnetic field associated with it also changes. 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 is called electromagnetic induction.

Concept Comprehension:- In electromagnetic induction the change in magnetic field induces a current in the conductor.

Solution 8
(i) Right hand thumb rule
(ii) Fleming’s left hand rule
(iii) Fleming’s right hand rule 

Solution 9

An electric generator converts mechanical energy into electrical energy. 

Concept Comprehension:- An electric generator consists of a rotating rectangular coil placed between the two poles of a permanent magnet.

·         The two rings R1 and R2 are internally attached to an axle. The axle may be mechanically rotated from outside to rotate the coil inside the magnetic field.·         

Outer ends of the two brushes B1 and B2 are connected to the galvanometer to show the flow of current in the given external circuit.·         When the axle is rotated, arm AB moves up (and the arm CD moves down) in the magnetic field produced by the permanent magnet. Let us say that the coil ABCD is rotated clockwise.·         

By applying Fleming’s right-hand rule, the induced currents are set up in these arms along the directions AB and CD. Thus, an induced current flows in the direction ABCD. If there are more turns in the coil, the current generated in each turn adds up to give a large current through the coil.·        

 After half rotation, CD and AB reverse direction, and thus, CD starts moving up and AB starts moving down. As a result, the directions of the induced currents in both the arms change.·         

Such a current which changes direction after equal intervals of time is called an alternating current. Thus, this device is called an AC generator.·         

To get a direct current, a split-ring type commutator must be used. The generator is thus called a DC generator.·         

The difference between direct and alternating currents is that the direct current always flows in one direction, whereas the alternating current reverses its direction periodically.

Solution 10

An electric short circuit occurs when the live wire and the neutral wire come in direct contact. This happens when the plastic insulation of live wire and neutral wire gets torn or when there is a fault in the electrical appliance.

Solution 11

The metallic body of electric appliances is connected to the Earth by means of earth wire so that any leakage of electric current is transferred to the ground. This prevents any electric shock to the user. That is why earthing of the electrical appliances is necessary.