Some devices in which electric motors are Water pumps, Electric fans, Electric mixers and Washing machines.

(i) The needle of the galvanometer shows a momentary deflection in a particular direction.

(ii) The needle of the galvanometer shows a momentarily in the opposite direction.

(iii) The needle of the galvanometer shows no deflection.

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.

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.

Answer

(i) Maxwell’s right hand thumb rule

(ii) Fleming’s left hand rule

(iii) Fleming’s right hand rule

Principle: An electric generator works on the principle of electromagnetic induction phenomenon. According to it, whenever a coil is rotated between the poles of a magnet, an induced current is set up in the coil, whose direction is given by Fleming�s right hand rule.

Diagram of Electric Generator

Working: Let in the beginning, as shown in Fig. brushes B1 and B2are kept pressed separately on rings R1 and R2 respectively. Let the axle attached to the rings is rotated such that arm AB of the coil moves up and arm CD moves down in the magnetic field. Due to rotation of arms AB and CD induced currents are set up in them. As per Fleming�s right hand rule induced currents in these arms are along the directions AB and CD. Thus an induced current flows along ABCD and current in the external circuit flows from B2 to B1.
After half a rotation, arm AB starts moving down and the arm CD upward. Therefore, directions of induced currents in these arms change. Thus net induced current now becomes in the direction DCBA. In the external circuit now current flows from B1 to B2. Thus after every half rotation current changes its direction and an alternating current is obtained from the generator.
Function of Brushes: Brushes are kept pressed on the two slip tings separately. Outer ends of the brushes are connected to the galvanometer (or the external load). Thus brushes help in transferring current from the coil ABCD to the external circuit.

If the insulation of the wires used in the circuit is damaged or the appliance used is faulty due to which the live wire and the neutral wire comes in direct contact as a result current in the circuit rises and the short circuit occurs.

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.

1. Choose the incorrect statement from the following regarding magnetic lines of field
(a) The direction of magnetic field at a point is taken to be the direction in which the north pole of a magnetic compass needle points
(b) Magnetic field lines are closed curves
(c) If magnetic field lines are parallel and equidistant, they represent zero field strength
(d) Relative strength of magnetic field is shown by the degree of closeness of the field lines

Ans. (c) If magnetic field lines are parallel and equidistant, they represent zero field strength Explanation: Magnetic field lines appear parallel when they are far from the magnet. But this does not mean that field strength is zero. No field line would be present where field strength becomes zero.

2. If the key in the arrangement (Figure 13.1) is taken out (the circuit is made open) and magnetic field lines are drawn over the horizontal plane ABCD, the lines are

(a) concentric circles
(b) elliptical in shape
(c) straight lines parallel to each other
(d) concentric circles near the point O but of elliptical shapes as we go away from it
Ans. (a) Concentric circles
Explanation: Magnetic field lines around a straight current carrying conductor are in the form of concentric circles.

3. A circular loop placed in a plane perpendicular to the plane of paper carries a current when the key is ON. The current as seen from points A and B (in the plane of paper and on the axis of the coil) is anti-clockwise and clockwise respectively.
The magnetic field lines point from B to A. The N-pole of the resultant magnet is on the face close to

(a) A
(b) B
(c) A if the current is small, and B if the current is large
(d) B if the current is small and A if the current is large
Ans. (a) A
Explanation: Magnetic field lines are in the direction from south pole to north pole. Point
A is showing north pole because field lines are pointing from B to A.

4. For a current in a long straight solenoid N- and S-poles are created at the two ends. Among the following statements, the incorrect statement is
(a) The field lines inside the solenoid are in the form of straight lines which indicates that the magnetic field is the same at all points inside the solenoid
(b) The strong magnetic field produced inside the solenoid can be used to magnetise a piece of magnetic material like soft iron, when placed inside the coil
(c) The pattern of the magnetic field associated with the solenoid is different from the pattern of the magnetic field around a bar magnet
(d) The N- and S-poles exchange position when the direction of current through the solenoid is reversed
Ans. (c) The pattern of the magnetic field associated with the solenoid is different from the pattern of the magnetic field around a bar magnet
Explanation: A solenoid behaves like a bar magnet. Hence, the pattern of the magnetic field associated with the solenoid is same as the pattern of the magnetic field around a bar magnet.

5. A uniform magnetic field exists in the plane of paper pointing from left to right as shown in Figure 13.3. In the field an electron and a proton move as shown. The electron and the proton experience
(a) forces both pointing into the plane of paper
(b) forces both pointing out of the plane of paper
(c) forces pointing into the plane of paper and out of the plane of paper, respectively
(d) force pointing opposite and along the direction of the uniform magnetic field respectively

Ans. (a) forces both pointing into the plane of paper
Explanation: Direction of current is opposite to the direction of movement of electron. So, current will move upwards. If index finger is showing the direction of magnetic field, ring finger is showing the direction of current, then direction of thumb is into the paper.

6. Commercial electric motors do not use
(a) an electromagnet to rotate the armature
(b) effectively large number of turns of conducting wire in the current carrying coil
(c) a permanent magnet to rotate the armature
(d) a soft iron core on which the coil is wound
Ans. (c) A permanent magnet to rotate the armature
Explanation: Electromagnet is used instead of permanent magnet in commercial electric motor.

7. In the arrangement shown in Figure 13.4 there are two coils wound on a nonconducting cylindrical rod. Initially the key is not inserted. Then the key is inserted and later removed. Then

(a) the deflection in the galvanometer remains zero throughout
(b) there is a momentary deflection in the galvanometer but it dies out shortly and there is no effect when the key is removed
(c) there are momentary galvanometer deflections that die out shortly; the deflections are in the same direction
(d) there are momentary galvanometer deflections that die out shortly; the deflections are in opposite directions

Ans. (d) there are momentary galvanometer deflections that die out shortly; the deflections are in opposite directions.

Explanation: When key is plugged, galvanometer shows momentary deflection in one direction. When the key is removed, galvanometer shows momentary deflection in opposite direction.

8. Choose the incorrect statement
(a) Fleming’s right-hand rule is a simple rule to know the direction of induced current
(b) The right-hand thumb rule is used to find the direction of magnetic fields due to current carrying conductors
(c) The difference between the direct and alternating currents is that the direct current always flows in one direction, whereas the alternating current reverses its direction periodically
(d) In India, the AC changes direction after every 1/50 second
Ans. (d) In India, the AC changes direction after every 1/50 second
Explanation: In India, the AC changes direction after every 1/100 second.

9. A constant current flows in a horizontal wire in the plane of the paper from east to west as shown in Figure 13.5. The direction of magnetic field at a point will be North to South

(a) directly above the wire
(b) directly below the wire
(c) at a point located in the plane of the paper, on the north side of the wire
(d) at a point located in the plane of the paper, on the south side of the wire
Ans. (b) directly below the wire
Explanation: Line WE shows a straight conductor through which current is moving from E to W. When seen from east, the magnetic field lines appear in clockwise direction, i.e. S to N above the wire and N to S below the wire. This is in accordance with Right Hand Thumb rule.

10. The strength of magnetic field inside a long current carrying straight solenoid is
(a) more at the ends than at the centre
(b) minimum in the middle
(c) same at all points
(d) found to increase from one end to the other


Ans. (c) Same at all points
Explanation: Magnetic field lines are straight and parallel inside the solenoid. This indicates a strong magnetic field. Hence, inside the solenoid, the magnetic field is same throughout.

11. To convert an AC generator into DC generator
(a) split-ring type commutator must be used
(b) slip rings and brushes must be used
(c) a stronger magnetic field has to be used
(d) a rectangular wire loop has to be used
Ans. (a) split-ring type commutator must be used
Explanation: Split ring commutator reverses the direction of current after each half turn of armature. This maintains a DC current.

12. The most important safety method used for protecting home appliances from short circuiting or overloading is
(a) earthing
(b) use of fuse
(c) use of stabilizers
(d) use of electric meter
Ans. (b) Use of fuse