Magnetic Effects of Electric Current MCQ
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
