Video Lecture
Theory For Making Notes
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Practice Questions (Level-1)
Q.1
A long vertical straight conductor (not shown) is placed at O in figure, carries an inward current of 5 A. A small straight wire X of length 0.03 m is placed along the tangent to the circle of centre O and radius 0.1m as shown. If X carries a current of 2A, the force on X in N is
(a) 9 ´ 10-7 (b) 6 ´ 10-7 (c) zero (d) 3 ´ 10-7
Ans. (c)
Q.2
A long current-carrying wire W is free to move when placed in a magnetic field B at right angles to the field as shown
(a) it will move along the magnetic field to the right
(b) it will move upwards towards you
(c) it will move downwards away from you
(d) it will not move at all
Ans. (c)
Q.3
A rectangular loop carrying a current I is situated near a long straight wire such that the wire is parallel to one of the sides of loop. If a steady current I is established in the wire as shown in fig., the loop will
(a) rotate about an axis parallel to the wire
(b) move away from the wire
(c) move towards the wire
(d) remain stationary
Ans : (c)
Q.4
A circular coil of wire carries a current. PQ is part of a very long wire carrying a current and passing close to the circular coil. If the direction of the current are as shown in the figure. Then the direction of the force acting on PQ will be
(a) parallel to PQ, towards P
(b) parallel to PQ, towards Q
(c) at right angles to PQ, to the right
(d) at right angles to PQ, to the left
Ans : (d)
Q.5
In the figure, X and Y are two long straight conductors each carrying a current of 2 A. The force on each conductor is F. When the current in each is changed to 1 A and reversed in direction, the force on each is now
(a) F/4 and unchanged direction
(b) F/2 and reversed direction
(c) F/2 and unchanged direction
(d) F/4 and reversed direction
Ans. (a)
Q.6
P, Q and R are long parallel straight wires equally spaced in air, carrying current as shown in the figure. What is the direction of the resultant force on Q?
(a) to the left
(b) to the right
(c) perpendicular to plane of paper and direction inside
(d) perpendicular to plane of paper and direction upward
Ans : (b)
7.
A 3.0 cm wire carrying a current of 10A is placed inside a solenoid perpendicular to its axis. The magnetic field inside the solenoid is given to the 0.27 T. What is the magnetic force on the wire?find the force experienced by the conductor.
(a) 8.1\times {{10}^{{2}}}N
(b) 7.5\times {{10}^{{-2}}}N
(c) 8.1\times {{10}^{{5}}}N
(d) 8.1\times {{10}^{{-2}}}N
Ans (d)
8.
A conductor of length 20 cm is placed
(i)
parallel
(a)zero
(b)3
(c)9
(d)15
Ans (a)
(ii)
perpendicular
(a)0.8N
(b)0.2N
(c)1.0N
(d)2.5N
Ans (a)
(iii)
inclined at an angle 30°, to a uniform magnetic field of 2T. If the charge of 10 C passes through it in 5s.
(a)0.4N
(b)0.8N
(c)1.5N
(d) 2.0N
Ans (a)
9.
A long horizontal rigidly supported wire carries a current of 100A. Directly above it and parallel to it is a fine wire that carries a current of 200A and weighs 0.05 N/m. How far above the lower wire should the fire wire be kept to support it by magnetic repulsion?
(a)2 cm
(b) 4 cm
(c)6 cm
(d)8 cm
Ans (d)
10.
Find the magnetic force on segment PQ, due to a current of 5A flowing in it, if it is placed in a magnetic field of 0.25 T.
(a)0.20 N
(b)0.28 N
(c)0.35 N
(d)0.50 N
Ans (b)
11.
A current of 10A flows through each of two parallel long wires. The wires are 5 cm apart. Calculate the force acting per unit length of each wire. Use the standard value of constant required.
(a) 5\times {{10}^{{-4}}}N{{m}^{{-1}}}
(b) 9\times {{10}^{{-4}}}N{{m}^{{-1}}}
(c) 3\times {{10}^{{-4}}}N{{m}^{{-1}}}
(d) 4\times {{10}^{{-4}}}N{{m}^{{-1}}}
Ans(d)
12.
A short conductor of length 5.0 cm is placed parallel to a long conductor of length 1.5 cm near its center. The conductors carry currents 4.0 A and 3.0 A respectively in the same direction. What is the total force experienced by the ling conductor, when they are 3.0 cm apart?
(a) 2\times {{10}^{{-2}}}N
(b) 4\times {{10}^{{6}}}N
(c) 5\times {{10}^{{-6}}}N
(d) 4\times {{10}^{{-6}}}N
Ans (d)
13.
Two straight long parallel wires A and B carry currents of 5A and 10A respectively in opposite directions. The distance between the two wires is 0.1 m. Calculate the force on a 0.2 m section of the sire B near its center.
(a) 5\times {{10}^{{-5}}}\,N
(b) 2\times {{10}^{{5}}}\,N
(c) 2\times {{10}^{{-5}}}\,N
(d) 5\times {{10}^{{5}}}\,N
Ans (c)
Practice Questions (Level-2)
Q.1
In figure shows three long straight wires P, Q and R carrying currents normal to the plane of the copper. All three currents have the same magnitude. Which arrow best shows the direction of the resultant force on the wire P?
(a) A (b) B (c) C (d) D
Ans. (c)
Q.2
A horizontal power cable carries a constant current into the plane of the paper. Which arrow shows the direction of the force on the cable caused by the earth’s magnetic field in a region where the field is at 70° to the horizontal?
(a) A (b) B (c) C (d) D
Ans. (d)
Q.3
A conducting circular loop of radius r carries a constant current l. It is placed in uniform magnetic field B such that B is perpendicular to the plane of the loop. The net magnetic force acting on the loop is
(a) I\,r\,B
(b) 2\pi \,\,r\,IB
(c) Zero
(d) \pi \,\,r\,IB
Ans. (c)
Q.4
P, Q and R are long parallel straight wires in air, carrying currents as shown in figure. What is the direction of the resultant force on
(a) To the left
(b) To the right
(c) Perpendicular to this page
(d) The same as that of the current in Q
Ans. (d)
Q.5
Find force on the segment CD as shown in figure, if B=0.15\,T and I=5\,A (\sin 65{}^\circ =0.9063)
(a) Zero
(b) 0.12 N
(c) 0.13 N
(d) 0.14 N
Ans. (d)
Q.6
An irregular closed loop carrying a current has such a shape that the entire loop cannot lie in a single plane. If this loop is placed in a uniform magnetic field, the force acting on the loop
(a) Must be zero
(b) Can never be zero
(c) Can be zero only for one particular direction of the
(d) Cannot be ascertained
Ans. (a)
Q.7
As shown in figure a bar of mass m is suspended by two springs. A magnetic field B is directed out of the paper. Each spring has spring factor K. What is the description of motion of bar when a current I is sent through it in the direction shown?
(a) The equilibrium is shifted upward by \frac{{BIL}}{{2k}}
(b) The equilibrium is shifted downward by \frac{{BIL}}{{2k}}
(c) The equilibrium is shifted downward by \frac{{BIl}}{{2(k+mg)}}
(d) The equilibrium is shifted upward by \frac{{BI\,l}}{{(2k+mg)}}
Ans. (a)