Practice Questions

Q21.A bar magnet is demagnetized by inserting it inside a solenoid of length 0.2 𝑚, 100 turns, and carrying a current of 5.2 𝐴. The coercivity of the bar magnet is: JEE Main 2019 (09 Jan Shift 1) JEE Main Previous Year Paper (1) 285 𝐴/ 𝑚 (2) 2600 𝐴/ 𝑚 (3) 520 𝐴/ 𝑚 (4) 1200 𝐴/ 𝑚

Q21.Two very long, straight, and insulated wires are kept at 90𝑜 angle from each other in xy - plane as shown in figure. These wires carry currents of equal magnitude I , whose direction are shown in the figure. The net magnetic field at point P will be: JEE Main 2019 (08 Apr Shift 2) JEE Main Previous Year Paper μ0I +μ0I (1) - 2πd ^x + ^y (2) πd ^z μ0I (3) ^x + ^y (4) Zero 2πd

Q21.A thin ring of 10 cm radius carries a uniformly distributed charge. The ring rotates at a constant angular speed of 40π rads-1 about its axis, perpendicular to its plane. Is the magnetic field its centre is 3.8 × 10-9T , then the charge carried by the ring is close to μ0 = 4π × 10-7N / A2 . (1) 4 × 10-5 C (2) 3 × 10-5 C (3) 2 × 10-6 C (4) 7 × 10-6 C

Q21.In the figure shown, a circuit contains two identical resistors with resistance R = 5 Ω and an inductance with L = 2 mH. An ideal battery of 15V is connected in the circuit. What will be the current through the battery long after the switch is closed? (1) 5.5 A (2) 6 A (3) 3 A (4) 7.5 A

Q22.A copper wire is wound on a wooden frame, whose shape is that of an equilateral triangle. If the linear dimension of each side of the frame is increased by a factor of 3, keeping the number of turns of the coil per unit length of the frame the same, then the self inductance of the coil: (1) decreases by a factor of 9 (2) increases by a factor of 27 (3) increases by a factor of 3 (4) decreases by a factor of 9√3

Q22.A rectangular coil (Dimension 5 cm × 2.5 cm ) with 100 turns, carrying a current of 3 A in the clock-wise direction, is kept centered at the origin and in the X −Z plane. A magnetic field of 1 T is applied along X −axis . If the coil is tilted through 45° about Z −axis , then the torque on the coil is: (1) 0.42 N m (2) 0.55 N m (3) 0.38 N m (4) 0.27 N m

Q22.The figure shows a square loop 𝐿 of side 5 cm which is connected to a network of resistances. The whole setup is moving towards the right with a constant speed of 1 cm s-1 . At some instant, a part of 𝐿 is in a uniform magnetic field of 1T perpendicular to the plane of the loop. If the resistance of 𝐿 is 1 . 7 Ω, the current in the loop at that instant will be close to: JEE Main 2019 (12 Apr Shift 1) JEE Main Previous Year Paper (1) 115 μA (2) 60 μA (3) 150 μA (4) 170 μA

Q22.A series AC circuit containing an inductor (20 mH), a capacitor (120 μF) and a resistor (60 Ω) is driven by an AC source of 24 V/50 Hz. The energy dissipated in the circuit in 60 s is: (1) 5.17 × 102 J (2) 3.39 × 103 J (3) 2.26 × 103 J (4) 5.65 × 102 J

Q22.A current loop, having two circular arcs joined by two radial lines is shown in the figure. It carries a current of 10 𝐴. The magnetic field at point 𝑂 will be close to: (1) 1.5 × 10-7 𝑇 (2) 1.0 × 10-7 𝑇 (3) 1.5 × 10-5 𝑇 (4) 1.0 × 10-5 𝑇

Q22.A plane electromagnetic wave having a frequency f = 23.9 GHz propagates along the positive z− direction in free space. The peak value of the Electric Field is 60 V/m. Which among the following is the acceptable magnetic field component in the electromagnetic wave? → → (1) (2) B = 2 × 10−7 sin(0.5 × 103z −1.5 × 1011t)ˆi B = 2 × 10−7 sin(1.5 × 103x + 0.5 × 1011t)ˆj → → (3) (4) B = 2 × 107 sin(0.5 × 103z + 1.5 × 1011t)ˆi B = 60 sin(0.5 × 103x + 1.5 × 1011t)ˆi

Q22.At some location the horizontal component of earth's magnetic field is 18 × 10−6 T. At this location, magnetic needle of length 0.12 m and pole strength 1.8 Am is suspended from its mid-point using a thread, it makes 45° angles with horizontal in equilibrium. To keep this needle horizontal, the vertical force that should be applied at one of its ends is: (1) 1.8 × 10−5 N (2) 3.6 × 10−5 N (3) 6. 5 × 10−5 N (4) 1.3 × 10−5 N

Q22.An electromagnetic wave of intensity 50Wm−2 enters in a medium of refractive index 'n' without any loss. The ratio of the magnitudes of electric fields, and the ratio of the magnitudes of magnetic fields of the wave before and after entering into the medium are respectively, given by : (1) ( √n1 , √n1 ) (2) (√n, √n) (3) (√n, √n1 ) (4) ( √n1 , √n)

Q22.An alternating voltage 𝑉( 𝑡) = 220sin100𝜋𝑡 volt is applied to a purely resistive load of 50 Ω . The time taken for the current to rise from half of the peak value to the peak value is: (1) 7.21 ms (2) 5 . 25 ms (3) 2.24 ms (4) 3.33 ms

Q22.A solid metal cube of edge length 2 cm is moving in the positive y-direction, at a constant speed of 6 m s−1 . There is a uniform magnetic field of 0.1 T in the positive z-direction. The potential difference between the two faces of the cube, perpendicular to the x-axis, is (1) 12 mV (2) 1 mV (3) 2 mV (4) 6 mV

Q22.The mean intensity of radiation on the surface of the Sun is about 108W/m2. The rms value of the corresponding magnetic field is closest to: (1) 1T (2) 10−2T (3) 102T (4) 10−4T

Q22.Given below in the left column are different modes of communication using the kinds of waves given in the right column. (1) Optical Fibre Communication (P) Ultrasound (2) Radar (Q) Infrared Light (3) Sonar (R) Microwaves (4) Mobile Phones (S) Radio Waves From the options given below, find the most appropriate match between entries in the left and the right column. (1) 1 −Q, 2 −S, 3 −P, 4 −R (2) 1 − S, 2 −Q, 3 −R, 4 −P (3) 1 −Q, 2 −S, 3 −R, 4 −P (4) 1 −R, 2 −P, 3 −S, 4 −Q

Q22.The magnitude of the magnetic field at the centre of an equilateral triangular loop of side 1 m which is carrying a current of 10 A is: [Take μ0 = 4π × 10-7 N A-2 ] (1) 3 μT (2) 1 μT (3) 18 μT (4) 9 μT

Q22.A very long solenoid of radius R is carrying current I(t) = kte−αt(k > 0), as a function of time (t ≥0). Counterclockwise current is taken to be positive. A circular conducting coil of radius 2R is placed in the equitorial plane of the solenoid and concentric with the solenoid. The current induced in the outer coil is correctly depicted, as a function of time, by: JEE Main 2019 (09 Apr Shift 2) JEE Main Previous Year Paper (1) (2) (3) (4)

Q22.A point source of light, S is placed at a distance L in front of the center of plane mirror of width d which is hanging vertically on a wall. A man walks in front of the mirror along a line parallel to the mirror, at a distance 2L as shown below. The distance over which the man can see the image of the light source in the mirror is: (1) 3d (2) 2d (3) d (4) d 2

Q22.Two magnetic dipoles X and Y are placed at a separation d , with their axes perpendicular to each other. The dipole moment of Y is twice that of X . A particle of charge q is passing through their mid-point P , at angle θ = 45o with the horizontal line, as shown in figure. What would be the magnitude of force on the particle at that instant? ( d is much larger than the dimension of the dipole) μ0 M × q𝑣 (1) 0 (2) √2 4π d 3 2 μ0 M μ0 2M (3) × q𝑣 (4) × q𝑣 4π d 3 4π d 3 2 2

Q23.A 27 mW laser beam has a cross-sectional area of 10 mm2 . The magnitude of the maximum electric field in this electromagnetic wave is given by: [Given permittivity of space ϵ0 = 9 × 10−12SI units, Speed of light c = 3 × 108 m/s] (1) 2kV/m (2) 0.7kV/m (3) 1kV/m (4) 1.4kV/m

Q23.An electromagnetic wave is represented by the electric filed →E = E0^nsin⁡ωt + 6𝑦- 8𝑧. Taking unit vectors in 𝑥, 𝑦 and 𝑧 directions to be ^𝑖, ^𝑗, ^𝑘, the directions of propogations ^s, is: (1) 3^𝑖- 4^𝑗 (2) -3^𝑖+ 4^𝑗 ^s = ^s = 5 5 4^𝑖- 3^𝑘 -4^𝑖- 3^𝑘 (3) (4) ^s = ^s = 5 5

Q23.A plano - convex lens (focal length f2 , refractive index μ2, radius of curvature R) fits exactly into a plano - concave lens (focal length f1, refractive index μ1, radius of curvature R). Their plane surfaces are parallel to each other. Then, the focal length of the combination will be: R (1) (2) f1 −f2 μ2 − μ1 (3) f1 + f2 (4) f1+f22f1f2

Q23.If the magnetic field of a plane electromagnetic wave is given by (The speed of light = 3 × 108 m/s) B = 100 × 10−6 sin[2π × 2 × 1015(t −xc )] then the maximum electric field associated with it is: (1) 3 × 104 N C−1 (2) 4 × 104 N C−1 (3) 4.5 × 104 N C−1 (4) 6 × 104 N C−1

Q23.A coil of self inductance 10 mH and resistance of 0.1 Ω is connected through a switch to a battery of internal resistance 0.9 Ω . After the switch is closed, the time taken for the current to attain 80% of the saturation value is: [ln5 = 1.6] (1) 0.103 s (2) 0.002 s (3) 0.324 s (4) 0.016 s