Practice Questions

Q18.To find the resistance of a galvanometer by the half deflection method the following circuit is used with resistances R1 = 9970 W, R2 = 30 W and R3 = 0 . The deflection in the galvanometer is d. With R3 = 107 W the deflection changed to d2 . The galvanometer resistance is approximately: (1) 107Ω (2) 137Ω (3) 107/2Ω (4) 77Ω

Q19.Choose the correct sketch of the magnetic field lines of a circular current loop shown by the dot and the cross ⊗. JEE Main 2013 (22 Apr Online) JEE Main Previous Year Paper (1) (2) (3) (4)

Q19.A rectangular loop of wire, supporting a mass m, hangs with one end in a uniform magnetic field →B pointing out of the plane of the paper. A clockwise current is set up such that i > mg/Ba, where a is the width of the JEE Main 2013 (23 Apr Online) JEE Main Previous Year Paper loop. Then : (1) The weight rises due to a vertical force caused by (2) The weight do not rise due to vertical for caused the magnetic field and work is done on the by the magnetic field and work is done on the system. system. (3) The weight rises due to a vertical force caused by (4) The weight rises due to a vertical force caused by the magnetic field but no work is done on the the magnetic field and work is extracted from the system. magnetic field.

Q19.A metallic rod of length l is tied to a string of length 2l and made to rotate with angular speed ω on a horizontal table with one end of the string fixed. If there is a vertical magnetic field B in the region, the e.m.f. induced across the ends of the rod is: (1) 4Bωl2 (2) 5Bωl2 2 2 (3) 2Bωl2 (4) 3Bωl2 2 2

Q20.A circular loop of radius 0. 3 cm lies parallel to a much bigger circular loop of radius 20 cm. The centre of the small loop is on the axis of the bigger loop. The distance between their centres is 15 cm. If a current of 2. 0 A flows through the smaller loop, then the flux linked with a bigger loop is: (1) 3 .3 ×10−11 weber (2) 6 .6 ×10−9 weber (3) 9 .1 ×10−11 weber (4) 6 × 10−11 weber

Q20.An electric current is flowing through a circular coil of radius R. The ratio of the magnetic field at the centre of the coil and that at a distance 2√2R from the centre of the coil and on its axis is : (1) 2√2 (2) 27 (3) 36 (4) 8

Q20.A current i is flowing in a straight conductor of length L. The magnetic induction at a point on its axis at a distance L from its centre will be : 4 (1) Zero (2) μ0i 2πL (3) μ0i (4) 4μ0i √2L √5πL

Q20.A particle of charge 16 × 10−16C moving with velocity 10 ms−1 along x-axis enters a region where magnetic field of induction →B is along the y-axis and an electric field of magnitude 104Vm−1 is along the negative z- axis. If the charged particle continues moving along x-axis, the magnitude of →B is : (1) 16 × 103 Wb m−2 (2) 2 × 103 Wb m−2 (3) 1 × 103 Wb m−2 (4) 4 × 103 Wb m−2

Q21.An LCR circuit as shown in the figure is connected to a voltage source Vac whose frequency can be varied. The frequency, at which the voltage across the resistor is maximum, is : (1) 902 Hz (2) 143 Hz (3) 23 Hz (4) 345 Hz

Q21.The amplitude of a damped oscillator decreases to 0.9 times its original magnitude in 5s. In another 10s it will decrease to α times its original magnitude, where α equals : JEE Main 2013 (07 Apr) JEE Main Previous Year Paper (1) 0.729 (2) 0.6 (3) 0.7 (4) 0.81

Q21.The plot given below is of the average power delivered to an LRC circuit versus frequency. The quality factor of the circuit is : (1) 5.0 (2) 2.0 (3) 2.5 (4) 0.4

Q22.In a series L −C −R circuit, C = 10−11 Farad, L = 10−5 Henry and R = 100Ohm, when a constant D.C. voltage E is applied to the circuit, the capacitor acquires a charge 10−9 C. The D.C. source is replaced by a sinusoidal voltage source in which the peak voltage E0 is equal to the constant D.C. voltage E. At resonance the peak value of the charge acquired by the capacitor will be : JEE Main 2013 (22 Apr Online) JEE Main Previous Year Paper (1) 10−15C (2) 10−6C (3) 10−10C (4) 10−8C

Q22.In the circuit shown here, the voltage across E and C are respectively 300 V and 400 V . The voltage E of the ac source is : (1) 400 Volt (2) 500 Volt (3) 100 Volt (4) 700 Volt

Q22.The magnetic field in a travelling electromagnetic wave has a peak value of 20 nT. The peak value of electric field strength is : (1) 9 V m−1 (2) 12 V m−1 (3) 3 V m−1 (4) 6 V m−1

Q23.A plane electromagnetic wave in a non-magnetic dielectric medium is given by →E = →E0 (4 × 10−7x −50t) with distance being in meter and time in seconds. The dielectric constant of the medium is : (1) 2.4 (2) 5.8 (3) 8.2 (4) 4.8

Q24.The focal length of the objective and the eyepiece of a telescope are 50 cm and 5 cm respectively. If the telescope is focussed for distinct vision on a scale distant 2 m from its objective, then its magnifying power will be : (1) −4 (2) −8 (3) +8 (4) −2

Q24.A light ray falls on a square glass slab as shown in the diagram. The index of refraction of the glass, if total internal reflection is to occur at the vertical face, is equal to : (1) (√2+1) (2) 2 √52 (3) 3 (4) 2 √32

Q24.Light is incident from a medium into air at two possible angles of incidence (A) 20∘ and (B) 40∘ . In the medium light travels 3.0 cm in 0.2 ns. The ray will : JEE Main 2013 (09 Apr Online) JEE Main Previous Year Paper (1) suffer total internal reflection in both cases (A) (2) suffer total internal reflection in case (B) only and (B) (3) have partial reflection and partial transmission in (4) have 100% transmission in case (A) case (B)

Q24.Diameter of a plano - convex lens is 6 cm and thickness at the centre is 3 mm. If the speed of light in the material of lens is 2 × 108 m s−1 , the focal length of the lens is: (1) 30 cm (2) 10 cm (3) 15 cm (4) 20 cm

Q25.The image of an illuminated square is obtained on a screen with the help of a converging lens. The distance of the square from the lens is 40 cm. The area of the image is 9 times that of the square. The focal length of the lens is : (1) 36 cm (2) 27 cm (3) 60 cm (4) 30 cm

Q25. n identical waves each of intensity I0 interfere with each other. The ratio of maximum intensities if the interference is (i) coherent and (ii) incoherent is : (1) n2 (2) n1 (3) 1 (4) n n2

Q25.A beam of unpolarised light of intensity I0 is passed through a polaroid A and then through another polaroid B which is oriented so that its principle plane makes an angle of 45o relative to that of A . The intensity of the emergent light is: (1) I0 (2) I0 4 8 (3) I0 (4) I02

Q26.Electrons are accelerated through a potential difference V and protons are accelerated through a potential difference 4 V. The de-Broglie wavelengths are λe and λp for electrons and protons respectively. The ratio of λe is given by: (given me is mass of electron and mp is mass of proton). λp (1) = λp λe = √mpme (2) λpλe √memp (3) = λp λe = 12 √memp (4) λpλe 2√mpme

Q27.In the Bohr's model of hydrogen-like atom the force between the nucleus and the electron is modified as e2 1 F = + β , where β is a constant. For this atom, the radius of the nth orbit in terms of the Bohr 4πε0 ( r2 r3 ) ε0h2 radius = is : (a0 mπe2 ) (1) rn = a0n −β (2) rn = a0n2 + β (3) rn = a0n2 −β (4) rn = a0n + β JEE Main 2013 (23 Apr Online) JEE Main Previous Year Paper

Q27.In the Bohr model an electron moves in a circular orbit around the proton. Considering the orbiting electron to be a circular current loop, the magnetic moment of the hydrogen atom, when the electron is in nth excited state, is : m e ) nh2π (1) ( 2me n2h2π ) (2) ( (3) ( 2me ) nh2π (4) ( me ) n2h2π JEE Main 2013 (09 Apr Online) JEE Main Previous Year Paper