Practice Questions

Q15.The time taken for the magnetic energy to reach 25% of its maximum value, when a solenoid of resistance R, inductance L is connected to a battery, is : (1) L ln5 (2) infinite R (3) R L ln2 (4) RL ln10

Q15.Figure A and B shown two long straight wires of circular cross-section ( a and b with a < b ), carrying current I which is uniformly distributed across the cross-section. The magnitude of magnetic field B varies with radius r and can be represented as: JEE Main 2021 (27 Jul Shift 2) JEE Main Previous Year Paper (1) (2) (3) (4)

Q15.An alternating current is given by the equation i = i1 sin ωt + i2 cos ωt. The rms current will be: (1) 1 (i1 + i2)2 (2) 1 (i1 + i2) √2 √2 2 (3) 1 1 1 2 (i21 + i22) 2 (4) √21 (i21 + i22)

Q15.The refractive index of a converging lens is 1. 4. What will be the focal length of this lens if it is placed in a medium of same refractive index ? (Assume the radii of curvature of the faces of lens are R1 and R2 respectively) (1) 1 (2) Infinite (3) R1R2 (4) Zero R1−R2

Q16.An oil drop of the radius 2 mm with a density 3 g cm−3 is held stationary under a constant electric field 3. 55 × 105 V m−1 in the Millikan's oil drop experiment. What is the number of excess electrons that the oil drop will possess? (consider g = 9. 81 m s−2). (1) 48. 8 × 1011 (2) 1. 73 × 1010 (3) 17. 3 × 1010 (4) 1. 73 × 1012

Q16.In a photoelectric experiment, ultraviolet light of wavelength 280 nm is used with lithium cathode having work function ϕ = 2. 5 eV. If the wavelength of incident light is switched to 400 nm, find out the change in the stopping potential. (h = 6. 63 × 10−34 J s, c = 3 × 108 m s−1) JEE Main 2021 (26 Aug Shift 1) JEE Main Previous Year Paper (1) 1. 1 V (2) 0. 6 V (3) 1. 3 V (4) 1. 9 V

Q16.The de Broglie wavelength of a proton and α -particle are equal. The ratio of their velocities is (1) 4 : 1 (2) 4 : 3 (3) 1 : 4 (4) 4 : 2

Q16.A light beam is described by E = 800 sin ω(t −xc ). An electron is allowed to move normal to the propagation of light beam with a speed of 3 × 107 m s−1 . What is the maximum magnetic force exerted on the electron? (1) 1. 28 × 10−21 N (2) 12. 8 × 10−18 N (3) 1. 28 × 10−18 N (4) 12. 8 × 10−17 N JEE Main 2021 (26 Aug Shift 2) JEE Main Previous Year Paper

Q16.What happens to the inductive reactance and the current in a purely inductive circuit if the frequency is halved ? (1) Both, inductive reactance and current will (2) Inductive reactance will be halved and be halved. current will be doubled.. (3) Inductive reactance will be doubled and (4) Both, inducting reactance and current will be current will be halved. doubled JEE Main 2021 (17 Mar Shift 2) JEE Main Previous Year Paper

Q16.Two plane mirrors M1 and M2 are at right angle to each other shown. A point source P is placed at a and 2a meter away from M1 and M2 respectively. The shortest distance between the images thus formed is : (Take √5 = 2. 3) (1) 2. 3 a (2) 2√10a (3) 4. 6a (4) 3a

Q16.In the given figure, a battery of emf E is connected across a conductor PQ of length l and different area of cross-sections having radii r1 and r2(r2 < r1). Choose the correct option as one moves from P to Q. (1) Drift velocity of electron increases. (2) Electric field decreases. (3) Electron current decreases. (4) All of these

Q16.A square loop of side 20 cm and resistance 1 Ω is moved towards right with a constant speed 𝑣0 . The right arm of the loop is in a uniform magnetic field of 5 T . The field is perpendicular to the plane of the loop and is going into it. The loop is connected to a network of resistors each of value 4Ω . What should be the value of 𝑣0 so that a steady current of 2 mA flows in the loop ? JEE Main 2021 (01 Sep Shift 2) JEE Main Previous Year Paper (1) 10-2 cm s-1 (2) 1 m s-1 (3) 1 cm s-1 (4) 102 m s-1

Q16.The focal length 𝑓 is related to the radius of curvature 𝑟 of the spherical convex mirror by: 1 1 (1) 𝑓= - 2𝑟 (2) 𝑓= + 2𝑟 (3) 𝑓= 𝑟 (4) 𝑓= - 𝑟

Q16.Given below are two statements : one is labelled as Assertion A and the other is labelled as Reason R. Assertion A : An electron microscope can achieve better resolving power than an optical microscope. Reason R : The de Broglie's wavelength of the electrons emitted from an electron gun is much less than wavelength of visible light. In the light of the above statements, choose the correct answer from the options given below: (1) A is false but R is true. (2) Both A and R are true and R is the correct explanation of A (3) Both A and R are true but R is NOT the correct (4) A is true but R is false. explanation of A

Q16.Given below are two statements : one is labeled as Assertion A and the other is labeled as Reason R. Assertion A : For a simple microscope, the angular size of the object equals the angular size of the image. Reason R: Magnification is achieved as the small object can be kept much closer to the eye than 25 cm and hence it subtends a large angle. In the light of the above statements, choose the most appropriate answer from the options given below: (1) A is false but R is true (2) Both A and R are true but R is NOT the correct explanation of A (3) A is true but R is false (4) Both A and R are true and R is the correct explanation of A →

Q16.The magnetic field vector of an electromagnetic wave is given by 𝐵= 𝐵0 ^i √2 represents unit vector along 𝑥 and 𝑦-axis respectively. At 𝑡= 0 s, two electric charges 𝑞1 of 4𝜋 coulomb and 𝜋 3𝜋 of 2𝜋 coulomb located at 0, 0, and 0, 0, respectively, have the same velocity of 0 . 5c ^i, (where 𝑞2 𝑘 𝑘, 𝑐 is the velocity of light ). The ratio of the force acting on charge 𝑞1 to 𝑞2 is : (1) 1 : √2 (2) 2√2 : 1 (3) √2 : 1 (4) 2 : 1

Q16.A 10 Ω resistance is connected across 220 V −50 Hz AC supply. The time taken by the current to change from its maximum value to the rms value is: (1) 2. 5 ms (2) 1. 5 ms (3) 3. 0 ms (4) 4. 5 ms

Q16.The magnetic susceptibility of a material of a rod is 499. Permeability in vacuum is 4π × 10−7 Hm−1. Absolute permeability of the material of the rod is (1) 4π × 10−4 Hm−1 (2) 2π × 10−4 Hm−1 (3) 3π × 10−4 Hm−1 (4) π × 10−4 Hm−1 JEE Main 2021 (20 Jul Shift 2) JEE Main Previous Year Paper

Q16.An object is placed beyond the centre of curvature C of the given concave mirror. If the distance of the object is d1 from C and the distance of the image formed is d2 from C, the radius of curvature of this mirror is: (1) 2 d1 d2 (2) 2 d1 d2 d1+d2 d1−d2 (3) d1 d2 (4) d1 d2 d1+d2 d1−d2

Q16.A particle of mass 4𝑀 at rest disintegrates into two particles of mass 𝑀 and 3𝑀, respectively, having non zero velocities. The ratio of de-Broglie wavelength of particle of mass 𝑀 to that of mass 3𝑀 will be: (1) 1: 3 (2) 3: 1 (3) 1: √3 (4) 1: 1

Q16.Curved surfaces of a plano-convex lens of refractive index μ1 and a plano-concave lens of refractive index μ2 have equal radius of curvature as shown in figure. Find the ratio of radius of curvature to the focal length of the combined lenses (1) 1 (2) 1 μ2−μ1 μ1−μ2 (3) μ2 −μ1 (4) μ1 −μ2

Q16.Region I and II are separated by a spherical surface of radius 25 cm. An object is kept in region I at a distance of 40 cm from the surface. The distance of the image from the surface is: (1) 55. 44 cm (2) 9. 52 cm (3) 18. 23 cm (4) 37. 58 cm

Q16.An electron of mass m and a photon have same energy E. The ratio of wavelength of electron to that of photon is : ( c being the velocity of light) (1) 1 2m 21 (2) 1 E 12 c ( E ) c ( 2m ) (3) E 21 (4) c(2mE) 12 ( 2 m )

Q16.A ray of light passes from a denser medium to a rarer medium at an angle of incidence i. The reflected and refracted rays make an angle of 90° with each other. The angle of reflection and refraction are respectively r and r′. The critical angle is given by, JEE Main 2021 (22 Jul Shift 1) JEE Main Previous Year Paper (1) sin−1(cot r) (2) tan−1(sin i) (3) sin−1(tanr′) (4) sin−1(tan r)

Q16.A 100Ω resistance, a 0. 1μF capacitor and an inductor are connected in series across a 250 V supply at variable frequency. Calculate the value of inductance of inductor at which resonance will occur. Given that the resonant frequency is 60 Hz. (1) 0. 70H (2) 70. 3 mH (3) 7. 03 × 10−5H (4) 70. 3 H