Practice Questions

Q18.A generator has armature resistance of 0.1Ω and develops an induced emf of 120 V when driven at its rated speed. Its terminal voltage when a current of 50 A is being drawn is (1) 120 V (2) 5 V (3) 115 V (4) 70 V

Q18.Two circuits (a) and (b) have charged capacitors of capacitance C, 2C and 3C with open switches. Charges on each of the capacitor are as shown in the figures. On closing the switches Circuit (a) Circuit (b) (1) No charge flows in (a) but charge flows from R (2) Charges flow from L to R in both (a) and (b) to L in (b) (3) Charges flow from R to L in (a) and from L to R (4) No charge flows in (a) but charge flows from L in (b) to R in (b)

Q18.A charge Q is uniformly distributed over the surface of non conducting disc of radius R. The disc rotates about an axis perpendicular to its plane and passing through its centre with an angular velocity ω. As a result of this rotation a magnetic field of induction B is obtained at the centre of the disc. If we keep both the amount of charge placed on the disc and its angular velocity to be constant and vary the radius of the disc then the variation of the magnetic induction at the centre of the disc will be represented by the figure (1) (2) (3) (4)

Q18.The resistance of a wire is R. It is bent at the middle by 180∘ and both the ends are twisted together to make a shorter wire. The resistance of the new wire is (1) 2R (2) R/2 (3) R/4 (4) R/8

Q19.Proton, Deuteron and alpha particle of the same kinetic energy are moving in circular trajectories in a constant magnetic field. The radii of proton, deuteron and alpha particle are respectively rp, rd and rα . Which one of the following relations is correct? (1) rα = rp = rd (2) rα = rp < rd (3) rα > rd > rp (4) rα = rd > rp

Q19.The magnetic force acting on charged particle of charge 2μC in magnetic field of 2T acting in y− direction, when the particle velocity is (2^i + 3^j) × 106 ms−1 is (1) 8 N in z-direction (2) 8 N in y-direction (3) 4 N in y-direction (4) 4 N in z-direction

Q19.In an experiment of potentiometer for measuring the internal resistance of primary cell a balancing length ℓ is obtained on the potentiometer wire when the cell is open circuit. Now the cell is short circuited by a resistance R. If R is to be equal to the internal resistance of the cell the balancing length on the potentiometer wire will be (1) ℓ (2) 2ℓ (3) ℓ/2 (4) ℓ/4 JEE Main 2012 (26 May Online) JEE Main Previous Year Paper

Q19.A proton and a deuteron are both accelerated through the same potential difference and enter in a magnetic field perpendicular to the direction of the field. If the deuteron follows a path of radius R, assuming the neutron and proton masses are nearly equal, the radius of the proton's path will be (1) √2R (2) R √2 (3) R (4) R 2

Q19.This question has Statement 1 and Statement 2. Of the four choices given after the Statements, choose the one that best describes the two Statements. Statement 1: The possibility of an electric bulb fusing is higher at the time of switching ON. Statement 2: Resistance of an electric bulb when it is not lit up is much smaller than when it is lit up. (1) Statement 1 is true, Statement 2 is false (2) Statement 1 is false, Statement 2 is true, Statement 2 is not a correct explanation of Statement 1. (3) Statement 1 is true, Statement 2 is true, Statement 2 is a correct explanation of Statement 1. (4) Statement 1 is false, Statement 2 is true.

Q20.A coil is suspended in a uniform magnetic field, with the plane of the coil parallel to the magnetic lines of force. When a current is passed through the coil it starts oscillating; it is very difficult to stop. But if an aluminium plate is placed near to the coil, it stops. This is due to : JEE Main 2012 (Offline) JEE Main Previous Year Paper (1) development of air current when the plate is (2) induction of electrical charge on the plate placed. (3) shielding of magnetic lines of force as aluminium (4) electromagnetic induction in the aluminium plate is a paramagnetic material. giving rise to electromagnetic damping.

Q20.Currents of a 10 ampere and 2 ampere are passed through two parallel thin wires A and B respectively in opposite directions. Wire A is infinitely long and the length of the wire B is 2 m. The force acting on the conductor B, which is situated at 10 cm distance from A will be (1) 8 × 10−5 N (2) 5 × 10−5 N (3) 8π × 10−7 N (4) 4π × 10−7 N

Q20.A bar magnet of length 6 cm has a magnetic moment of 4 J T−1 . Find the strength of magnetic field at a distance of 200 cm from the centre of the magnet along its equatorial line. (1) 4 × 10−8 tesla (2) 3.5 × 10−8 tesla (3) 5 × 10−8 tesla (4) 3 × 10−8 tesla

Q20.A coil of self inductance L is connected at one end of two rails as shown in figure. A connector of length l, mass m can slide freely over the two parallel rails. The entire set up is placed in a magnetic field of induction B going into the page. At an instant t = 0 an initial velocity v0 is imparted to it and as a result of that it starts JEE Main 2012 (19 May Online) JEE Main Previous Year Paper moving along x-axis. The displacement of the connector is represented by the figure. (1) (2) (3) (4)

Q20.The circuit in figure consists of wires at the top and bottom and identical springs as the left and right sides. The wire at the bottom has a mass of 10 g and is 5 cm long. The wire is hanging as shown in the figure. The springs stretch 0.5 cm under the weight of the wire and the circuit has a total resistance of 12Ω. When the lower wire JEE Main 2012 (12 May Online) JEE Main Previous Year Paper is subjected to a static magnetic field, the springs, stretch an additional 0.3 cm. The magnetic field is (1) 0.6 T and directed out of page (2) 1.2 T and directed into the plane of page (3) 0.6 T and directed into the plane of page (4) 1.2 T and directed out of page

Q21.An electromagnetic wave in vacuum has the electric and magnetic fields →E and →B, which are always → perpendicular to each other. The direction of polarization is given by →X and that of wave propagation by k. Then : (1) →X∥→B and →k∥→B × →E (2) →X∥→E and →k∥→E × →B (3) →X∥→B and →k∥→E × →B (4) →X∥→E and →k∥→B × →E

Q21.This question has Statement 1 and Statement 2. Of the four choices given after the Statements, choose the one that best describes the two Statements. Statement 1: A charged particle is moving at right angle to a static magnetic field. During the motion the kinetic energy of the charge remains unchanged. Statement 2: Static magnetic field exert force on a moving charge in the direction perpendicular to the magnetic field. (1) Statement 1 is false, Statement 2 is true. (2) Statement 1 is true, Statement 2 is true, Statement 2 is not the correct explanation of Statement 1. (3) Statement 1 is true, Statement 2 is false. (4) Statement 1 is true, Statement 2 is true, Statement 2 is the correct explanation of Statement 1.

Q21.The velocity of certain ions that pass undeflected through crossed electric field E = 7.7kV/m and magnetic field B = 0.14 T is (1) 18 km/s (2) 77 km/s (3) 55 km/s (4) 1078 km/s

Q21.Magnetic flux through a coil of resistance 10Ω is changed by Δϕ in 0.1 s. The resulting current in the coil varies with time as shown in the figure. Then |Δϕ| is equal to (in weber) (1) 6 (2) 4 (3) 2 (4) 8

Q21.This question has Statement 1 and Statement 2. Of the four choices given after the Statements, choose the one that best describes the two Statements. Statement 1: Self inductance of a long solenoid of length L, total πμ0N 2r2 number of turns N and radius r is less than L . Statement 2: The magnetic induction in the solenoid in Statement 1 carrying current I is μ0NIL in the middle of the solenoid but becomes less as we move towards its ends. (1) Statement 1 is true, Statement 2 is false. (2) Statement 1 is true, Statement 2 is true, Statement 2 is the correct explanation of Statement 1. (3) Statement 1 is false, Statement 2 is true. (4) Statement 1 is true, Statement 2 is true, Statement 2 is not the correct explanation of Statement 1.

Q22.In an LCR circuit shown in the following figure, what will be the readings of the voltmeter across the resistor and ammeter if an a.c. source of 220 V and 100 Hz is connected to it as shown? JEE Main 2012 (07 May Online) JEE Main Previous Year Paper (1) 800 V, 8 A (2) 110 V, 1.1 A (3) 300 V, 3 A (4) 220 V, 2.2 A

Q22.A resistance R and a capacitance C are connected in series to a battery of negligible internal resistance through a key. The key is closed at t = 0 . If after t sec the voltage across the capacitance was seven times the voltage across R, the value of t is (1) 3RCℓn2 (2) 2RCℓn2 (3) 2RC ln 7 (4) 3RCℓn7

Q22.An electromagnetic wave with frequency ω and wavelength λ travels in the +y direction. Its magnetic field is along +x-axis. The vector equation for the associated electric field (of amplitude E0 ) is JEE Main 2012 (19 May Online) JEE Main Previous Year Paper 2π (1) →E = −E0 cos (ωt + λ y)^x (2) →E = E0 cos (ωt −2πλ y)^x 2π (4) →E = −E0 cos (ωt + (3) →E = E0 cos (ωt −2πλ y)^z λ y)^z

Q22.A radio transmitter transmits at 830kHz. At a certain distance from the transmitter magnetic field has amplitude 4.82 × 10−11 T. The electric field and the wavelength are respectively (1) 0.014 N/C, 36 m (2) 0.14 N/C, 36 m (3) 0.14 N/C, 360 m (4) 0.014 N/C, 360 m

Q22.An object 2.4 m in front of a lens forms a sharp image on a film 12 cm behind the lens. A glass plate 1 cm thick, of refractive index 1.50 is interposed between lens and film with its plane faces parallel to film. At what distance (from lens) should object be shifted to be in sharp focus on film? (1) 7.2 m (2) 2.4 m (3) 3.2 m (4) 5.6 m

Q23.The frequency of X -rays; γ -rays and ultraviolet rays are respectively a, b and c then (1) a < b; b > c (2) a > b; b > c (3) a < b < c (4) a = b = c