JEE MAIN - Physics (2019 - 9th January Evening Slot)

1
A rod of length 50 cm is pivoted at one end. It is raised such that if makes an angle of 30o from the horizontal as shown and released from rest. Its angular speed when it passes through the horizontal (in rad s$$-$$1) will be (g = 10 ms$$-$$2)

JEE Main 2019 (Online) 9th January Evening Slot Physics - Rotational Motion Question 175 English
Answer
(B)
$$\sqrt {30} $$
2
A parallel plate capacitor with square plates is filled with four dielecytrics of dielectrics constants K1, K2, K3, K4 arranged as shown in the figure. The effective dielectric constant K will be :

JEE Main 2019 (Online) 9th January Evening Slot Physics - Capacitor Question 124 English
Answer
(A)
$$K = {{{K_1}{K_2}} \over {{K_1} + {K_2}}} + {{{K_3}{K_4}} \over {{K_3} + {K_4}}}$$
3
In the given circuit the the internal resistance of the 18 V cell is negligible. If R1 = 400 $$\Omega $$, R3 = 100 $$\Omega $$ and R4 = 500 $$\Omega $$ and the reading of an ideal voltmeter across R4 is 5V, then the value of R2 will be :

JEE Main 2019 (Online) 9th January Evening Slot Physics - Current Electricity Question 270 English
Answer
(A)
300 $$\Omega $$
4
Ge and Si diodes start conducting at 0.3 V and 0.7 V respectively. In the following figure if Ge diode connection are reversed, the value of V0 changes by : (assume that the Ge diode has large breakdown voltage)

JEE Main 2019 (Online) 9th January Evening Slot Physics - Semiconductor Question 163 English
Answer
(D)
0.4 V
5
A particle is executing simple harmonic motion (SHM) of amplitude A, along the x-axis, about x = 0. When its potential Energy (PE) equals kinetic energy (KE), the position of the particle will be :
Answer
(C)
$${A \over {\sqrt 2 }}$$
6
The magnetic field associated with a light wave is given, at the origin, by B = B0 [sin(3.14 $$ \times $$ 107)ct + sin(6.28 $$ \times $$ 107)ct]. If this light falls on a silver plate having a work function of 4.7 eV, what will be the maximum kinetic energy of the photo electrons ?
(Take c = 3 $$ \times $$ 108 ms$$-$$1, h = 6.6 $$ \times $$ 10$$-$$34J-s)
Answer
(D)
7.72 eV
7
In a car race on straight road, car A takes a time t less than car B at the finish and passes finishing point with a speed '$$\upsilon $$' more than that of car B. Both the cars start from rest and travel with constant acceleration a1 and a2 respectively. Then '$$\upsilon $$' is equal to :
Answer
(C)
$$\sqrt {{a_1}{a_2}} t$$
8
Charge is distributed within a sphere of radius R with a volume charge density $$\rho \left( r \right) = {A \over {{r^2}}}{e^{ - {{2r} \over s}}},$$ where A and a are constants. If Q is the total charge of this charge distribution, the radius R is :
Answer
(B)
$${a \over 2}$$ log $$\left( {{1 \over {1 - {Q \over {2\pi aA}}}}} \right)$$
9
The pitch and the number of divisions, on the circular scale, for a given screw gauge are 0.5 mm and 100 respectively. When the screw gauge is fully tightened without any object, the zero of its circular scale lies 3 divisions below the mean line.

The readings of the main scale and the circular scale, for a thin sheet, are 5.5 mm and 48 respectively, the thickness of this sheet is :
Answer
(C)
5.725 mm
10
A rod of mass 'M' and length '2L' is suspended at its middle by a wire. It exhibits torsional oscillations; If two masses each of 'm' are attached at distance 'L/2' from its centre on both sides, it reduces the oscillation frequency by 20%. The value of radio m/M is close to :
Answer
(C)
0.37
11
One of the two identical conducting wires of length L is bent in the form of a circular loop and the other one into a circular coil of N identical turns. If the same current is passed in both, the radio of the magnetic field at the central of the loop (BL) to that at the center of the coil (BC), i.e. $${{{B_L}} \over {{B_C}}}$$ will be :
Answer
(D)
$${1 \over {{N^2}}}$$
12
Two plane mirrors are inclined to each other such that a ray of light incident on the first mirror (M1) and parallel to the second mirror (M2) is finally reflected from the second mirror (M2) parallel to the first mirror (M1). The angle between the two mirrors will be :
Answer
(B)
60o
13
In a Young's double slit experiment, the slits are placed 0.320 mm apart. Light of wavelength $$\lambda $$ = 500 nm is incident on the slits. The total number of bright fringes that are observed in the angular range $$-$$ 30o$$ \le $$$$\theta $$$$ \le $$30o is :
Answer
(D)
641
14
A 15 g mass of nitrogen gas is enclosed in a vessel at a temperature 27oC. Amount of heat transferred to the gas, so that rms velocity of molecules is doubled, is about : [Take R = 8.3 J/K mole]
Answer
(C)
10 kJ
15
A mass of 10 kg is suspended vertically by a rope from the roof. When a horizontal force is applied on the rope at some point. the rope deviated at an angle of 45o at the roof point. If the suspended mass is at equilibrium, the magnitude of the force applied is (g = 10 ms$$-$$2
Answer
(D)
100 N
16
A particle is executing simple harmonic motion (SHM) of amplitude A, along the x-axis, about x = 0. When its potential Energy (PE) equals kinetic energy (KE), the position of the particle will be :
Answer
(C)
$${A \over {\sqrt 2 }}$$
17
The energy required to take a satellite to a height 'h' above Earth surface (radius of Earth = 6.4 $$ \times $$ 103 km) is E1 and kinetic energy required for the satellite to be in a circular orbit at this height is E2. The value of h for which E1 and E2 are equal, is
Answer
(B)
3.2 $$ \times $$ 103 km
18
The top of a water tank is open to air and its water level is mainted. It is giving out 0.74 m3 water per minute through a circular opening of 2 cm radius in its wall. The depth of the center of the opening from the level of water in the tank is close to :
Answer
(B)
4.8 m
19
Expression for time in terms of G(universal gravitional constant), h (Planck constant) and c (speed of light) is proportional to :
Answer
(C)
$$\sqrt {{{Gh} \over {{c^5}}}} $$
20
Two point charges q1$$\left( {\sqrt {10} \mu C} \right)$$ and q2($$-$$ 25 $$\mu $$C) are placed on the x-axis at x = 1 m and x = 4 m respectively. The electric field (in V/m) at a point y = 3 m on y-axis is,
[take $${1 \over {4\pi { \in _0}}}$$ = 9 $$ \times $$ 109 Nm2C$$-$$2]
Answer
(A)
$$\left( {63\widehat i - 27\widehat j} \right) \times {10^2}$$
21
A particle having the same charge as of electron moves in a ciurcular path of radius 0.5 cm under the influence of a magnetic field 0f 0.5 T. If an electric field of 100 V/m makes it to move in a straight path, then the mass of the particle is (Given charge of electron = 1.6 $$ \times $$ 10$$-$$19C)
Answer
(D)
2.0 $$ \times $$ 10$$-$$24 kg
22
The energy associated with electric field is (UE) and with magnetic field is (UB) for an electromagnetic wave in free space. Then :
Answer
(D)
$${U_E} = {U_B}$$
23
A force acts on a 2 kg object so that its position is given as a function of time as x = 3t2 + 5. What is the work done by this force in first 5 seconds ?
Answer
(D)
900 J
24
A power transmission line feeds input power at 2300 V to a srep down transformer with its primary windings having 4000 turns. The output power is delivered at 230 V by the transformer. If the current in the primary of the transformer is 5A and its efficiency is 90%, the output current would be :
Answer
(B)
45 A
25
The position co-ordinates of a particle moving in a 3-D coordinate system is given by
x = a cos$$\omega $$t
y = a sin$$\omega $$t and
z = a$$\omega $$t

The speed of the particle is :
Answer
(A)
$$\sqrt 2 \,a\omega $$