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

1
A thin smooth rod of length L and mass M is rotating freely with angular speed $$\omega $$0 about an axis perpendicular to the rod and passing through its center. Two beads of mass m and negligible size are at the center of the rod initially. The beads are free to slide along the rod. The angular speed of the system , when the beads reach the opposite ends of the rod, will be :-
Answer
(C)
$${{M{\omega _0}} \over {M + 6m}}$$
2
A string 2.0 m long and fixed at its ends is driven by a 240 Hz vibrator. The string vibrates in its third harmonic mode. The speed of the wave and its fundamental frequency is :-
Answer
(D)
320m/s, 80 Hz
3
In a conductor, if the number of conduction electrons per unit volume is 8.5 × 1028 m–3 and mean free time is 25ƒs (femto second), it's approximate resistivity is :-
(me = 9.1 × 10–31 kg)
Answer
(A)
10–8 $$\Omega $$m
4
The parallel combination of two air filled parallel plate capacitors of capacitance C and nC is connected to a battery of voltage, V. When the capacitors are fully charged, the battery is removed and after that a dielectric material of dielectric constant K is placed between the two plates of the first capacitor. The new potential difference of the combined system is :-
Answer
(C)
$${{(n+1)V} \over {K + n}}$$
5
Two materials having coefficients of thermal conductivity '3K' and 'K' and thickness 'd' and '3d', respectively, are joined to form a slab as shown in the figure. The temperatures of the outer surfaces are '$$\theta $$2' and '$$\theta $$1' respectively, ($$\theta $$2 > $$\theta $$1). The temperature at the interface is :-

JEE Main 2019 (Online) 9th April Evening Slot Physics - Heat and Thermodynamics Question 310 English
Answer
(A)
$${{{\theta _1}} \over {10}} + {{9{\theta _2}} \over {10}}$$
6
A He+ ion is in its first excited state. Its ionization energy is :-
Answer
(A)
13.60 eV
7
A wedge of mass M = 4m lies on a frictionless plane. A particle of mass m approaches the wedge with speed v. There is no friction between the particle and the plane or between the particle and the wedge. The maximum height climbed by the particle on the wedge is given by :-
Answer
(D)
$${{2{v^2}} \over {5g}}$$
8
A particle of mass 'm' is moving with speed '2v' and collides with a mass '2m' moving with speed 'v' in the same direction. After collision, the first mass is stopped completely while the second one splits into two particles each of mass 'm', which move at angle 45° with respect to the origianl direction. The speed of each of the moving particle will be :-
Answer
(A)
2 $$\sqrt2$$v
9
A wooden block floating in a bucket of water has 4/5 of its volume submerged. When certain amount of an oil is poured into the bucket, it is found that the block is just under the oil surface with half of its volume under water and half in oil. The density of oil relative to that of water is :-
Answer
(C)
0.6
10
A test particle is moving in a circular orbit in the gravitational field produced by a mass density $$\rho (r) = {K \over {{r^2}}}$$ . Identify the correct relation between the radius R of the particle's orbit and its period T
Answer
(D)
T/R is a constant
11
The position vector of a particle changes with time according to the relation $$\overrightarrow r (t) = 15{t^2}\widehat i + (4 - 20{t^2})\widehat j$$
What is the magnitude of the acceleration at t = 1 ?
Answer
(A)
50
12
The specific heats, CP and CV of a gas of diatomic molecules, A, are given (in units of J mol–1 K–1) by 29 and 22, respectively. Another gas of diatomic molecules, B, has the corresponding values 30 and 21. If they are treated as ideal gases, then :-
Answer
(B)
A has a vibrational mode but B has none
13
Four point charges –q, +q, +q and –q are placed on y-axis at y = –2d, y = –d, y = +d and y = +2d, respectively. The magnitude of the electric field E at a point on the x-axis at x = D, with D >> d, will behave as :-
Answer
(C)
$$E \propto {1 \over D^4}$$
14
A moving coil galvanometer has a coil with 175 turns and area 1 cm2. It uses a torsion band of torsion constant 10–6 N-m/rad. The coil is placed in a maganetic field B parallel to its plane. The coil deflects by 1° for a current of 1 mA. The value of B (in Tesla) is approximately :-
Answer
(D)
10–3
15
50 W/m2 energy density of sunlight is normally incident on the surface of a solar panel. Some part of incident energy (25%) is reflected from the surface and the rest is absorbed. The force exerted on 1m2 surface area will be close to (c = 3 × 108 m/s) :-
Answer
(A)
20 × 10–8 N
16
The area of a square is 5.29 cm2. The area of 7 such squares taking into account the significant figures is :-
Answer
(A)
37.0 cm2
17
The logic gate equivalent to the given logic circuit is :- JEE Main 2019 (Online) 9th April Evening Slot Physics - Semiconductor Question 153 English
Answer
(D)
OR
18
Moment of inertia of a body about a given axis is 1.5 kg m2. Initially the body is at rest. In order to produce a rotational kinetic energy of 1200 J, the angular accleration of 20 rad/s2 must be applied about the axis for a duration of :-
Answer
(D)
2 s
19
A particle 'P' is formed due to a completely inelastic collision of particles 'x' and 'y' having de-Broglie wavelengths '$$\lambda $$x' and '$$\lambda $$y' respectively. If x and y were moving in opposite directions, then the de-Broglie wavelength of 'P' is :-
Answer
(B)
$${{{\lambda _x}{\lambda _y}} \over {\left| {{\lambda _x} - {\lambda _y}} \right|}}$$
20
A convex lens of focal length 20 cm produces images of the same magnification 2 when an object is kept at two distances x1 and x2 (x1 > x2) from the lens. The ratio of x1 and x2 is :-
Answer
(A)
3 : 1
21
Diameter of the objective lens of a telescope is 250 cm. For light of wavelength 600nm. coming from a distant object, the limit of resolution of the telescope is close to :-
Answer
(A)
3.0 × 10–7 rad
22
A massless spring (k = 800 N/m), attached with a mass (500 g) is completely immersed in 1 kg of water. The spring is stretched by 2 cm and released so that it starts vibrating. What would be the order of magnitude of the change in the temperature of water when the vibrations stop completely ? (Assume that the water container and spring receive negligible heat and specific heat of mass = 400 J/kg K, specific heat of water = 4184 J/kg K)
Answer
(C)
10–5K
23
A very long solenoid of radius R is carrying current I(t) = kte–at(k > 0), as a function of time (t $$ \ge $$ 0). counter clockwise current is taken to be positive. A circular conducting coil of radius 2R is placed in the equatorial 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 :-
Answer
(A)
JEE Main 2019 (Online) 9th April Evening Slot Physics - Electromagnetic Induction Question 95 English Option 1
24
The position of a particle as a function of time t, is given by
x(t) = at + bt2 – ct3
where a, b and c are constants. When the particle attains zero acceleration, then its velocity will be :
Answer
(C)
$$a + {{{b^2}} \over {3c}}$$
25
A thin convex lens L (refractive index = 1.5) is placed on a plane mirror M. When a pin is placed at A, such that OA = 18 cm, its real inverted image is formed at A itself, as shown in figure. When a liquid of refractive index μ1 is put between the lens and the mirror, The pin has to be moved to A', such that OA' = 27 cm, to get its inverted real image at A' itself. The value of μ1 will be :- JEE Main 2019 (Online) 9th April Evening Slot Physics - Geometrical Optics Question 164 English
Answer
(D)
4 / 3
26
The resistance of a galvanometer is 50 ohm and the maximum current which can be passed through it is 0.002 A. What resistance must be connected to it in order to convert it into an ammeter of range 0 – 0.5 A ?
Answer
(B)
0.2 ohm
27
A metal wire of resistance 3 $$\Omega $$ is elongated to make a uniform wire of double its previous length. This new wire is now bent and the ends joined to make a circle. If two points on this circle make an angle 60° at the centre, the equivalent resistance between these two points will be :-
Answer
(D)
5 / 3 $$\Omega $$
28
Two coils 'P' and 'Q' are separated by some distance. When a current of 3 A flows through coil 'P', a magnetic flux of 10–3 Wb passes through 'Q'. No current is passed through 'Q'. When no current passes through 'P' and a current of 2 A passes through 'Q', the flux through 'P' is :-
Answer
(C)
6.67 × 10–4 Wb