JEE MAIN - Physics (2025 - 29th January Evening Shift)

1

JEE Main 2025 (Online) 29th January Evening Shift Physics - Center of Mass and Collision Question 5 English

Three equal masses $m$ are kept at vertices $(A, B, C)$ of an equilateral triangle of side a in free space. At $t=0$, they are given an initial velocity $\overrightarrow{V_A}=V_0 \overrightarrow{A C}, \overrightarrow{V_B}=V_0 \overrightarrow{B A}$ and $\overrightarrow{V_C}=V_0 \overrightarrow{C B}$. Here, $\overrightarrow{A C}, \overrightarrow{C B}$ and $\overrightarrow{B A}$ are unit vectors along the edges of the triangle. If the three masses interact gravitationally, then the magnitude of the net angular momentum of the system at the point of collision is :

Answer
(D)
$\frac{\sqrt{3}}{2}$ a $\mathrm{m}_0$
2
In an experiment with photoelectric effect, the stopping potential,
Answer
(A)
is $\left(\frac{1}{e}\right)$ times the maximum kinetic energy of the emitted photoelectrons
3
The number of spectral lines emitted by atomic hydrogen that is in the 4th energy level, is
Answer
(B)
6
4
The difference of temperature in a material can convert heat energy into electrical energy. To harvest the heat energy, the material should have
Answer
(A)
low thermal conductivity and high electrical conductivity
5

Given below are two statements. One is labelled as Assertion (A) and the other is labelled as Reason (R).

Assertion (A) : With the increase in the pressure of an ideal gas, the volume falls off more rapidly in an isothermal process in comparison to the adiabatic process.

Reason (R) : In isothermal process, PV = constant, while in adiabatic process $PV^{\gamma}$ = constant. Here $\gamma$ is the ratio of specific heats, P is the pressure and V is the volume of the ideal gas.

In the light of the above statements, choose the correct answer from the options given below:

Answer
(D)
Both (A) and (R) are true and (R) is the correct explanation of (A)
6
A sand dropper drops sand of mass m(t) on a conveyer belt at a rate proportional to the square root of speed (v) of the belt, i.e., $\frac{dm}{dt} \propto \sqrt{v}$. If P is the power delivered to run the belt at constant speed then which of the following relationship is true?
Answer
(D)
$P^2 \propto v^5$
7
A plane electromagnetic wave propagates along the + x direction in free space. The components of the electric field, $\vec{E}$ and magnetic field, $\vec{B}$ vectors associated with the wave in Cartesian frame are
Answer
(C)
$E_y, B_z$
8
A cup of coffee cools from 90°C to 80°C in t minutes when the room temperature is 20°C. The time taken by the similar cup of coffee to cool from 80°C to 60°C at the same room temperature is:
Answer
(A)
$\frac{13}{5}t$
9
A convex lens made of glass (refractive index = 1.5) has focal length 24 cm in air. When it is totally immersed in water (refractive index = 1.33), its focal length changes to
Answer
(A)
96 cm
10
Two bodies A and B of equal mass are suspended from two massless springs of spring constant k1 and k2, respectively. If the bodies oscillate vertically such that their amplitudes are equal, the ratio of the maximum velocity of A to the maximum velocity of B is
Answer
(B)
$ \sqrt{\frac{k_1}{k_2}} $
11

Match List - I with List - II.

List - I List - II
(A) Young’s Modulus (I) M L-1 T-1
(B) Torque (II) M L-1 T-2
(C) Coefficient of Viscosity (III) M-1 L3 T-2
(D) Gravitational Constant (IV) M L2 T-2

Choose the correct answer from the options given below:

Answer
(D)
(A)-(II), (B)-(IV), (C)-(I), (D)-(III)
12

Match List - I with List - II.

List - I List - II
(A) Magnetic induction (I) Ampere meter2
(B) Magnetic intensity (II) Weber
(C) Magnetic flux (III) Gauss
(D) Magnetic moment (IV) Ampere meter

Choose the correct answer from the options given below:

Answer
(A)
(A)-(III), (B)-(IV), (C)-(II), (D)-(I)
13
JEE Main 2025 (Online) 29th January Evening Shift Physics - Geometrical Optics Question 21 English

Two concave refracting surfaces of equal radii of curvature and refractive index 1.5 face each other in air as shown in figure. A point object O is placed midway, between P and B. The separation between the images of O, formed by each refracting surface is :

Answer
(B)
0.114R
14

The truth table for the circuit given below is:

JEE Main 2025 (Online) 29th January Evening Shift Physics - Semiconductor Question 8 English
Answer
(D)
ABY
000
011
101
110
15
JEE Main 2025 (Online) 29th January Evening Shift Physics - Heat and Thermodynamics Question 23 English

A poly-atomic molecule $\left(C_V=3 R, C_P=4 R\right.$, where $R$ is gas constant) goes from phase space point $\mathrm{A}\left(\mathrm{P}_{\mathrm{A}}=10^5 \mathrm{~Pa}, \mathrm{~V}_{\mathrm{A}}=4 \times 10^{-6} \mathrm{~m}^3\right)$ to point $\mathrm{B}\left(\mathrm{P}_{\mathrm{B}}=5 \times 10^4 \mathrm{~Pa}, \mathrm{~V}_{\mathrm{B}}=6 \times 10^{-6} \mathrm{~m}^3\right)$ to point $\mathrm{C}\left(\mathrm{P}_{\mathrm{C}}=10^4\right.$ $\mathrm{Pa}, \mathrm{V}_C=8 \times 10^{-6} \mathrm{~m}^3$ ). A to $B$ is an adiabatic path and $B$ to $C$ is an isothermal path.

The net heat absorbed per unit mole by the system is :

Answer
(B)
$450 \mathrm{R}(\ln 4-\ln 3)$
16

Two identical symmetric double convex lenses of focal length f are cut into two equal parts L1, L2 by AB plane and L3, L4 by XY plane as shown in figure respectively. The ratio of focal lengths of lenses L1 and L3 is

JEE Main 2025 (Online) 29th January Evening Shift Physics - Geometrical Optics Question 20 English
Answer
(A)
1 : 2
17

A point charge causes an electric flux of $-2 \times 10^4 \mathrm{Nm}^2 \mathrm{C}^{-1}$ to pass through a spherical Gaussian surface of 8.0 cm radius, centred on the charge. The value of the point charge is :

(Given $\epsilon_0=8.85 \times 10^{-12} \mathrm{C}^2 \mathrm{~N}^{-1} \mathrm{~m}^{-2}$ )

Answer
(B)
$-17.7 \times 10^{-8} \mathrm{C}$
18

A capacitor, $C_1 = 6 \mu F$ is charged to a potential difference of $V_0 = 5V$ using a 5V battery. The battery is removed and another capacitor, $C_2 = 12 \mu F$ is inserted in place of the battery. When the switch 'S' is closed, the charge flows between the capacitors for some time until equilibrium condition is reached. What are the charges ($q_1$ and $q_2$) on the capacitors $C_1$ and $C_2$ when equilibrium condition is reached.

JEE Main 2025 (Online) 29th January Evening Shift Physics - Capacitor Question 10 English
Answer
(A)
$q_1 = 10 \mu C, \ q_2 = 20 \mu C$
19

An electric dipole is placed at a distance of 2 cm from an infinite plane sheet having positive charge density $\sigma_{\mathrm{o}}$. Choose the correct option from the following.

JEE Main 2025 (Online) 29th January Evening Shift Physics - Electrostatics Question 17 English
Answer
(B)
Potential energy of dipole is minimum and torque is zero.
20

Given below are two statements. One is labelled as Assertion (A) and the other is labelled as Reason (R).

Assertion (A): JEE Main 2025 (Online) 29th January Evening Shift Physics - Center of Mass and Collision Question 4 English Three identical spheres of same mass undergo one dimensional motion as shown in figure with initial velocities $v_{\mathrm{A}}=5 \mathrm{~m} / \mathrm{s}, v_{\mathrm{B}}=2 \mathrm{~m} / \mathrm{s}, v_{\mathrm{C}}=4 \mathrm{~m} / \mathrm{s}$. If we wait sufficiently long for elastic collision to happen, then $v_{\mathrm{A}}=4 \mathrm{~m} / \mathrm{s}, v_{\mathrm{B}}=2 \mathrm{~m} / \mathrm{s}$, $v_{\mathrm{C}}=5 \mathrm{~m} / \mathrm{s}$ will be the final velocities.

Reason (R): In an elastic collision between identical masses, two objects exchange their velocities.

In the light of the above statements, choose the correct answer from the options given below:

Answer
(C)
(A) is false but (R) is true
21

$\text { A physical quantity } Q \text { is related to four observables } a, b, c, d \text { as follows : }$

$Q = \frac{ab^4}{cd}$

where, $\mathrm{a}=(60 \pm 3) \mathrm{Pa} ; \mathrm{b}=(20 \pm 0.1) \mathrm{m} ; \mathrm{c}=(40 \pm 0.2) \mathrm{Nsm}^{-2}$ and $\mathrm{d}=(50 \pm 0.1) \mathrm{m}$, then the percentage error in Q is $\frac{x}{1000}$, where $x=$ _________ .

Answer
7700
22
Two cars P and Q are moving on a road in the same direction. Acceleration of car P increases linearly with time whereas car Q moves with a constant acceleration. Both cars cross each other at time t = 0, for the first time. The maximum possible number of crossing(s) (including the crossing at t = 0) is ________.
Answer
3
23
Two planets, $A$ and $B$ are orbiting a common star in circular orbits of radii $R_A$ and $R_B$, respectively, with $R_B=2 R_A$. The planet $B$ is $4 \sqrt{2}$ times more massive than planet $A$. The ratio $\left(\frac{\mathrm{L}_{\mathrm{B}}}{\mathrm{L}_{\mathrm{A}}}\right)$ of angular momentum $\left(L_B\right)$ of planet $B$ to that of planet $A\left(L_A\right)$ is closest to integer ________.
Answer
8
24

A parallel plate capacitor consisting of two circular plates of radius 10 cm is being charged by a constant current of 0.15 A . If the rate of change of potential difference between the plates is $7 \times 10^8 \mathrm{~V} / \mathrm{s}$ then the integer value of the distance between the parallel plates is

$\left(\right.$ Take, $\left.\epsilon_0=9 \times 10^{-12} \frac{\mathrm{~F}}{\mathrm{~m}}, \pi=\frac{22}{7}\right)$ ____________ $\mu \mathrm{m}$.

Answer
1320
25
The magnetic field inside a 200 turns solenoid of radius 10 cm is $2.9 \times 10^{-4} ~\mathrm{Tesla}$. If the solenoid carries a current of 0.29 A , then the length of the solenoid is _______ $\pi \mathrm{cm}$.
Answer
8