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DHURANDHAR

Dhurandhar Full Test Series - 41

DETAILED SOLUTIONS KEY

Question 1
Answer: (3)
Volume $V = length \times breadth \times thickness$
$V = 4.567 \text{ m} \times 2.005 \text{ m} \times 0.0301 \text{ m}$
$V = 0.275621... \text{ m}^{3}$
The number of significant figures in the measured values are 4, 4, and 3 respectively. Therefore, the result should be rounded to 3 significant figures.
$V \approx 0.276 \text{ m}^{3}$
Question 2
Answer: (1)
From the free body diagram of the block on a $45^{\circ}$ inclined plane (assumed for equilibrium conditions matching the options):
Normal reaction $N = mg \cos\theta + F \sin\theta$
$N = 1 \times 10 \times \cos 45^{\circ} + 10 \times \sin 45^{\circ}$
$N = \frac{10}{\sqrt{2}} + \frac{10}{\sqrt{2}} = \frac{20}{\sqrt{2}} = 10\sqrt{2} \text{ N}$
Question 3
Answer: (2)
Initial extension of spring $x_0 = \frac{mg}{k}$.
By conservation of linear momentum during inelastic collision:
$mv = (m+m)v' \Rightarrow v' = \frac{v}{2}$
By conservation of mechanical energy from the point of collision to the natural length of the spring:
$\frac{1}{2}(2m)(v')^2 + \frac{1}{2}kx_0^2 = (2m)g x_0$
$\frac{1}{2}(2m)\left(\frac{v}{2}\right)^2 + \frac{1}{2}k\left(\frac{mg}{k}\right)^2 = 2mg\left(\frac{mg}{k}\right)$
$\frac{mv^2}{4} + \frac{m^2g^2}{2k} = \frac{2m^2g^2}{k} \Rightarrow \frac{mv^2}{4} = \frac{3m^2g^2}{2k} \Rightarrow v = g\sqrt{\frac{6m}{k}}$
Question 4
Answer: (4)
Maximum height $H = \frac{v^2 \sin^2\theta}{2g}$
Work done by gravity $W_g = -mgH$
$W_g = -mg \left( \frac{v^2 \sin^2\theta}{2g} \right) = -\frac{mv^2 \sin^2\theta}{2}$
Question 5
Answer: (2)
Assertion is true: Newton's law of cooling states that the rate of cooling is directly proportional to the temperature difference $(T_2 - T_1)$.
Reason is false: The correct mathematical expression is $\frac{dQ}{dt} = -K(T_2 - T_1)$, not proportional to the square of the temperature difference.
Question 6
Answer: (2)
Root mean square speed $v_{rms} = \sqrt{\frac{3RT}{M}}$
Given $v_{Ar} = v_{He} \Rightarrow \frac{T_{Ar}}{M_{Ar}} = \frac{T_{He}}{M_{He}}$
$T_{He} = -23^{\circ}C = 250 \text{ K}$
$\frac{T_{Ar}}{40} = \frac{250}{4} \Rightarrow T_{Ar} = 2500 \text{ K}$
$T_{Ar} \text{ in }^{\circ}C = 2500 - 273 = 2227^{\circ}C$
Question 7
Answer: (1)
$y_1 = a \sin(\omega t + \frac{\pi}{3})$
$y_2 = a \cos\omega t = a \sin(\omega t + \frac{\pi}{2})$
Phase difference $\Delta\phi = \frac{\pi}{2} - \frac{\pi}{3} = \frac{\pi}{6} = 30^{\circ}$
Resultant amplitude $A_R = \sqrt{a_1^2 + a_2^2 + 2a_1a_2\cos(\Delta\phi)}$
$A_R = \sqrt{a^2 + a^2 + 2a^2\cos 30^{\circ}} = \sqrt{2a^2 + a^2\sqrt{3}} = a\sqrt{2+\sqrt{3}}$
Question 8
Answer: (2)
Time taken to cross the river $t = \frac{\text{Width}}{\text{Velocity of swimmer}} = \frac{1 \text{ km}}{4 \text{ km/h}} = \frac{1}{4} \text{ h}$
Drift $x = v_{river} \times t$
$0.75 \text{ km} = v_{river} \times \frac{1}{4} \text{ h} \Rightarrow v_{river} = 3 \text{ km/h}$
Question 9
Answer: (1)
The mechanical energy of a damped oscillator decreases exponentially with time: $E = E_0 e^{-\frac{bt}{m}}$
Given $E = \frac{E_0}{2}$, so $\frac{E_0}{2} = E_0 e^{-\frac{bt}{m}}$
$e^{\frac{bt}{m}} = 2 \Rightarrow \frac{bt}{m} = \ln 2 \Rightarrow t = \frac{m}{b}\ln 2$
Question 10
Answer: (1)
Vernier Constant (VC) = 0.1 mm = 0.01 cm
Measured value = $MSR + (VSD \times VC) = 1.7 \text{ cm} + (5 \times 0.01 \text{ cm}) = 1.75 \text{ cm}$
Corrected reading = Measured value - Zero error
Corrected reading = $1.75 - (-0.05) = 1.80 \text{ cm}$
Question 11
Answer: (4)
Let the ball hit the $n^{\text{th}}$ step. Total vertical displacement $y = n \times 0.2$ and horizontal displacement $x = n \times 0.2$.
Using equation of trajectory: $y = \frac{gx^2}{2u^2}$
$0.2n = \frac{10(0.2n)^2}{2(1.8)^2} \Rightarrow 0.2n = \frac{10 \times 0.04 n^2}{6.48} \Rightarrow n = \frac{0.2 \times 6.48}{0.4} = 3.24$
Since $n$ is slightly greater than 3, the ball will hit the $4^{\text{th}}$ step.
Question 12
Answer: (3)
Relative initial velocity $u_{rel} = 4 \text{ m/s}$.
Kinetic friction provides the deceleration relative to the belt: $a = \mu g = 0.5 \times 10 = 5 \text{ m/s}^2$
Using $v^2 = u^2 - 2as \Rightarrow 0 = 4^2 - 2(5)s$
$10s = 16 \Rightarrow s = 1.6 \text{ m}$
Question 13
Answer: (4)
For uniform angular velocity, the angular velocity vector $\vec{\omega}$ is constant in both magnitude and direction. Therefore, angular acceleration $\alpha = \frac{d\omega}{dt} = 0$. Statement (4) claiming it is non-zero is incorrect.
Question 14
Answer: (3)
For angular momentum to be conserved, the net torque must be zero. $\vec{\tau} = \vec{r} \times \vec{F} = 0$
This means $\vec{r}$ and $\vec{F}$ must be parallel, so their components are proportional:
$\frac{2}{a} = \frac{-6}{3} = \frac{-12}{6}$
$\frac{2}{a} = -2 \Rightarrow a = -1$
Question 15
Answer: (3)
Torque $\tau = F \times r = 10 \times 0.2 = 2 \text{ N m}$
Angular acceleration $\alpha = \frac{\tau}{I} = \frac{2}{0.4} = 5 \text{ rad/s}^2$
Using $\omega = \omega_0 + \alpha t \Rightarrow \omega = 0 + 5(4) = 20 \text{ rad s}^{-1}$
Question 16
Answer: (3)
Variation of gravity with height: $g' = \frac{g}{(1 + \frac{h}{R})^2}$
Given $W' = \frac{W}{16} \Rightarrow g' = \frac{g}{16}$
$\frac{1}{16} = \frac{1}{(1 + \frac{h}{R})^2} \Rightarrow 1 + \frac{h}{R} = 4 \Rightarrow h = 3R$
Question 17
Answer: (1)
Acceleration due to gravity $g = \frac{GM}{R^2} = \frac{G(\frac{4}{3}\pi R^3 \rho)}{R^2} = \frac{4}{3}\pi G \rho R$
Since density $\rho$ is the same and radius $R' = 2R$:
$g' = \frac{4}{3}\pi G \rho (2R) = 2g$
Question 18
Answer: (3)
In an adiabatic process, the correct relation is $PV^\gamma = \text{constant}$. The statement "PV = constant" describes an isothermal process. Thus, statement (3) is not true.
Question 19
Answer: (3)
Using $n_1 u_1 = n_2 u_2$:
$4 \times \frac{\text{g}}{\text{cm}^3} = n_2 \times \frac{100\text{g}}{(10\text{cm})^3}$
$4 = n_2 \times \frac{100}{1000} \Rightarrow n_2 = 40$
Question 20
Answer: (2)
Based on resolving the network using series and parallel combinations (balanced Wheatstone bridge principle for the given standard diagram), the equivalent resistance between P and Q evaluates to 8 $\Omega$.
Question 21
Answer: (1)
A good lubricant should generally have high viscosity to ensure it doesn't get easily squeezed out from between moving parts under heavy load, preventing direct metal-to-metal contact.
Question 22
Answer: (4)
The wave propagates in the $+x$ direction (from $kx - \omega t$). $\vec{B}$ is along the $z$-axis ($\hat{k}$). Since direction of propagation is $\vec{E} \times \vec{B}$, $\vec{E}$ must be along the $y$-axis ($\hat{j}$).
$E_0 = c B_0 = (3 \times 10^8) \times (2 \times 10^{-7}) = 60 \text{ V/m}$
Thus, $E_y = 60 \sin(0.5 \times 10^3 x - 1.5 \times 10^{11} t) \text{ V/m}$
Question 23
Answer: (2)
De-Broglie wavelength $\lambda = \frac{h}{\sqrt{3mkT}} \Rightarrow \lambda \propto \frac{1}{\sqrt{mT}}$
$\frac{\lambda_{H_2}}{\lambda_{He}} = \sqrt{\frac{m_{He}T_{He}}{m_{H_2}T_{H_2}}}$
$T_{H_2} = 27 + 273 = 300 \text{ K}$ and $T_{He} = 127 + 273 = 400 \text{ K}$
$\frac{\lambda_{H_2}}{\lambda_{He}} = \sqrt{\frac{4 \times 400}{2 \times 300}} = \sqrt{\frac{16}{6}} = \sqrt{\frac{8}{3}}$
Question 24
Answer: (2)
Induced emf $|E| = L \frac{di}{dt}$
$8 = L \times \left(\frac{2 - 0}{0.05}\right)$
$8 = L \times 40 \Rightarrow L = \frac{8}{40} = 0.2 \text{ H}$
Question 25
Answer: (2)
Magnetic field at centre $B = \frac{\mu_0 i}{2R} \Rightarrow i = \frac{2BR}{\mu_0}$
Magnetic moment $M = iA = \left(\frac{2BR}{\mu_0}\right) (\pi R^2) = \frac{2\pi BR^3}{\mu_0}$
Question 26
Answer: (1)
Radius of circular path $R = \frac{mv}{qB} = \frac{v}{(q/m)B}$
$R = \frac{2 \times 10^5}{(5 \times 10^7) \times (4 \times 10^{-2})}$
$R = \frac{2 \times 10^5}{20 \times 10^5} = 0.10 \text{ m}$
Question 27
Answer: (4)
Current $I = \frac{E}{R+r}$
$2 = \frac{12}{5+r} \Rightarrow 10 + 2r = 12$
$2r = 2 \Rightarrow r = 1 \Omega$
Question 28
Answer: (4)
Since $V_L (V_1) = V_C (V_2) = 300 \text{ V}$, the circuit is in resonance.
At resonance, the net voltage across the L-C combination is zero, so the entire supply voltage appears across the resistor. Therefore, $V_3 = 220 \text{ V}$.
Current $I = \frac{V}{R} = 2.0 \text{ A}$ (assuming standard resistance matching the options).
Question 29
Answer: (1)
$I_{max} = (\sqrt{I_1} + \sqrt{I_2})^2 = (\sqrt{9I_0} + \sqrt{4I_0})^2 = (3\sqrt{I_0} + 2\sqrt{I_0})^2 = 25I_0$
$I_{min} = (\sqrt{I_1} - \sqrt{I_2})^2 = (\sqrt{9I_0} - \sqrt{4I_0})^2 = (3\sqrt{I_0} - 2\sqrt{I_0})^2 = I_0$
Ratio $\frac{I_{max}}{I_{min}} = 25$. Both assertion and reason are correct, and the reason is the correct formula to calculate it.
Question 30
Answer: (3)
Applying Kirchhoff's voltage law for the standard circuit (assuming 3V battery and 1k$\Omega$ resistor):
$V_{net} = V_{battery} - V_{diode} = 3\text{V} - 0.5\text{V} = 2.5\text{V}$
$I = \frac{V_{net}}{R} = \frac{2.5\text{V}}{1000\Omega} = 2.5 \text{ mA}$
Question 31
Answer: (1)
When the P and N ends of a P-N junction diode are simply joined by a wire without any external battery, the net electromotive force is zero. Hence, there will be no steady current in the circuit.
Question 32
Answer: (1)
For Total Internal Reflection (TIR) to occur at the hypotenuse face of the isosceles right-angled prism, the angle of incidence ($45^{\circ}$) must be greater than or equal to the critical angle $\theta_c$.
$45^{\circ} \ge \theta_c \Rightarrow \sin 45^{\circ} \ge \sin\theta_c$
$\frac{1}{\sqrt{2}} \ge \frac{1}{\mu} \Rightarrow \mu \ge \sqrt{2}$. Least value is $\sqrt{2}$.
Question 33
Answer: (1)
(A) Biot-Savart's law $\rightarrow$ (iv) $\vec{B}=\frac{\mu_{0}i}{4\pi}\int\frac{dl\sin\theta}{r^{2}}\hat{n}$
(B) Ampere's circuit law $\rightarrow$ (iii) $\oint\vec{B}\cdot\vec{dl}=\mu_{0}\Sigma i$
(C) Force between parallel conductors $\rightarrow$ (i) $\frac{\mu_{0}l_{1}l_{2}}{2\pi d}$
(D) Lorentz force $\rightarrow$ (ii) $q[\vec{E}+(\vec{V}\times\vec{B})]$
Question 34
Answer: (4)
Statement-I is incorrect because the intensity of unpolarised light reduces to half ($I = I_0/2$) after passing through a polariser.
Statement-II is correct; polarisation is an exclusive property of transverse waves and proves the transverse nature of light.
Question 35
Answer: (3)
When the reflected and refracted rays are perpendicular, the light must be incident at Brewster's angle.
According to Brewster's law: $\tan \theta_p = \mu$
$\tan \theta_p = 1.73 = \sqrt{3} \Rightarrow \theta_p = 60^{\circ}$
Question 36
Answer: (1)
Kinetic energy gained $K = qV = (2e)V$.
At distance of closest approach $r_0$, Kinetic Energy = Potential Energy:
$2eV = \frac{1}{4\pi\epsilon_0} \frac{(2e)(Ze)}{r_0}$
$r_0 = \frac{1}{4\pi\epsilon_0} \frac{Z e}{V} = (9 \times 10^9) \frac{1.6 \times 10^{-19} Z}{V} = 14.4 \times 10^{-10} \frac{Z}{V} \text{ m} = 14.4 \frac{Z}{V} \mathring{A}$
Question 37
Answer: (2)
From the standard logic gate symbols shown in the problem figure, X represents a NAND gate and Y represents an OR gate.
Question 38
Answer: (1)
The power of the exponent must be dimensionless. $[\frac{\alpha z}{K\theta}] = 1$
$[\alpha] \frac{[L]}{[ML^2T^{-2}]} = 1 \Rightarrow [\alpha] = [MLT^{-2}]$
Now, $[p] = \frac{[\alpha]}{[\beta]} \Rightarrow [ML^{-1}T^{-2}] = \frac{[MLT^{-2}]}{[\beta]}$
$[\beta] = \frac{[MLT^{-2}]}{[ML^{-1}T^{-2}]} = [L^2] = [M^0L^2T^0]$
Question 39
Answer: (1)
The given charges form an electric dipole along the z-axis. The xy-plane ($z=0$) is the equatorial plane of this dipole.
Electric potential at any point on the equatorial plane is zero. Moving a charge between any two points in this plane requires zero work ($W = q\Delta V = 0$).
Question 40
Answer: (3)
Energy of incident photon $E = \frac{12400}{\lambda(\mathring{A})} \text{ eV} = \frac{12400}{4100} \approx 3.02 \text{ eV}$
For photoelectric emission, $E > \text{Work Function } (W)$.
$3.02 \text{ eV} > 1.92 \text{ eV}$ (Metal A) and $3.02 \text{ eV} > 2 \text{ eV}$ (Metal B).
Both A and B will emit photoelectrons.
Question 41
Answer: (1)
Balmer series wavelengths:
$H_\alpha$ (3 to 2) is longest wavelength $\approx 656.3 \text{ nm}$ (A $\rightarrow$ III)
$H_\beta$ (4 to 2) is $\approx 486.1 \text{ nm}$ (B $\rightarrow$ IV)
$H_\gamma$ (5 to 2) is $\approx 434.1 \text{ nm}$ (C $\rightarrow$ II)
$H_\delta$ (6 to 2) is $\approx 410.2 \text{ nm}$ (D $\rightarrow$ I)
Question 42
Answer: (2)
The potential difference between two concentric conducting spheres depends only on the charge of the inner sphere. Adding charge to the outer shell changes the absolute potential of both spheres by the same amount, keeping the potential difference constant at V.
Question 43
Answer: (4)
By applying Kirchhoff's circuit laws to the parallel branches shown in the figure, the node potential difference $V_A - V_B$ evaluates to 2 V.
Question 44
Answer: (2)
Initial charge $Q_1 = CV = 10\mu C$
Final charge $Q_2 = (KC)V = K(CV) = K(10\mu C)$
Charge that flowed $\Delta Q = Q_2 - Q_1 \Rightarrow 20\mu C = K(10\mu C) - 10\mu C$
$20 = 10(K - 1) \Rightarrow K - 1 = 2 \Rightarrow K = 3$
Question 45
Answer: (2)
A. Magnetic induction (B) is measured in Gauss (III).
B. Magnetic intensity (H) is measured in Ampere/meter (IV).
C. Magnetic flux ($\Phi$) is measured in Weber (II).
D. Magnetic moment (M) is measured in Ampere meter$^2$ (I).
Matching options correctly yields (A)-(III), (B)-(IV), (C)-(II), (D)-(I).