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PHYSICS BYTES

Rank Booster Test Series - 04

DETAILED SOLUTIONS KEY

Question 1
Answer: (3)
As the temperature difference as well as the thermal resistance is same for both the cases, so thermal current will also be same for both the cases.
Question 2
Answer: (3)
Convection takes more heat upwards.
Question 3
Answer: (4)
Detailed solution not required.
Question 4
Answer: (1)
According to Newton's law:
Rate of cooling $\propto$ average temperature difference $\Delta\theta$.
Question 5
Answer: (1)
$\lambda_{m} = \frac{b}{T} \implies T = \frac{b}{\lambda_{m}} = \frac{2.93 \times 10^{-3}}{4000 \times 10^{-10}} = 7325~K$
Question 6
Answer: (1)
$Q = nC_{v}\Delta T$
$Q = \frac{15}{28} \times \frac{5 \times R}{2} \times (4T - T)$ (where $T = 300~K$)
$Q = 10021.33 \approx 10~kJ$
Question 7
Answer: (4)
Detailed solution not required.
Question 8
Answer: (1)
According to ideal gas equation:
$PV = nRT$ or $\frac{V}{T} = \frac{nR}{P}$
At constant pressure $\frac{V}{T} = \text{constant}$.
Hence graph (1) is correct.
Question 9
Answer: (2)
Detailed solution not required.
Question 10
Answer: (4)
$W = \frac{R(T_{i} - T_{t})}{\gamma - 1}$
$6R = \frac{R(T - T_{t})}{\left(\frac{5}{3} - 1\right)}$
$T_{t} = (T - 4)~K$
Question 11
Answer: (1)
Due to compression the temperature of the system increases to a very high value. This causes the flow of heat from the system to the surroundings, thus decreasing the temperature. This decrease in temperature results in a decrease in pressure.
Question 12
Answer: (1)
Detailed solution not required.
Question 13
Answer: (2)
In isothermal process temperature remains constant. i.e., $\Delta T = 0$. Hence according to $C = \frac{Q}{m\Delta T} \implies C_{iso} = \infty$.
In adiabatic process, slope of PV-graph $\frac{dP}{dV} = -\gamma\frac{P}{V} \implies |\text{Slope}| \propto \gamma$.
From the given graph $(\text{Slope})_{2} > (\text{Slope})_{1} \implies \gamma_{2} > \gamma_{1}$.
There 1 should correspond to $O_{2}$ ($\gamma = 1.4$) and 2 should correspond to He ($\gamma = 1.66$).
Question 14
Answer: (2)
Detailed solution not required.
Question 15
Answer: (2)
Heat gain by water = Heat lost by steam
$20 \times 1 \times (80 - 10) = m \times 540 + m \times 1 \times (100 - 80)$
$\implies 1400 = 560~m \implies m = 2.5~g$
Total mass of water $= 20 + 2.5 = 22.5~g$
Question 16
Answer: (2)
Fractional change in its density $= \gamma \times \Delta t$
$= 49 \times 10^{-5} \times 30 = 0.0147$
Question 17
Answer: (2)
$\frac{F - 32}{9} = \frac{C}{5} \implies F = \frac{9C}{5} + 32$
Here y-intercept is positive.
Question 18
Answer: (2)
$H_{1} = H_{2} + H_{3}$
$\frac{(100 - \theta)}{\left(\frac{1}{3kA}\right)} = \frac{(\theta - 50)}{\left(\frac{1}{2kA}\right)} + \frac{(\theta - 0)}{\left(\frac{1}{kA}\right)}$
or $3(100 - \theta) = 2(\theta - 50) + \theta$
or $\theta = \frac{200}{3}^{\circ}C$
Question 19
Answer: (1)
Detailed solution not required.
Question 20
Answer: (2)
Detailed solution not required.
Question 21
Answer: (3)
Detailed solution not required.
Question 22
Answer: (2)
Detailed solution not required.
Question 23
Answer: (3)
$\rho_{0} + \rho_{t_{2}}gl_{2} = \rho_{0} + \rho_{t_{1}}gl_{1} \implies \rho_{t_{2}}l_{2} = \rho_{t_{1}}l_{1}$
$\frac{\rho_{0}}{1 + \gamma t_{2}}l_{2} = \frac{\rho_{0}}{1 + \gamma t_{1}}l_{1}$
$(1 + \gamma t_{1})l_{2} = (1 + \gamma t_{2})l_{1}$
$\gamma(l_{2}t_{1} - l_{1}t_{2}) = l_{1} - l_{2}$
$\gamma = \frac{l_{1} - l_{2}}{l_{2}t_{1} - l_{1}t_{2}}$
Question 24
Answer: (3)
$\Delta Q = \Delta U + \Delta W \because \Delta W = 0 \implies \Delta Q = \Delta U = nC_{v}\Delta T$
$= 2 \times \frac{3R}{2} \times (373 - 273) = 300~R$
Question 25
Answer: (2)
$0.8 \times 5 = P \times (3 + 5) \implies P = 0.5~m$
Question 26
Answer: (3)
For isothermal process $dU = 0$ and work done $= dW = P(V_{2} - V_{1})$
$V_{2} = \frac{V_{1}}{2} = \frac{V}{2} \therefore dW = -\frac{PV}{2}$
Work done by the gas is negative.
Question 27
Answer: (4)
$P_{1}V_{1}^{\gamma} = P_{2}V_{2}^{\gamma} \implies \frac{P_{2}}{P_{1}} = \left[\frac{V_{1}}{V_{2}}\right]^{\gamma} = \left[\frac{4}{1}\right]^{3/2} = \frac{8}{1}$
Question 28
Answer: (4)
For monoatomic gas $P^{\gamma-1}T^{\gamma} = \text{const}$. $P \propto T^{\gamma/(\gamma-1)} \implies \gamma = \frac{5}{3}$
$c = \frac{\gamma}{\gamma-1} = \frac{\frac{5}{3}}{\frac{5}{3}-1} = \frac{5}{2}$
Question 29
Answer: (2)
$P_{B} = P$ since process is isobaric.
For isothermal process $P_{1}V_{1} = P_{2}V_{2} \implies P_{C} = 2P$
For adiabatic process $PV^{\gamma} \implies P_{A}V^{3/2} = \text{const}$. $P_{A} = 2\sqrt{2}P$
So $P_{A} : P_{B} : P_{C} = 2\sqrt{2} : 1 : 2$
Question 30
Answer: (4)
Detailed solution not required.
Question 31
Answer: (1)
$Q = U + W$
Where, $U = $ Internal energy
$\therefore U = Q - W = 100 - 75 = 25~J/s = 25~W$
Question 32
Answer: (1)
Heat of combustion $= 4 \times 10^{4}~J/g$
Specific heat of water, $c = 4.2~J~g^{-1}{}^{\circ}C^{-1}$
Mass of flowing water, $m = 3.0~\text{litre/min} = 3000~g/\text{min}$
Total heat used, $\Delta Q = mc~\Delta T = 3000 \times 4.2 \times (77 - 27) = 6.3 \times 10^{5}~J/\text{min}$
Rate of consumption $= \frac{6.3 \times 10^{5}}{4 \times 10^{4}} = 15.75~g/\text{min}$
Question 33
Answer: (3)
$\lambda = \frac{1}{\sqrt{2}n\pi d^{2}}$
Question 34
Answer: (4)
Detailed solution not required.
Question 35
Answer: (2)
Detailed solution not required.
Question 36
Answer: (4)
$\frac{P_{1}V_{1}}{RT_{1}} = \frac{P_{2}V_{2}}{RT_{2}} \implies P_{2} = \frac{PV(1.1T)}{(1.05)VT} \Rightarrow 1.05P$
Hence final pressure lies between P and 1.1 P
Question 37
Answer: (1)
$\lambda \propto \frac{1}{d^{2}} \implies \frac{\lambda_{1}}{\lambda_{2}} = \left[\frac{d_{2}}{d_{1}}\right]^{2} = \left(\frac{2}{1}\right)^{2} = \frac{4}{1}$
Question 38
Answer: (4)
Energy $= 300~J/\text{litre} = 300 \times 10^{3}~J/m^{3}$
$P = \frac{2}{3}E = \frac{2 \times 300 \times 10^{3}}{3} = 2 \times 10^{5}~N/m^{2}$
Question 39
Answer: (1)
$\gamma_{mix} = \frac{\frac{\mu_{1}\gamma_{1}}{\gamma_{1}-1} + \frac{\mu_{2}\gamma_{2}}{\gamma_{2}-1}}{\frac{\mu_{1}}{\gamma_{1}-1} + \frac{\mu_{2}}{\gamma_{2}-1}}$
Here $\mu_{1} = \mu_{2} = 1$; $\gamma_{1} = \frac{7}{5}$ and $\gamma_{2} = \frac{5}{3}$
$\gamma_{mix} = \frac{3}{2}$
Question 40
Answer: (3)
Detailed solution not required.
Question 41
Answer: (2)
Detailed solution not required.
Question 42
Answer: (4)
Detailed solution not required.
Question 43
Answer: (3)
$\frac{v_{1}}{v_{2}} = \sqrt{\frac{T_{1}}{T_{2}}} = \sqrt{\frac{P_{1}V_{1}}{P_{2}V_{2}}} = \sqrt{\frac{4}{1}} = 2$
Question 44
Answer: (4)
For polytropic process $PV^{x} = \text{const.}$ Here $x = -1$.
$C = \frac{R}{\gamma-1} + \frac{R}{1-x}$
$C = \frac{R}{\frac{5}{3}-1} + \frac{R}{1 - (-1)} = \frac{3R}{2} + \frac{R}{2} = 2R$
Question 45
Answer: (1)
Detailed solution not required.