NEET Thermal Physics MCQs & PYQs

Practice NEET Thermal Physics Questions

Question 91:

moderate

The power radiated by a black body is $P$ and it radiates maximum energy at wavelength, $\lambda_0$. If the temperature of the black body is now changed so that it radiates maximum energy at wavelength $\frac{3}{4}\lambda_0$, the power radiated by it becomes $nP$. The value of $n$ is: (2018)

From Wien's displacement law, $T \propto 1/\lambda_{max}$. Thus $T'/T = \lambda_0 / (3\lambda_0/4) = 4/3$. According to Stefan's law, Power $P \propto T^4$. So, $P'/P = (T'/T)^4 = (4/3)^4 = 256/81$.

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Question 92:

moderate

A spherical black body with a radius of $12\text{ cm}$ radiates $450\text{ watt}$ power at $500\text{ K}$. If the radius were halved and the temperature doubled, the power radiated in watt would be: (2017-Delhi)

Power radiated $P = \sigma A T^4 = \sigma (4\pi r^2) T^4 \Rightarrow P \propto r^2 T^4$. Given $r' = r/2$ and $T' = 2T$. Thus, $P' = P(1/2)^2 (2)^4 = P(1/4)(16) = 4P = 4 \times 450 = 1800\text{ W}$.

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Question 93:

moderate

A black body is at a temperature of $5760\text{ K}$. The energy of radiation emitted by the body at wavelength $250\text{ nm}$ is $U_1$, at wavelength $500\text{ nm}$ is $U_2$ and that at $1000\text{ nm}$ is $U_3$. Wien’s constant, $b = 2.88 \times 10^6\text{ nmK}$. Which of the following is correct? (2016 – I)

From Wien's displacement law, $\lambda_{max} = \frac{b}{T} = \frac{2.88 \times 10^6}{5760} = 500\text{ nm}$. This means maximum energy is radiated at $500\text{ nm}$. Therefore, the energy $U_2$ is maximum, so $U_2 > U_1$ and $U_2 > U_3$.

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Question 94:

moderate

On observing light from three different stars P, Q and R, it was found that intensity of violet color is maximum in the spectrum of P, the intensity of green color is maximum in the spectrum of R and the intensity of red color is maximum in the spectrum of Q. If $T_P$, $T_Q$ and $T_R$ are the respective absolute temperatures of P, Q and R then it can be concluded from the above observations that: (2015)

The wavelengths corresponding to maximum intensity are $\lambda_P < \lambda_R < \lambda_Q$ because violet has the shortest wavelength and red has the longest. From Wien's law, $T \propto 1/\lambda_{max}$. Thus, the temperatures are in the reverse order: $T_P > T_R > T_Q$.

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Question 95:

moderate

A piece of iron is heated in a flame. It first becomes dull red then becomes reddish yellow and finally turns to white hot. The correct explanation for the above observation is possible by using: (2013)

As the temperature of the iron increases, the wavelength at which it emits maximum energy decreases, according to Wien's displacement law ($lambda_{max} T = text{constant}$). This causes the color to shift from longer wavelengths (red) to shorter wavelengths (yellow, then all visible mixing to white).

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Question 96:

moderate

Gravitational force is required for: (2000)

Convection involves the macroscopic movement of fluid which relies on density differences. These differences in density lead to buoyant forces, which require gravity to operate.

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Question 97:

moderate

A cup of coffee cools from $90^\circ\text{C}$ to $80^\circ\text{C}$ in $t$ minutes, when the room temperature is $20^\circ\text{C}$. The time taken by a similar cup of coffee to cool from $80^\circ\text{C}$ to $60^\circ\text{C}$ at a room temperature same at $20^\circ\text{C}$ is: (2021)

Using average form of Newton's law of cooling: $\frac{90-80}{t} = K(\frac{90+80}{2}-20) \Rightarrow \frac{10}{t} = K(65)$. For second case: $\frac{80-60}{t'} = K(\frac{80+60}{2}-20) \Rightarrow \frac{20}{t'} = K(50)$. Dividing the two equations yields $t' = \frac{13}{5}t$.

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Question 98:

moderate

A body cools from a temperature $3T$ to $2T$ in $10$ minutes. The room temperature is $T$. Assume that Newton’s law of cooling is applicable. The temperature of the body at the end of next $10$ minutes will be: (2016 – II)

By Newton's law of cooling: $\frac{3T-2T}{10} = K(\frac{3T+2T}{2}-T) \Rightarrow \frac{T}{10} = K(1.5T)$. For next 10 mins: $\frac{2T-T'}{10} = K(\frac{2T+T'}{2}-T)$. Substituting $K = \frac{1}{15}$, we get $2T - T' = \frac{1}{15}(0.5T' + T) \times 10$. Solving gives $T' = \frac{3}{2}T$.

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Question 99:

moderate

A black body has wavelength $\lambda_m$ corresponding to maximum energy at $2000 \text{ K}$. Its wavelength corresponding to maximum energy at $3000 \text{ K}$ will be: (2001)

According to Wien's displacement law, $\lambda_{m1} T_1 = \lambda_{m2} T_2$. $\lambda_m (2000) = \lambda_{m2} (3000) \Rightarrow \lambda_{m2} = \frac{2000}{3000} \lambda_m = \frac{2}{3} \lambda_m$.

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Question 100:

moderate

A sphere maintained at temperature $600 \text{ K}$, has cooling rate $R$ in an external environment of $200 \text{ K}$ temperature. If its temperature, falls to $400 \text{ K}$ then its cooling rate will be: (1999)

Cooling rate $\propto (T^4 - T_0^4)$. $\frac{R'}{R} = \frac{400^4 - 200^4}{600^4 - 200^4} = \frac{2^4 - 1^4}{3^4 - 1^4} = \frac{15}{80} = \frac{3}{16}$. Since $\frac{3}{16} = \frac{9}{48}$, the correct cooling rate would be $\frac{3}{16}R$. The closest option is incorrectly printed as 9/27, the answer is 3/16 R

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