Thermal Properties of Matter NEET PYQ – Important Previous Year Questions for NEET Physics Preparation

Q 1 :
Three identical heat conducting rods are connected in series as shown in the figure. The rods on the sides have thermal conductivity 2 K while that in the middle has thermal conductivity K. The left end of the combination is maintained at temperature $3 T$ and the right end at $T$ . The rods are thermally insulated from outside. In steady state, temperature at the left junction is $T_{1}$ and that at the right junction is $T_{2}$ . The ratio $\frac{T_{1}}{T_{2}}$ is [2025]

[IMAGE 98]

$\frac{5}{3}$
$\frac{5}{4}$
$\frac{3}{2}$
$\frac{4}{3}$

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(1)

[IMAGE 99]

According to heat transfer, at junction $T_{1}$

$\frac{2 K A (3 T - T_{1})}{L} = \frac{K A (T_{1} - T_{2})}{L}$

Since all identical rods are connected in series,

So, A and L are same.

$∴$ $6 T - 2 T_{1} = T_{1} - T_{2}$

$6 T = 3 T_{1} - T_{2}$ ...(i)

At junction $T_{2},$

$\frac{K A (T_{1} - T_{2})}{L} = \frac{2 K A (T_{2} - T)}{L}$

or $T_{1} - T_{2} = 2 T_{2} - 2 T$

or $2 T = 3 T_{2} - T_{1}$ ...(ii)

From eqn. (i) and (ii),

$\frac{T_{1}}{T_{2}} = \frac{5}{3}$

Q 2 :
Given below are two statements: One is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: Houses made of concrete roofs overlaid with foam keep the room hotter during summer.
Reason R: The layer of foam insulation prohibits heat transfer, as it contains air pockets.
In the light of the above statements, choose the most appropriate answer from the options given below: [2024]

A is true but R is false.
A is false but R is true.
Both A and R are true and R is the correct explanation of A.
Both A and R are true but R is not the correct explanation of A.

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(2)

Houses made of concrete roofs generally keep the room hotter during summer, but the roofs overlaid with foam make the room cooler. As this layer of foam prohibits heat transfer as it contains air pockets. So, assertion is false but reason is true.

Q 3 :
The mechanical quantity, which has dimensions of reciprocal of mass $M^{- 1}$ is [2023]

angular momentum
coefficient of thermal conductivity
torque
gravitational constant

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(4)

Angular momentum, $L = [M L^{2} T^{- 1}]$

Coefficient of thermal conductivity, $K = [M L T^{- 3} K^{- 1}]$

Gravitational constant, $G = [M^{- 1} L^{- 3} T^{- 2}]$

So, gravitational constant has dimensions of reciprocal of mass.

Q 4 :
The energy that will be ideally radiated by a 100 kW transmitter in 1 hour is [2022]

$36 \times 10^{7}$ $J$
$36 \times 10^{4}$ $J$
$36 \times 10^{5}$ $J$
$1 \times 10^{5}$ $J$

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(1)

Power, $P = 100 KW = 100 \times 10^{3} W$

Time, $t = 1$ $hr = 3600$ $s$

Energy, $E = power \times time$

$E = 100 \times 10^{3} \times 3600$

$E = 36 \times 10^{7}$ $J$

Q 5 :
The power radiated by a black body is $P$ and it radiates maximum energy at wavelength, $λ_{0}$ . If the temperature of the black body is now changed so that it radiates maximum energy at wavelength $\frac{3}{4} λ_{0}$ , the power radiated by it becomes $n P$ . The value of $n$ is [2018]

$\frac{3}{4}$
$\frac{4}{3}$
$\frac{256}{81}$
$\frac{81}{256}$

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(3)

From Wien’s law, $λ_{max} T = constant$

So, $λ_{{max}_{1}} T_{1} = λ_{{max}_{2}} T_{2}$

$\Rightarrow λ_{0} T = \frac{3 λ_{0}}{4} T' \Rightarrow \frac{T'}{T} = \frac{4}{3}$

According to Stefan–Boltzmann law, energy emitted per unit time by a black body is $A e σ T^{4}$ , i.e., power radiated.

$∴$ $P \propto T^{4}$

So, $\frac{P'}{P} = {(\frac{T'}{T})}^{4} \Rightarrow n = {(\frac{4}{3})}^{4} = \frac{256}{81}$

Q 6 :
Two rods A and B of different materials are welded together as shown in figure. Their thermal conductivities are $K_{1}$ and $K_{2}$ . The thermal conductivity of the composite rod will be [2017]

[IMAGE 100]

$\frac{3 (K_{1} + K_{2})}{2}$
$K_{1} + K_{2}$
$2 (K_{1} + K_{2})$
$\frac{K_{1} + K_{2}}{2}$

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(4)

Equivalent thermal conductivity of the composite rod in parallel combination will be,

$K = \frac{K_{1} A_{1} + K_{2} A_{2}}{A_{1} + A_{2}} = \frac{K_{1} + K_{2}}{2}$

Q 7 :
A spherical black body with a radius of 12 cm radiates 450 watt power at 500 K. If the radius were halved and the temperature doubled, the power radiated in watt would be [2017]

450
1000
1800
225

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(3)

According to Stefan-Boltzmann law, rate of energy radiated by a black body is given as

$E = σ A T^{4} = σ 4 π R^{2} T^{4}$

Given $E_{1} = 450 W, T_{1} = 500 K, R_{1} = 12 cm$

$R_{2} = \frac{R_{1}}{2}, T_{2} = 2 T_{1}, E_{2} = ?$

$\frac{E_{2}}{E_{1}} = \frac{σ 4 π R_{2}^{2} T_{2}^{4}}{σ 4 π R_{1}^{2} T_{1}^{4}} = {(\frac{R_{2}}{R_{1}})}^{2} {(\frac{T_{2}}{T_{1}})}^{4} \Rightarrow \frac{E_{2}}{E_{1}} = \frac{1}{4} \times 16 = 4$

$E_{2} = E_{1} \times 4 = 450 \times 4 = 1800$ $W$

Q 8 :
A black body is at a temperature of 5760 K. The energy of radiation emitted by the body at wavelength 250 nm is $U_{1}$ , at wavelength 500 nm is $U_{2}$ and that at 1000 nm is $U_{3}$ . Wien’s constant, $b = 2.88 \times 10^{6} nm$ $K$ . Which of the following is correct? [2016]

$U_{1} > U_{2}$
$U_{2} > U_{1}$
$U_{1} = 0$
$U_{3} = 0$

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(2)

According to Wien's displacement law

$λ_{m} = \frac{b}{T} = \frac{2.88 \times 10^{6} nm K}{5760 K} = 500$ $nm$

[IMAGE 101]

Clearly from graph $U_{2} > U_{3}$ and $U_{2} > U_{1}$

Q 9 :
The two ends of a metal rod are maintained at temperatures $100 ° C$ and $110 ° C$ . The rate of heat flow in the rod is found to be $4.0$ $J/s$ . If the ends are maintained at temperatures $200 ° C$ and $210 ° C$ , the rate of heat flow will be [2015]

8.0 $J / s$
4.0 $J / s$
44.0 $J / s$
16.8 $J / s$

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Topic Question Set

Q 3 :
The mechanical quantity, which has dimensions of reciprocal of mass $M^{- 1}$ is [2023]

Q 4 :
The energy that will be ideally radiated by a 100 kW transmitter in 1 hour is [2022]

Q 5 :
The power radiated by a black body is $P$ and it radiates maximum energy at wavelength, $λ_{0}$ . If the temperature of the black body is now changed so that it radiates maximum energy at wavelength $\frac{3}{4} λ_{0}$ , the power radiated by it becomes $n P$ . The value of $n$ is [2018]

Q 6 :
Two rods A and B of different materials are welded together as shown in figure. Their thermal conductivities are $K_{1}$ and $K_{2}$ . The thermal conductivity of the composite rod will be [2017]

[IMAGE 100]

Q 7 :
A spherical black body with a radius of 12 cm radiates 450 watt power at 500 K. If the radius were halved and the temperature doubled, the power radiated in watt would be [2017]

Q 8 :
A black body is at a temperature of 5760 K. The energy of radiation emitted by the body at wavelength 250 nm is $U_{1}$ , at wavelength 500 nm is $U_{2}$ and that at 1000 nm is $U_{3}$ . Wien’s constant, $b = 2.88 \times 10^{6} nm$ $K$ . Which of the following is correct? [2016]

Q 9 :
The two ends of a metal rod are maintained at temperatures $100 ° C$ and $110 ° C$ . The rate of heat flow in the rod is found to be $4.0$ $J/s$ . If the ends are maintained at temperatures $200 ° C$ and $210 ° C$ , the rate of heat flow will be [2015]

(2)

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Topic Question Set

Q 3 : The mechanical quantity, which has dimensions of reciprocal of mass M-1 is [2023]

Q 4 : The energy that will be ideally radiated by a 100 kW transmitter in 1 hour is [2022]

Q 5 : The power radiated by a black body is P and it radiates maximum energy at wavelength, λ0. If the temperature of the black body is now changed so that it radiates maximum energy at wavelength 34λ0, the power radiated by it becomes nP. The value of n is [2018]

Q 6 : Two rods A and B of different materials are welded together as shown in figure. Their thermal conductivities are K1 and K2. The thermal conductivity of the composite rod will be [2017] [IMAGE 100]

Q 7 : A spherical black body with a radius of 12 cm radiates 450 watt power at 500 K. If the radius were halved and the temperature doubled, the power radiated in watt would be [2017]

Q 8 : A black body is at a temperature of 5760 K. The energy of radiation emitted by the body at wavelength 250 nm is U1, at wavelength 500 nm is U2 and that at 1000 nm is U3. Wien’s constant, b=2.88×106nm K. Which of the following is correct? [2016]

Q 9 : The two ends of a metal rod are maintained at temperatures 100°C and 110°C. The rate of heat flow in the rod is found to be 4.0J/s. If the ends are maintained at temperatures 200°C and 210°C, the rate of heat flow will be [2015]

(2)

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Q 3 :
The mechanical quantity, which has dimensions of reciprocal of mass $M^{- 1}$ is [2023]

Q 4 :
The energy that will be ideally radiated by a 100 kW transmitter in 1 hour is [2022]

Q 5 :
The power radiated by a black body is $P$ and it radiates maximum energy at wavelength, $λ_{0}$ . If the temperature of the black body is now changed so that it radiates maximum energy at wavelength $\frac{3}{4} λ_{0}$ , the power radiated by it becomes $n P$ . The value of $n$ is [2018]

Q 6 :
Two rods A and B of different materials are welded together as shown in figure. Their thermal conductivities are $K_{1}$ and $K_{2}$ . The thermal conductivity of the composite rod will be [2017]

[IMAGE 100]

Q 7 :
A spherical black body with a radius of 12 cm radiates 450 watt power at 500 K. If the radius were halved and the temperature doubled, the power radiated in watt would be [2017]

Q 8 :
A black body is at a temperature of 5760 K. The energy of radiation emitted by the body at wavelength 250 nm is $U_{1}$ , at wavelength 500 nm is $U_{2}$ and that at 1000 nm is $U_{3}$ . Wien’s constant, $b = 2.88 \times 10^{6} nm$ $K$ . Which of the following is correct? [2016]

Q 9 :
The two ends of a metal rod are maintained at temperatures $100 ° C$ and $110 ° C$ . The rate of heat flow in the rod is found to be $4.0$ $J/s$ . If the ends are maintained at temperatures $200 ° C$ and $210 ° C$ , the rate of heat flow will be [2015]