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Physics MCQs for NEET — Practice Questions with Answers

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A sample of gas expands from volume V₁ to V₂. The amount of work done by the gas is greatest when the expansion is

Isothermal

Isobaric

Adiabatic

Equal in all cases

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In a reversible isochoric change -

ΔW = 0

ΔQ = 0

ΔT = 0

ΔU = 0

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Explanation

$Δ V = 0 \Rightarrow P Δ V = 0 \Rightarrow Δ W = 0$

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Entropy of a thermodynamic system does not change when this system is used for -

Conduction of heat from a hot reservoir to a cold reservoir

Conversion of heat into work isobarically

Conversion of heat into internal energy isochorically

Conversion of work into heat isochorically

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Explanation

Entropy does not change when there is no heat exchange in a process.

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Which is incorrect ?

In an isobaric process, Δp = 0

In an isochoric process, ΔW = 0

In an isothermal process, ΔT = 0

In an isothermal process, ΔQ = 0

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Explanation

In isothermal process ΔQ ≠ 0.

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A Carnot engine working between 300 K and 600K has work output of 800 J per cycle. What is amount of heat energy supplied to the engine from source per cycle ?

1800 J/cycle

1000 J/cycle

2000 J/cycle

1600 J/cycle

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Explanation

$η = \frac{T_{1} - T_{2}}{T_{1}} = \frac{W}{Q}$ ⇒ $Q = (\frac{T_{1}}{T_{1} - T_{2}}) W$

$= \frac{600}{(600 - 300)} \times 800$ =1600 J

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If the door of a refrigerator is kept open, then which of the following is true ?

Room is cooled

Room is heated

Room is either cooled or heated

Room is neither cooled nor heated

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Explanation

In a refrigerator, the heat dissipated in the atmosphere is more than that taken from the cooling chamber, therefore the room is heated if the door of a refrigerator is kept open.

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In a cyclic process, the internal energy of the gas -

Increases

Decreases

Remains constant

Becomes zero

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Explanation

Internal energy is a state function.

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For a reversible process, necessary condition is -

In the whole cycle of the system, the loss of any type of heat energy should be zero

That the process should be too fast

That the process should be slow so that the working substance should remain in thermal and mechanical equilibrium with the surroundings

The loss of energy should be zero and it should be quasistatic

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Explanation

For a reversible process $\int \frac{d Q}{T} = 0$

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A Carnot's engine used first an ideal monoatomic gas then an ideal diatomic gas. If the source and sink temperature are 411°C and 69°C respectively and the engine extracts 1000 J of heat in each cycle, then area enclosed by the PV diagram is -

100 J

300 J

500 J

700 J

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Explanation

$η = 1 - \frac{T_{2}}{T_{1}} = 1 - \frac{(273 + 69)}{(273 + 411)} = 0 .5$

⇒ Work done $= η \times Q = 0 .5 \times 1000 = 500 J$

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The temperature of sink of Carnot engine is 27°C. Efficiency of engine is 25%. Then temperature of source is -

227°C

327°C

127°C

27°C

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Explanation

$η = 1 - \frac{T_{2}}{T_{1}}$ ⇒ $\frac{25}{100} = 1 - \frac{300}{T_{1}}$ ⇒ $\frac{1}{4} = 1 - \frac{300}{T_{1}}$

$T_{1} = 400 K = 127 ° C$

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