﻿ NEET Important Questions of Thermodynamics | Zigya

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# Thermodynamics

#### Multiple Choice Questions

1.

A Carnot engine operating between temperatures T1 and T2 has efficiency 1/6. When T2 is lowered by 62 K, its efficiency increases to 1/3. Then T1 and T2 are, respectively

• 372 K and 330 K

• 330 K and 268 K

• 310 K and 248 K

• 372 K and 310 K

D.

372 K and 310 K

The efficiency  is given by,

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2.

The above p-v diagram represents the thermodynamic cycle of an engine, operating with an ideal monoatomic gas. The amount of heat extracted from the source in a single cycle is

• povo

• 4povo

B.

Heat is extracted from the source in path DA and AB is

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3.

One mole of diatomic ideal gas undergoes a cyclic process ABC as shown in the figure. The process BC is adiabatic. The temperatures at A, B and C are 400 K, 800 K and 600 K respectively. Choose the correct statement:

• The change in internal energy in the process AB is -350 R.

• The change in internal energy in the process BC is -500 R.

• The change in internal energy in the whole cyclic process is 250 R.

• The change in internal energy in the process CA is 700 R.

D.

The change in internal energy in the process CA is 700 R.

According to first law of thermodynamics,
(i) change in internal energy from A to B i.e,

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4.

Consider a spherical shell of radius R at temperature T. The black body radiation inside it can be considered as an ideal gas of photons with internal energy per unit volume  and pressure. If the shell now undergoes an adiabatic expansion the relation between T and R is

• T ∝ e-R

• T ∝ e-3R

• T ∝ (1/R)

• T ∝(1/R3)

C.

T ∝ (1/R)

According to given equation,

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5.

A Carnot engine, whose efficiency is 40%, takes in heat from a source maintained at a temperature of 500 K It is desired to have an engine of efficiency 60%. Then, the intake temperature for the same exhaust (sink) temperature must be

• the efficiency of Carnot engine cannot be made larger than 50%

• 1200 K

• 750 K

• 600 K

C.

750 K

Efficiency

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6.

Three perfect gases at absolute temperature T1, T2 and T3 are mixed. The masses of molecules are m1,m2 and m3 and the number of molecules is n1,n2 and n3 respectively.Assuming no loss of energy, the final temperature of the mixture is

A.

For adiabatic process i.e., no heat change

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7.

Consider an ideal gas confined in an isolated closed chamber. As the gas undergoes an adiabatic expansion the average time of collision between molecules increases as Vq , where V is the volume of the gas. The value of q is:

C.

For an adiabatic process TVγ-1 = constant
We know that average time of collision between molecules

Where n= number of molecules per unit volume

vrms = rms velocity of molecules

Thus, we can write
n =K1V-1 and Vrms  = K2T1/2
Where K1 and K2 are constants.
For adiabatic process TVγ-1 = constant. Thus we can write

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8.

A solid body of constant heat capacity 1 J/°C is being heated by keeping it in contact with reservoirs in two ways:
(i) Sequentially keeping in contact with 2 reservoirs such that each reservoir supplies the same amount of heat.
(ii) Sequentially keeping in contact with 8 reservoirs such that each reservoir supplies the same amount of heat. In both the cases body is brought from an initial temperature 100°C to final temperature 200°C. Entropy change of the body in the two cases respectively is:

• ln2,4ln2

• ln2,ln2

• ln2,2ln2

• 2ln2,8ln2

B.

ln2,ln2

Since entropy is a state function, therefore a change in entropy in both the processes must be same .

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9.

Helium gas goes through a cycle ABCDA (consisting of two isochoric and two isobaric lines) as shown in the figure. Efficiency of this cycle is nearly:(Assume the gas to be close to ideal gas)

• 15.4%

• 9.1%

• 10.5%

• 12.5%

A.

15.4%

The efficiency of a process is defined as the ratio of work done to energy supplied.
Here,
Where Cp and Cv are two heat capacities (molar)

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10.

An ideal gas enclosed in a vertical cylindrical container supports a freely moving piston of mass M. The piston and cylinder have equal cross sectional area A. When the piston is in equilibrium, the volume of the gas is V0 and its pressure is P0. The piston is slightly displaced from the equilibrium position and released. Assuming that the system is completely isolated from its surrounding, the piston executes a simple harmonic motion with frequency

C.

FBD of piston at equilibrium
⇒ Patm A + mg = P0A

FBD of piston when piston is pushed down a distance x

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