Physics

Thermodynamics and Gas Laws

616 Questions

Thermodynamics and gas laws questions test the understanding of ideal gas behavior, work done during thermodynamic processes, and specific heat ratios. Key areas include isothermal, adiabatic, and isobaric expansions along with real gas deviations. These mathematical physics concepts are standard in engineering and general science competitive exams.

Ideal gas equationIsothermal and adiabatic processesThermodynamic workGas kinetic theoryReal gas behavior

Thermodynamics and Gas Laws Questions

Multiple choice

Nitrogen at an initial state of 10 bar, 1 m3 and 300 K is expanded isothermally to a final volume of 2 m3. The p-v-T relation is $\bigg(p+\frac{a}{v^2}\bigg)v-RT$, where a > 0, the final pressure

  1. will be slightly less than 5 bar

  2. will be slightly more than 5 bar

  3. will be exactly 5 bar

  4. cannot be ascertained in the absence of the value of a
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$\text{Here a>0, so above equation shows that $p_2$ is greater than 5 and +ve.}$

Multiple choice

All the molecules of a gas are in constant motion. Which of the following statements regarding the nature of this motion is incorrect?

  1. As the gas molecules are elastic in nature there is no loss of energy when the molecules collide. But some energy is transferred during the time of collision.

  2. All the molecules of a gas are in constant motion.

  3. There is a large distance between the gaseous molecules.

  4. The kinetic energy of all the gaseous molecules is independent of the absolute temperature.
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The average kinetic energy of all the gaseous molecules is directly proportional to the absolute temperature. The increase in temperature increases the kinetic energy of the molecules and hence they move at greater speed.

Multiple choice

What will be the effect on temperature of an ideal gas when some work is done during the adiabatic expansion and if the expansion process is reversible?

  1. It increases

  2. It decreases

  3. It may increase or decrease

  4. It will remain unchanged
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The adiabatic (no heat exchanged) expansion of a gas may be carried out in a number of ways. The change in temperature experienced by the gas during expansion depends not only on the initial and final pressure, but also on the manner in which the expansion is carried out. If the expansion process is reversible. The gas does positive work during the expansion, and its temperature decreases. 

Multiple choice physics heat and energy zeroth law of thermodynamics

For 2 gases in thermal equilibrium with each other , Pressure of the gases 

  1. varies as speed of the gas molecules

  2. varies as square root of the gas molecules

  3. varies inversely as the gas molecules

  4. remains same
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

For the ideal gas equation $PV=nRT$

if T is constant the pressure is also constant
hence  when the gases in thermal equilibrium have same temperature their pressure remains same .
 hence option D is correct.

Multiple choice physics heat and energy zeroth law of thermodynamics

When a gas  is in thermal equilibrium, its molecules

  1. have the same average kinetic energy.

  2. have different energies which remains constant.

  3. have a certain constant energy.

  4. do not collide with one another.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

When a gas is in thermal equilibrium, the temperature throughout the gas is the same. (Zeroth law of thermodynamics)
The average kinetic energy of the gas molecules is directly proportional to the absolute temperature of the gas. I.e. different sections of the gas will all have the same average kinetic energy.

Multiple choice

Takes up a definite amount of space

  1. Plasma

  2. Solid

  3. Liquid

  4. Gas
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

A solid has a definite shape and a definite volume. Liquids have a definite volume but not a definite shape, and gases have neither.

Multiple choice modelling gases - the kinetic model ideal gases kinetic theory of gases physics

Statement 1: The internal energy of a perfect gas is entirely kinetic and depends only on absolute temperature of the gas and not on its pressure or volume.
Statement 2: A perfect gas is heated keeping pressure constant and later at constant volume. For the same amount of heat the temperature of the gas at constant pressure is lower than that at constant volume.

  1. Statement 1 is true, statement 2 is true and statement 2 is correct explanation of statement 1

  2. statement 1 is true, statement 2 is false

  3. Statement 1 is true, Statement 2 is true but Statement 2 is not the correct explanation of Statement 1

  4. Statement 1 is false, Statement 2 is true
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Statement 1 is true as internal energy of an ideal gas depends only on temperature. Statement 2 is true because at constant pressure, some heat is used for work (expansion), leaving less for temperature rise compared to constant volume. However, Statement 2 does not explain Statement 1.

Multiple choice modelling gases - the kinetic model ideal gases kinetic theory of gases physics

The average pressure of an ideal gas is  

  1. $\rho = (1/3)\, mn \, V^2 _{av}$
  2. $\rho = (1/2)\, mn \, V _{av}$
  3. $\rho = (1/4)\, mn \, V^2 _{av}$
  4. $\rho = (1/3)\, mn \, V _{av}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

From the kinetic theory of gases, the pressure exerted by an ideal gas is given by P = (1/3) * rho * v_rms^2, where rho is density (m*n) and v_rms^2 is the mean square speed (V^2_av).

Multiple choice modelling gases - the kinetic model ideal gases kinetic theory of gases physics

The equation of state of some gases can be expressed as (p+av2)(v−b)=RT Here p is pressure, v is volume , a,b,R are constants. The dimensions of ′a′ are

  1. $ML^{-5}T^{-2}$
  2. $ML^{-1} T^{-2}$
  3. $M^o L^3T^o$
  4. $M^B L^oT^o$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Here $P$ and $a/{ V }^{ 2 }$ are added that means they have same dimensions.

Therefore,
$\left[ { M }^{ 1 }{ L }^{ -1 }{ T }^{ -2 } \right] =\dfrac { a }{ \left[ { M }^{ 0 }{ L }^{ 6 }{ T }^{ 0 } \right]  } $
$a=\dfrac { { M }^{ 1 }{ L }^{ -1 }{ T }^{ -2 } }{ { M }^{ 0 }{ L }^{ 6 }{ T }^{ 0 } } $
$a={ M }^{ 1 }{ L }^{ 5 }{ T }^{ -2 }$

Multiple choice modelling gases - the kinetic model ideal gases kinetic theory of gases physics

According to kinetic theory of gases,

  1. The velocity of molecules decreases for each collision

  2. The pressure exerted by a diatomic gas is proportional to teh mean velocity of the molecule

  3. The K.E of the gas decreases on expansion at constant temperature

  4. The mean translational KE of a diatomic gas increases with increase in absolute temperature
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Options:
(A): One can not say velocity of molecule decreases  in each collision.
(B): The pressure exerted by a diatomic gas is proportional to rms speed, not mean speed.
(C): Since temp is constant, KE will be constant.
(D): Mean translational KE is proportional to temp, hence will increase with increase in absolute temp.

Multiple choice modelling gases - the kinetic model ideal gases kinetic theory of gases physics

Consider the following statements for air molecules in an air tight container.

  1. the average speed of molecules is larger than root mean square speed.

  2. mean free path of molecules is larger than the mean distance between molecules

  3. mean free path of molecules increases with temperature.

  4. the rms speed of nitrogen molecules is smaller
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The mean free path lambda = 1 / (sqrt(2) * pi * d^2 * n). Since n = P / (kT), lambda = kT / (sqrt(2) * pi * d^2 * P). Thus, lambda is directly proportional to temperature T.

Multiple choice modelling gases - the kinetic model ideal gases kinetic theory of gases physics

Gas exerts pressure on the walls of container because the molecules:

  1. Are losing their Kinetic Energy

  2. Are getting stuck to the walls

  3. Are transferring their momentum to walls

  4. Are accelerated towards walls
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Pressure is force per unit area

and force is nothing but the rate of change of momentum
When the molecules collide with wall their direction get changed so their momentum get changed and this difference
 in momentum after and before the collision is given to the walls so walls feel a force consequently Pressure.
Energy remains constant because collisions are supposed to be $elastic$
Option C is correct.