I quote from NCERT - thermodynamics chapter - page 302:

"'Now, the system may go from initial state to final state in a number of ways. For example, to change the state of a gas from (P1,V1) to (P2,V2), we can first change the volume of the gas from V1 to V2, keeping it's pressure constant i.e. we can first go to the stateP1, V2 and then change the pressure of the gas from P1 to P2, keeping the volume constant, to take the gas to P2, V2. Alternatively, we can first keep the volume constant and then keep the pressure constant."'

My question is, when they say they'll keep either of the 2 : pressure or volume constant, and change the other, they change either the temperature(T) or the amount of gas taken(n) to achieve this, right ?

Because I don't see any other way this could be possible - changing pressure and keeping volume constant or vice versa.

Please explain... it wuld be great help.

thanks.

You're on the right track in your understanding of how thermodynamic processes work! When we talk about changing the state of a gas while keeping either pressure or volume constant, we indeed have to consider how temperature and the amount of gas can change to facilitate these transitions. Let's break this down step by step.

Understanding the Basics of Gas Laws

In thermodynamics, the behavior of gases is often described by the ideal gas law, which is expressed as:

PV = nRT

Here, P is pressure, V is volume, n is the number of moles of gas, R is the ideal gas constant, and T is temperature. This equation shows the relationship between these four variables.

Changing Volume at Constant Pressure

When you change the volume of a gas while keeping the pressure constant, you can indeed change the temperature or the amount of gas. For instance:

• If you increase the volume (V), to maintain constant pressure (P), the temperature (T) must also increase. This is due to the fact that, according to the ideal gas law, if V increases and P remains constant, T must rise to balance the equation.
• Alternatively, if you decrease the volume, the temperature must decrease if the amount of gas (n) remains constant.

Changing Pressure at Constant Volume

Conversely, if you keep the volume constant and change the pressure, you can again adjust the temperature or the amount of gas:

• Increasing the pressure while keeping the volume constant will require an increase in temperature. This is because, with a fixed volume, more gas molecules collide with the walls of the container more frequently, raising the pressure.
• If you were to decrease the pressure while keeping the volume constant, you would need to decrease the temperature or reduce the amount of gas in the container.

Real-World Examples

To visualize this, think about a balloon. When you heat a balloon (increasing temperature), it expands (increasing volume) if the pressure is constant. If you were to squeeze the balloon (decreasing volume), the pressure inside increases, and if you want to keep the pressure constant, you would need to heat it up to allow it to expand again.

Conclusion

In summary, when you keep either pressure or volume constant while changing the other, you are indeed altering the temperature or the amount of gas in the system. This interplay is fundamental to understanding how gases behave under different conditions. So, your intuition about needing to change temperature or the amount of gas is absolutely correct!