# Constant pressure and constant volume?

• Federer33
In summary, the concept of a thermodynamic system existing simultaneously under both constant volume and constant pressure at a particular instant is possible but uncommon. This would require a material with zero thermal expansion or a system with constant temperature. However, in most cases, there will be changes in other parameters such as the number of particles.
Federer33
Can a thermodynamic system simultaneously exist under both constant volume and constant pressure at a particular instant. I mean , when the system is under constant pressure condition, can we impose constant volume on the system. Please clear my doubt. I don't know if it's a silly question, but eating my brain.

It's possible, but pretty uncommon. Equivalently this says that the thermal expansion (∂V/∂T)P vanishes. If you Google "zero thermal expansion" you'll find a few articles where such a material has been created. Otherwise the only time the thermal expansion vanishes is at absolute zero.

Sure. It just means that you also have constant temperature.

Wouldn't a sealed uncooled canister of gas, such as a helium tank at a party store, be at a constant pressure and constant volume? Or is there something else to the problem I don't see?

Federer33 said:
Can a thermodynamic system simultaneously exist under both constant volume and constant pressure at a particular instant.
If by "constant" you mean constant in time, the specification underlined will make it a nonsense.
Either you mean "uniform" (that means no spatial variation) or you mean a process and not an instant.

A process with no change in pressure or volume may be possible, for a general system. Other parameters may change. For ideal gas you may have a system which variable number of particles.

## 1. What is the difference between constant pressure and constant volume?

Constant pressure refers to a thermodynamic process where the system remains at a constant pressure while undergoing a change in volume. On the other hand, constant volume refers to a process where the system remains at a constant volume while undergoing a change in pressure. In other words, in constant pressure, the pressure is held constant while the volume can change, and in constant volume, the volume is held constant while the pressure can change.

## 2. How do you calculate the work done in a constant pressure process?

The work done in a constant pressure process can be calculated by multiplying the change in volume by the external pressure. This is because in a constant pressure process, the external pressure is equal to the pressure inside the system, and work is defined as the force applied over a distance.

## 3. What is an example of a constant volume process?

An example of a constant volume process is an isochoric process, where the volume remains constant while the temperature and pressure can change. This can be seen in a closed container of gas that is heated, causing an increase in pressure while the volume remains the same.

## 4. Why is constant pressure more common in real-life scenarios?

Constant pressure processes are more common in real-life scenarios because most systems, such as engines and power plants, operate at a constant pressure. This allows for a steady flow of energy and control over the system. Additionally, in open systems, like the Earth's atmosphere, the pressure is not constant but changes gradually, making constant pressure a more accurate representation of real-life processes.

## 5. How does the ideal gas law apply to constant pressure and constant volume processes?

The ideal gas law, PV = nRT, applies to both constant pressure and constant volume processes. In a constant pressure process, the ideal gas law can be rearranged to find the final volume or temperature, while in a constant volume process, it can be used to find the final pressure or temperature. However, the ideal gas law assumes that the pressure and volume are directly proportional, which may not always be the case in real-life scenarios.

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