Does the pressure of a gas always decrease as the volume increases?

In summary, the pressure of the gas decreases as the volume increases because the pressure is a force and according to Newton's law, every force has an equal and opposite force.
  • #1
gkangelexa
81
1
PV = nRT

When a gas in a cylinder expands, the gas pushes the piston upward against a constant opposing external atmospheric pressure P. The work done by the gas on the surroundings is w= -P[itex]\Delta[/itex]V

[itex]\Delta[/itex]V is the change in volume

My question is:
Assuming the Temperature stays constant (as my book does), then as the gas expands (the volume increases) then the Pressure must also decrease, right? bc PV = nRT and everything else is constant...

But pressure is a force, and according to Newton, every force has an equal and opposing force. So the pressure that the gas exerts must equal the atmospheric pressure, right?
But how can this be true if the pressure of the gas is decreasing as the volume is increasing?
 
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  • #2
Let's suppose T and n fixed. If you don't apply any other forces from the outside besides the atmospheric pressure, the expansion can happen only if the pressure of the perfect gas is greater than the atmospheric pressure.
The work done by the gas can be correctly calculated using your formula, but in this case p is not the pressure of the gas during the expansion. You're right when you say that in this case if the volume increases the pressure decreases.
If the pressure of the gas is equal to the atmospheric pressure, the system is in equilibrium and there would be no expansion.
 
Last edited:
  • #3
vuellesse said:
Let's suppose T and n fixed. If you don't apply any other forces from the outside besides the atmospheric pressure, the expansion can happen only if the pressure of the perfect gas is greater than the atmospheric pressure.
The work done by the gas can be correctly calculated using your formula, but in this case p is not the pressure of the gas during the expansion. You're right when you say that in this case if the volume increases the pressure decreases.
If the pressure of the gas is equal to the atmospheric pressure, the system is in equilibrium and there would be no expansion.


I'm confused because of Newton's law where it says that the force of A on B is equal and opposite to the force of B on A.
So shouldn't the pressure of the gas on the atmosphere equal the pressure of the atmosphere on the gas?
 
  • #4
Let's think the cylinder as if it has one base free to move. The gas inside the cylinder applies a pressure p on the base (so the force is F_in=p S, where S is the area of the base). Because of Newton's law, the base itself applies on the gas the same force (in opposite direction). Moreover, the gas outside the cylinder applies a pressure p_atm on the base (so the force this time is F_out= - p_atm S; the minus sign is needed because the two forces are opposite in direction), while the base applies on the outside gas (- F_out). Newton's law is not violated. In conclusion, on the base the total force is F=F_in - F_out=(p - p_atm) S, so the expansion happens when the two pressures are different.

In other words, the two forces (equal and opposite) considered by Newton's law are not to be applied on the same object (otherwise they always cancel each other and there would be no motion at all).

ps Sorry for my English, I hope I made myself clear enough.
 
  • #5
Remember these forces act on a piston. The piston may have a mass 'm' or be considered mass-less. The force applied by the gas on the piston is P*A, but since P is a function of volume like you stated, the force is a variable force. The other force applied on the piston is from the atmospheric pressure and is Patm*A. Notice in reality there would also be a friction force between the piston and the cylinder wall. So summing forces on the piston would result in:

A*(nRT/V)-Patm*A = m*a

Now if m=0 or a=0 then you arrive at the steady state where the pressure of the gas is equal to atmospheric pressure.
 
  • #6
Wow. Yes it is clear now. Thank you!
 

Related to Does the pressure of a gas always decrease as the volume increases?

1. What is gas pressure and how is it measured?

Gas pressure is defined as the force exerted by gas molecules per unit area of a container. It is typically measured in units of pounds per square inch (psi) or pascals (Pa). It can be measured using a variety of instruments such as a barometer, manometer, or pressure gauge.

2. How does temperature affect the pressure of a gas?

According to the ideal gas law, as temperature increases, the pressure of a gas also increases, assuming the volume and number of gas molecules remain constant. This is because at higher temperatures, gas molecules have more kinetic energy and collide with the container walls more frequently, resulting in a higher pressure.

3. What is Boyle's Law and how does it relate to gas pressure?

Boyle's Law states that at a constant temperature, the pressure of a gas is inversely proportional to its volume. In other words, as the volume of a gas decreases, its pressure increases, and vice versa.

4. Can work be done by or on a gas?

Yes, work can be done by or on a gas. When a gas expands, it does work on its surroundings, and when a gas is compressed, work is done on the gas.

5. How is the work done by a gas calculated?

The work done by a gas can be calculated using the equation W = PΔV, where W is the work done, P is the pressure of the gas, and ΔV is the change in volume of the gas. This equation is derived from the definition of work, which is force multiplied by distance.

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