How Does Compressing a Syringe Affect an Ideal Gas?

In summary, the conversation discusses the ideal gas law and how changes in volume can affect pressure, with the constraint of temperature being held constant. It also addresses the concept of an ideal gas not occupying space and how this relates to the ideal gas law. The discussion concludes with a clarification on the definition of an ideal gas and how it can be visualized.
  • #1
adam878787878
11
0

Homework Statement



if you have an ideal gas in a syinge, then you compress the syringe to a point, then that means that Volume has decreased. Consequentally, Pressure must increase to compensate (oin order for PV=nRT to be true) right?
I think this is wrong but I am not undersnating why..
please help ! thanks


Homework Equations





The Attempt at a Solution

 
Physics news on Phys.org
  • #2
Think of it like this. Pressure is caused by molecules of the gas hitting the sides of the container. So, if you decrease the volume of the container, the gas molecules will have no choice but to hit the walls more often. Thus, if pressure is caused by molecules hitting off the walls, the pressure must increase is the molecules are hitting the wall more. This is why the pressure of a gas will increase if you decrease the volume of its container.

Does this make sense?
 
  • #3
well, this homework questions says: "explain why the following statement is incorrect"...

"in the ideal gas law, P=nRt/V, so the pressure is inversely proportional to the volume. If you decrease the volume, the pressure has to go up"

That looks pretty correct to me! so i am stumped
 
  • #4
Hmmm. Yeah, that is the ideal gas law, and the ideal gas law is correct! I'm lost. Maybe ask your instructor about it. It seems to me that the question is either stated incorrectly, or we are missing part of it.
 
  • #5
it ends by asking "What is the flaw in the student's reasoning?"

so the statement is wrong in someway, and I quoted it verbatim from the book.
any ideas?
 
  • #6
Ahh. I think I see the catch here!:smile:

Yes, when we decrease the volume, something must change to keep the equation true, but does this change have to be in the pressure? What other quantity in the ideal gas law is not always fixed?
 
  • #7
adam878787878 said:
well, this homework questions says: "explain why the following statement is incorrect"...

"in the ideal gas law, P=nRt/V, so the pressure is inversely proportional to the volume. If you decrease the volume, the pressure has to go up"

That looks pretty correct to me! so i am stumped


The problem is the "... the pressure has to go up..."

It does not necessarily have to go up and if it does go up, it does not have to go up like one over the volume.
 
  • #8
ah ok i see.
n is not fixed also (so, could halve the gas and halve volume then pressure would stay same.)
thanks!
 
  • #9
adam878787878 said:
ah ok i see.
n is not fixed also (so, could halve the gas and halve volume then pressure would stay same.)
thanks!

There are only three state variables in the Ideal Gas Law, N is not one of them.

The statement in the book is correct only if you add the constraint of the temperature being held constant (Isothermal).

CS
 
  • #10
isnt n moles of the gas? If you let 1/2 the gas escape, and half the volume, then P would stay same right?
if you can't change moles of gas, then how many moles of gas in ideal gas?
?
 
  • #11
adam878787878 said:
isnt n moles of the gas? If you let 1/2 the gas escape, and half the volume, then P would stay same right?
if you can't change moles of gas, then how many moles of gas in ideal gas?
?

The original question you posed asks about a cylinder being depressed (a syringe). It is implied that the gas cannot escape (or at least it seems that way to me), hence the number of moles is constant. The only variables are the pressure, volume (of the cylinder or syringe in this case), and the temperature. I believe the point they want you to understand is that the temperature may also be varied which would result in a pressure change also.

CS
 
  • #12
i have another ideal gas law question.
how can we say that a mole of any ideal gas will "occupy" 22.4L , when by definition of ideal gas, they don't occupy any space?
 
  • #13
think about this and ur doubts will be cleared..if the space occupied by molecules of a ideal gas is negligible in comparison to the total volume then ..the p vs v graph of experimental data and that calculated by boyle's law should coincide.

THIS IS NOT THE CASE...
 
  • #14
adam878787878 said:
i have another ideal gas law question.
how can we say that a mole of any ideal gas will "occupy" 22.4L , when by definition of ideal gas, they don't occupy any space?

I'm not sure what you mean by an ideal gas being defined as to not occupy space.

Here is some more information on the Ideal Gas Law that may be of some help.

An ideal gas is defined as one in which all collisions between atoms or molecules are perfectly eleastic and in which there are no intermolecular attractive forces. One can visualize it as a collection of perfectly hard spheres which collide but which otherwise do not interact with each other. In such a gas, all the internal energy is in the form of kinetic energy and any change in internal energy is accompanied by a change in temperature.

Source: http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/idegas.html#c4

CS
 

FAQ: How Does Compressing a Syringe Affect an Ideal Gas?

What is the Ideal Gas Law?

The Ideal Gas Law is a mathematical equation that describes the behavior of ideal gases under various conditions. It states that the pressure, volume, and temperature of a gas are all related and can be calculated using the equation PV = nRT, where P is the gas pressure, V is the volume, n is the number of moles of gas, R is the universal gas constant, and T is the temperature in Kelvin.

Why is the Ideal Gas Law important?

The Ideal Gas Law is important because it allows scientists to predict the behavior of gases and make calculations about their properties. It is also the basis for many other gas laws and serves as a fundamental concept in the study of thermodynamics and fluid mechanics.

What are the assumptions of the Ideal Gas Law?

The Ideal Gas Law assumes that the gas particles are in constant, random motion and do not interact with each other. It also assumes that the volume of the gas particles is negligible compared to the volume of the container and that there are no intermolecular forces present.

How does the Ideal Gas Law relate to real gases?

While the Ideal Gas Law is a good approximation for many gases at low pressures and temperatures, it does not accurately describe the behavior of real gases. Real gases have volume and intermolecular forces that affect their behavior, so the Ideal Gas Law needs to be modified to account for these factors.

What happens when one of the variables in the Ideal Gas Law is held constant?

If one of the variables in the Ideal Gas Law is held constant, then a change in one of the other variables will result in a proportional change in the remaining variable. For example, if the temperature is held constant and the pressure is increased, then the volume of the gas will decrease in order to maintain a constant value for the product PV.

Similar threads

Ideal Gas Law and Pressure at 80°C
Replies
2
Views
2K
What would be a real-life example of the ideal gas law?
Replies
3
Views
9K
Exploring the Ideal Gas Law: A Balloon Problem
Replies
5
Views
2K
Ideal Gas Law -- Isobaric Epansion followed by....
Replies
9
Views
2K
Minimum heat removed from gas to restore its state
Replies
5
Views
2K
Thermodynamics problem (ideal gas law, kinetic theory, processes, etc.)
Replies
18
Views
1K
What Happens When Gas Expands at Constant Pressure?
Replies
12
Views
6K
Why Might Statement (b) Be Incorrect in Ideal Gas Processes?
Replies
2
Views
1K
Force acting on the area of the cylinder due to gas
Replies
3
Views
2K
Ideal Gas Law in "alternate" universe
Replies
4
Views
2K

Hot Threads

Recent Insights

Back
Top