Calculating Pressure and Force from He Atoms Colliding with a Wall?

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Discussion Overview

The discussion revolves around calculating pressure and force exerted by helium atoms colliding with a wall in a square box at a specified temperature and collision rate. Participants explore the necessary equations and concepts related to momentum, mass, and the relationships between these variables in the context of gas behavior.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant expresses uncertainty about the variable N in the equation PV=Nmv², questioning how to determine the number of helium atoms without additional information.
  • Another participant suggests that the mass m can be derived from the molar mass and Avogadro's number.
  • A different participant proposes using gas density instead of volume to find the necessary parameters for the calculations.
  • One participant suggests approaching the problem from first principles, focusing on momentum change during collisions and how that relates to force.
  • There is a discussion about the momentum of a single molecule and how to calculate the total force based on the number of collisions per second.
  • Participants debate whether the number of collisions corresponds to the number of molecules, leading to a calculation of mass based on the number of moles and the molar mass of helium.
  • Clarification is provided that m in the equation refers to the mass of a single helium atom, which should be expressed in kilograms for the calculations.

Areas of Agreement / Disagreement

Participants generally agree on the need to clarify the definitions of variables and the relationships between them, but there is no consensus on the exact method to determine the mass of helium or the interpretation of the number of collisions.

Contextual Notes

Some participants note limitations in the information provided, such as the lack of explicit details on the amount of helium gas and the need to express mass in appropriate units for calculations.

higherme
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Homework Statement



A square box contain He at 25C. If the atoms collide with wall perpendicularly (90 degrees) at rate of 4.0E22 times/s, calculate pressure and force exerted on wall given area of wall 100cm^2 and speed of atoms =600 m/s

Homework Equations


PV=Nmv^2

The Attempt at a Solution


I'm not really sure what the N is... the question doesn't give the amount of He gas, so how do i know how many atoms there are? and the same for m...
otherwise, i can find V from area given; velocity also given, then i can find pressure
 
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actually, for m, i can get from the molar mass and avogadro's number right?
but i still don't get what N is...
 
You don't need volume, gas density will be enough.

--
methods
 
higherme said:

Homework Statement



A square box contain He at 25C. If the atoms collide with wall perpendicularly (90 degrees) at rate of 4.0E22 times/s, calculate pressure and force exerted on wall given area of wall 100cm^2 and speed of atoms =600 m/s

Homework Equations


PV=Nmv^2

The Attempt at a Solution


I'm not really sure what the N is... the question doesn't give the amount of He gas, so how do i know how many atoms there are? and the same for m...
otherwise, i can find V from area given; velocity also given, then i can find pressure

You could go to first principles:
.What is the momentum of one molecule on hitting the wall?
.What is the momentum when it bounces off?
.What is the momentum change?(remember momentum is a vector)
.Force is rate of change of momentum i.e.the total momentum change in 1 second so what is the force?
 
Dadface said:
You could go to first principles:
.What is the momentum of one molecule on hitting the wall?
.What is the momentum when it bounces off?
.What is the momentum change?(remember momentum is a vector)
.Force is rate of change of momentum i.e.the total momentum change in 1 second so what is the force?


ok, so the momentum of 1 molecule hitting the wall is mv(mass x velocity)
when bounce off, the v is going to be in opposite direction so, change in momentum will be
mv - m(-v) = 2mv

so force = 2mv / time ? is this correct?
I have no idea what's next ...
 
So this is what i have:

F = change in momentum /time
F = 2mv /t

velocity is given in the question=600m/s , time is given = 4.0X10^22 times/s
m is I'm guessing the molar mass?
 
higherme said:
So this is what i have:

F = change in momentum /time
F = 2mv /t

velocity is given in the question=600m/s , time is given = 4.0X10^22 times/s
m is I'm guessing the molar mass?
Your equation gives the change in momentum of a single molecule in one second i.e. the force exerted by the single molecule.If there were N molecules the total force would be
given by F=N2mv/t=2mv*N/t where N/t=number of collisions per second(4.0*10^22)
 
Dadface said:
Your equation gives the change in momentum of a single molecule in one second i.e. the force exerted by the single molecule.If there were N molecules the total force would be
given by F=N2mv/t=2mv*N/t where N/t=number of collisions per second(4.0*10^22)


okay, that makes sense. But i still don't really get how to find the mass of He, which has to be in grams right...?
 
does the number of collision equals the number of molecules of He? If so, then i can get the # of moles from avogadro's number. Then with the moles, i can get grams of He using molar mass of He... can i do that?

m = 4.0X10^22 molecules * (1 mol/6.022E10^23 molecules) * (4.003 g/mol) He
=.266 g He
can someone double check.. thanks!
 
  • #10
m in the equation stands for the mass of a single helium atom which you can find from the mass of one mol of He divided by Avagadros number.To find F in N and P in N/m^2(Pa) you need to express m in Kg.
 

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