Force exerted on walls of metal box

[Rubidium]

force exerted on walls of metal box...please help!

1. Homework Statement
A cubic metal box that has 20-cm-long edges contains air at a pressure of 1.0 atm and a temperature of 300 K. The box is sealed so that the enclosed volume remains constant, and it is heated to a temperature of 400 K. Find the force due to the internal air pressure on each wall of the box.




2. Homework Equations
PV=nRT




3. The Attempt at a Solution
F=[tex]\left|[/tex][tex]\Delta[/tex][tex]\vec{p}[/tex]/[tex]\Deltat[/tex][tex]\right|[/tex]
v=[tex]\sqrt{3RT/M}[/tex]=[tex]\sqrt{3(8.314 J/molxK)(300K)/0.029kg/mol)}[/tex]=507.957 m/s
Delta[/tex][tex]\vec{p}[/tex]/[tex]=mv^2=(9423.9987g)(507.957m/s)=4.787x10^6
I just need delta t

 

[Dick]

Forget about v. You don't need to count individual particles, PV=nRT will give you the pressure. Pressure is force per unit area.

 

[Rubidium]

How do I use all the information given in the problem to find that?

 

[Dick]

PV=nRT. How many variables in this equation change when the temperature is varied in your problem?

 

[Rubidium]

I think the pressure would change but the volume remains constant in the box. Is the pressure given in the problem negligible or should I be able to use it?

 

[Dick]

Only P and T change, right. But you are given the initial P is 1atm. What's the final P? You certainly shouldn't neglect the pressure change!

 

[Rubidium]

got it...thanks!

 

[MeKnos]

I'm having severe trouble on this one:

I know that P1(T1)/T2 = P2

so P2 x (length)^2 x (6 sides) = Internal Pressure Force.

But I'm not getting the right values. Even if I just do P2 x (Length^2) i don't get the right answer.

 

[Dick]

I'm having severe trouble on this one:

I know that P1(T1)/T2 = P2

so P2 x (length)^2 x (6 sides) = Internal Pressure Force.

But I'm not getting the right values. Even if I just do P2 x (Length^2) i don't get the right answer.

Show your whole solution. We can't really be expected to guess what you did wrong.

 

[MeKnos]

101300 (400) / 300 = P2 = 135093

135093 Pa x (.20^2) x 6 = 32422 N

and the other way 135093 Pa (.2^2) = 5403 N

neither worked

 

[Dick]

101300 (400) / 300 = P2 = 135093

135093 Pa x (.20^2) x 6 = 32422 N

and the other way 135093 Pa (.2^2) = 5403 N

neither worked

I don't know. I got 5403N as well. Significant figures issue??