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Stuck with physics gas questions

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WoAiMeiMei

180 posts since Jul '07
- 09 Apr `08, 8:07PM
  
  Completed:
  C19, P13 *after long confusion*
  C19, P38
  C19, P43
  
  for P38 i i think you use Vavg and Vrms equations and rearrange to get m1/m2 ratio..... but im not quite sure how.
  
  ahh!
  
  Edited by WoAiMeiMei 11 Apr `08, 7:09AM
newcomer

ALL YOUR BASES ARE BELONG TO ME

6,573 posts since Apr '05
- 09 Apr `08, 8:18PM
  
  damn i wish i listened to class on gases.
Moderator

eagle

16,361 posts since Aug '01
- 09 Apr `08, 10:09PM
  
  i try out tmr night... got 1 jap oral test and 1 eng prof presentation tmr...
WoAiMeiMei

180 posts since Jul '07
- 09 Apr `08, 10:16PM
  
  cool.. these questions are giving me the sh-
  
  >.<
Moderator

^tamago^

我又郁闷了...

49,423 posts since Sep '03
- 10 Apr `08, 1:11PM
  
  V₁ = 0.20m³
  P₂ = 101.3 kPa
  P₁ = 150 kPa + P₂
  = 261.3 kPa
  
  Since this is an isothermal process, ΔT = 0
  Δ(PV) = ΔQ = nRΔT = 0
  P₁V₁ = P₂V₂
  V₂ = V₁P₁/P₂ = 0.20m³ × 261.3/101.3 = 0.516m³
  W₁ = Work done by air = nRT ln(V_2/V₁) = P₁V₁ ln(V₂/V₁) = 0.20m³ × 261.3 kPa × ln(0.516m³/0.2m³) = 49.5kJ
  
  During the cooling process, pressure is constant.
  W₂ = Work done by air = p(V₁ - V₂) = 101.3kPa × (0.20m³ - 0.516m³) = -32.0kJ
  
  ∴ Total work done by air = W₁ + W₂ = 49.5kJ - 32.0kJ = 17.5kJ (3 s.f.)
  
  Edited by ^tamago^ 10 Apr `08, 5:57PM
WoAiMeiMei

180 posts since Jul '07
- 10 Apr `08, 5:19PM
  
  i cant get the right answer with 17509 Joules... argh!
weewee

398 posts since Aug '03
- 10 Apr `08, 5:27PM
  
  I dunno the formula but maybe tamago should use nRT In(V2-V1) instead of nRT ln(V2/V1).
WoAiMeiMei

180 posts since Jul '07
- 10 Apr `08, 5:42PM
  
  -
  
  omg! wait i go recalc...
  
  you applied the pressures to the volume calcs..
  
  CRAP!
  
  i got 17.5206 kJ
  
  .. still wrong
  
  ahhh!!! soo angry at this stupid work
  
  Edited by WoAiMeiMei 10 Apr `08, 5:48PM
Moderator

^tamago^

我又郁闷了...

49,423 posts since Sep '03
- 10 Apr `08, 5:54PM
  
  ....
  
  Edited by ^tamago^ 10 Apr `08, 5:56PM

WoAiMeiMei

180 posts since Jul '07

10 Apr `08, 5:56PM

                        V1 = 200L

                        P2 = 101.3 kPa

                        P1 = 261.3 kPa

                        V2 = 516L

                        

                        W1 by air= nRT ln(V2/V1) = P1V1 ln(V2/V1) = 261.3 x 200 x ln (516/200)= 49531J

                        W2 on air = p(V2 - V1) = 101.3 × (516-200)) = 32010.8

                        

                        Total Work = W1 - W2 = 49513 - 32010.8

                        = 17502.2J

                        

                        is still wrong.. TT

WoAiMeiMei

180 posts since Jul '07
- 10 Apr `08, 6:15PM
  
  Originally posted by ^tamago^:
  
  ....
  
  editted.. just saw sth, be back soon
  
  Edited by WoAiMeiMei 10 Apr `08, 6:17PM
Moderator

^tamago^

我又郁闷了...

49,423 posts since Sep '03
- 10 Apr `08, 6:17PM
  
  yeah positive. what's ur model answer?
WoAiMeiMei

180 posts since Jul '07
- 10 Apr `08, 6:21PM
  
  heh... guess what my friend sent me
  
  let me go through it first
  
  yes!
  
  it's correct >_<
  
  ans = 15664.277 J
  
  Edited by WoAiMeiMei 10 Apr `08, 6:25PM
Moderator

^tamago^

我又郁闷了...

49,423 posts since Sep '03
- 10 Apr `08, 6:21PM
  
  Originally posted by weewee:
  
  I dunno the formula but maybe tamago should use nRT In(V2-V1) instead of nRT ln(V2/V1).
  
  nope.... cos u intergrate 1/V dV you will get ln v_f - ln v_i = ln(v_f/v_i).
Moderator

^tamago^

我又郁闷了...

49,423 posts since Sep '03
- 10 Apr `08, 6:24PM
  
  the variables look different. hmmm.
WoAiMeiMei

180 posts since Jul '07
- 10 Apr `08, 6:28PM
  
  yeah, the variables i use are randomly generated specially for me! yay !
  
  the one in the solution is the one given in the original textbook question >_<
weewee

398 posts since Aug '03
- 10 Apr `08, 7:27PM
  
  Originally posted by ^tamago^:
  
  nope.... cos u intergrate 1/V dV you will get ln v_f - ln v_i = ln(v_f/v_i).
  
  Yeah as i state, i dunno the formula. TS's formula is wrong in #1 post.
  
  Chapter 19, P13, i think you need to find the volume using the P1V1 = P2V2... then use the W = nRT ln (Vf - Vi)
  which is also W = PV lv (Vf - Vi)
Moderator

^tamago^

我又郁闷了...

49,423 posts since Sep '03
- 10 Apr `08, 7:34PM
  
  Originally posted by weewee:
  
  Yeah as i state, i dunno the formula. TS's formula is wrong in #1 post.
  
  orh. paiseh. ><
weewee

398 posts since Aug '03
- 10 Apr `08, 7:39PM
  
  I am so ashamed to say i have never heard of this formula. Is this A level physics?
Moderator

eagle

16,361 posts since Aug '01
- 10 Apr `08, 8:24PM
  
  Originally posted by weewee:
  
  I am so ashamed to say i have never heard of this formula. Is this A level physics?
  
  Uni Year 1 engineering physics...
  
  btw, are all the questions solved already?
  
  Edited by eagle 10 Apr `08, 8:24PM
WoAiMeiMei

180 posts since Jul '07
- 10 Apr `08, 8:24PM
  
  it's umm...
  
  ... kinda >_<
  
  the other questions are soooo argh
Moderator

eagle

16,361 posts since Aug '01
- 10 Apr `08, 8:27PM
  
  I think I forgot all the formulas >_<
Moderator

eagle

16,361 posts since Aug '01
- 10 Apr `08, 8:37PM
  
  C19 P38
  
  temperature the same, kinetic energy of molecules should be the same
  
  thus, (0.5)(m1)(v1) = (0.5)(m2)(v2)
  
  but v2 = 2v1,
  
  m1/m2 = v2/v1 = 2
  
  Not confirmed though....
WoAiMeiMei

180 posts since Jul '07
- 10 Apr `08, 8:39PM
  
  nah 2 isnt the answer... there must be a reason why 5p is there
WoAiMeiMei

180 posts since Jul '07
- 10 Apr `08, 8:41PM
  
  looking at some formula...
  
  p = (nMv^2)/3V , with v = root mean square speed
  
  v(rms) = Sq root of (3RT/M)
  
  V(avg) is top line of :
  
  Edited by WoAiMeiMei 10 Apr `08, 8:43PM

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