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Prelim paper question... - HSN forum

# Prelim paper question...

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### #1Pyrefly

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Posted 17 May 2006 - 07:48 AM

Right was looking through some of the questions we has in our second prelim and noticed one i still dont understand. Its from the multi choice so...i would end up guessing. So..er..please help

A fixed mass of an ideal gas has a volume V and an absolute temperature T. Its pressure is doubled by changing its volume and temperature to:
A V/2 and 4T
B V/4 and T/2
C 2V and T/4
D 2V and T/2
E 4V and 2T

### #2ice_illusion

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Posted 17 May 2006 - 08:53 AM

The equation is PV/T. I think it's easier just to go through them.

A would make it 8 times the pressure. (half the volume doubles, 4 times the temperature quadruples)
B would make it double.
C would make it 8 times less.
...

It's B becuase
Volume: 2V halves the pressure as the gas has more room, V/2 Doubles.
Temperature: 2T doubles the pressure as the particles speed up and hit the walls twice as often. T/2 lowers temperature lowering kinetic energy and pressure.

This is really hard to explain. You have to imagine those gas model pictures.

### #3Pyrefly

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Posted 17 May 2006 - 08:57 AM

Ya..im a little confused by your explanation. You did get the right answer

I decided to just use hypothetical values till i worked out what doubled P (and got the answer B)

Thanks alot

### #4ice_illusion

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Posted 17 May 2006 - 08:58 AM

That works too. Sometimes its the best way to answer those little thingies.
Good luck.

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