inaccensa

10+ Year Member
Sep 5, 2008
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Medical Student
A force is applied to a container of gas reducing its volume
by half. The temperature of the gas:
A. decreases.
B. increases.
C. remains constant.
D. The temperature change depends upon the amount of
force used.

I thought that I'm lets says applying force, ie- pressure, pressure is increasing and since the volume is decreasing, the temp will remain constant. The ans is increases.
 

ezsanche

10+ Year Member
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Aug 15, 2007
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If you increase the pressure the temperature should increase because the molecules are moving faster than when they were moving before.
 

musafirah

im so cereal right now
Jun 14, 2009
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Medical Student
the temperature increases because the internal energy increases, due to pV work done on the system. internal energy is proportional to temperature, and if you do work on a system you increase the temperature.
it says a force is applied. you can assume the force is a CONSTANT. thus pressure is constant, NOT increasing.

whenever a question says a force is applied to compress gas, e.g. a piston.. it means pV work. pV work is always constant pressure, changing volume. so the temp change depends only on if the work is being done ON the system, or BY the system. in this case work is done ON the system, and so is positive work, the energy increases.
 
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inaccensa

inaccensa

10+ Year Member
Sep 5, 2008
511
1
Status
Medical Student
so the change in volume has no effect?

W= PV ( change in V)
 

musafirah

im so cereal right now
Jun 14, 2009
307
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Medical Student
noo, i'm saying that the pressure is constant. you guys were saying that since volume decreased, pressure must increase. but pressure is given by the force applied.. it is a constant. VOLUME is the only thing that changed. the volume change is essentially the displacement over which the work is done. the increase in temperature is because energy is added to the system when work is done on it (the work is the change in volume times constant pressure)