Gases in a Cylinder

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ilovemcat

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Hey guys,

I have a few questions regarding those thermo problems with a gas in a cylinder doing work.

Can anyone clarify what the "P" term in w=-PdeltaV is? I'm assuming it's the external pressure you're working against. So for example, if the gas is doing work (increasing volume of container) at atmospheric pressure - the P represents 1 atm? Does that sound right?

But what about the value for a gas being compressed? Would the "external" pressure in this case be the pressure of the gas (since the surroundings is doing work on the container)? If so, wouldn't the pressure be constantly increasing - if that's the case ...what value would we use for pressure as a gas is being compressed?

I was told for the MCAT, pressure is normally constant (or assumed). Otherwise we'd have to use calculus to figure out the amount of work done. That's why I'm a little confused about the second situation.

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Remember in both compression and expansion the pressure inside and out equilibrate to reach a final volume, so they are the same.

You only need calculus in certain instances, like adiabatic processes IIRC. It's been a while since pchem, but isothermal processes mean pressure and volume are independent, thus you can just do an algebraic P delta V to find work.
 
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