TBR Physics Discrete #95 (from older book)

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cristiano

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How much work is done on a system when an ideal gas is expanded adiabatically from 0.4 liters to 2.4 liter under a constant pressure of 950.0 torr?


A. 5.00 liter.atmospheres
B. 2.50 liter.atmospheres
C, -2.50 liter'atmospheres
D. -5.00 liter.atmospheres


I understand the math part and can get 2.5 L.atm but I don't understand why the answer is C -2.50 L.atm

I thought that if the gas is expanding, then work done by the system is negative (since it's pushing out the piston or whatever), and therefore work done on the system is positive.

I've been having a hard time with this concept overall so if anybody can help me out it'd be great.

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How much work is done on a system when an ideal gas is expanded adiabatically from 0.4 liters to 2.4 liter under a constant pressure of 950.0 torr?


A. 5.00 liter.atmospheres
B. 2.50 liter.atmospheres
C, -2.50 liter'atmospheres
D. -5.00 liter.atmospheres


I understand the math part and can get 2.5 L.atm but I don't understand why the answer is C -2.50 L.atm

I thought that if the gas is expanding, then work done by the system is negative (since it's pushing out the piston or whatever), and therefore work done on the system is positive.

I've been having a hard time with this concept overall so if anybody can help me out it'd be great.

You got the bolded part wrong - an expanding system is doing positive work on the surroundings. Which is another way of saying that the work on the system is negative.

The way that I remember is that the work done by the system is PΔV. If the volume is increasing, ΔV is positive and the work is positive.
 
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