Question

when a gas is filled in a cylinder fitted with a movable piston is allowed to expand, workdone by gas is positive. explain.​

Answer 1

PHYSICS

An ideal gas is enclosed in a cylinder with a movable piston on top of it. The piston has a mass of 8000g and an area of 5.00cm

2

and is free to slide up and down, keeping the pressure of the gas constant. How much work is done on the gas as the temperature of 0.200mol of the gas is raised from 20.0

∘

C to 300

∘

C?

January 17, 2020avatar

Urmi Balla

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VIDEO EXPLANATION

ANSWER

The gas is expanding and doing positive quantity of work in lifting the piston. The work on the gas is this number of joules but negative.

The original internal energy of the gas is some multiple of nRT

i

=(0.2mol)(8.314J/molK)(293K)=10

3

J.

The work done by the gas does not come just from the internal energy, but from some of the energy that must be put into the sample by heating to raise its temperature. But still we can estimate the work as on the order of −1kJ.

The integral of PdV is easy because the pressure is constant. Then the equation of state will let us evaluate what we need about the pressure and the change in volume.

For constant pressure,

−∫

i

f

PdV=−PΔV=−P(V

f

−V

i

)

Rather than evaluating the pressure numerically from atmospheric pressure plus the pressure due to the weight of the piston, we can just use the ideal gas law to express in the volumes obtaining

W=−P(

P

nRT

h

−

P

nRT

c

)−nR(T

h

−T

c

)

Therefore, W=−nRΔT

=−(0.200mol)(8.314J/molK)(280K)

=−466J

We did not need to use the mass of the piston or its face area. The answer would be the same if the gas had to lift a heavier load over a smaller volume change or a lighter load through a larger volume change. The calculation turned out to be simple, but only because the pressure stayed constant during the expansion. Do not try to use −PΔV for work in any case other than constant pressure, and never try to use −VΔP.

January 17, 2020

Answer 2

Answer:

yasa

Explanation:

the work done by gas is positive because the piston pusshes the gas so negative so gas is positive lol!