Question

A. A small, closed chamber of gas is heated. When the gas in the
chamber expands, it does 5 J of work on a piston. The gas has
an initial energy of 8 J and a final energy of 30 J. Considering
the equation for the first law of thermodynamics (AU = Q + W.
does the work done in this scenario have a positive or negative
value? Explain.
B. How much heat is added to the gas in the chamber?

Answer 1

A.

The work has a negative value.

Since the gas in the chamber expands, it increases in volume and does positive work on the piston since the change in volume is positive.

Work, W = pΔV. Since the gas expands, ΔV > 0. So, W > 0. Thus Work done by the gas is positive.

Since in ΔU = Q + W, W here is work done by the surroundings, W is negative since it is the opposite of the work done on the surroundings by the gas..

So, the work has a negative value.

B.

The heat added to the gas chamber is 17 J

From the first law of thermodynamics, ΔU = Q + W where ΔU = internal energy change = U₂ - U₁ where U₁ = initial energy = 8 J and U₂ = final energy = 30 J, Q = heat added to the gas chamber and W = work on piston = - 5 J

ΔU = Q + W

U₂ - U₁  = Q + W

30 J - 8 J = Q + (-5 J)

22 J = Q - 5 J

Q = 22 J + 5 J

Q = 27 J

The heat added to the gas chamber is 27 J.

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