When the first law of thermodynamics (ΔU = Q + W) is applied to a system S, the variables Q and W stand for

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BlackTom AI · Accepted Answer

When applying the first law of thermodynamics, ΔU = Q + W, the symbols Q and W denote specific energy transfers with respect to the system S.

First, consider Q: this represents heat transferred between the system and its surroundings. A positive Q indicates heat flowing into the system, raising its internal energy, while a negative Q would indicate heat leaving the system. In this equation form, Q is consistently the heat flow into S.

Next, consider W: this term accounts for mechanical work performed on the system. A positive W means work is done on the system by the surroundings, which tends to increase the system's internal energy, whereas negative W would indicate the system performing work on the surroundings, which would decrease its internal energy. In the given expression, W specifically stands for the work done on S.

Together, ΔU = Q + W expresses that changes in the system's internal energy come from both heat added to the system and work done on the system, using the convention that positive Q is into the system and positive W is work done on the system. If one encounters a different sign convention (for example, W representing work done by the system), then the equation would be written as ΔU = Q − W, but that is not the form given in this question.

Thus, the statement that best matches the provided equation is that Q is the heat flow into S and W is the work done on S, by the convention used in ΔU = Q + W.

SPHY001 Practice Quiz - Unit 15 | LA055

When the first law of thermodynamics (ΔU = Q + W) is applied to a system S, the variables Q and W stand for

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