Learning Goal: To understand the first law of thermodynamics and its origin. By

Question

Learning Goal:

To understand the first law of thermodynamics and its origin.

By relating heat, internal energy, and work, the first law lays the groundwork for thermodynamics, a field that also explains the conversion of heat into mechanical energy.

Like the law of mechanical energy conservation that it generalizes, the first law of thermodynamics relates the changes in energy that occur from the beginning to the end of some process. The first law of thermodynamics involves changes in the following physical quantities:

W: work done by the system on the outside world,

Q: heat added to the system by the outside world, and

U: internal energy change of the system.

You need to look carefully at the wording used here; some other disciplines (such as chemistry) use a different sign convention for W.

Part A

Which of the following is the sign convention that results from these definitions?

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Correct

Part B

Using your knowledge of energy conservation, express Q in terms of U and W.

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Learning Goal:

To understand the first law of thermodynamics and its origin.

The first law of thermodynamics generalizes the concept of energy conservation to include heat energy. You probably already appreciate that loss of total mechanical energy (e.g., from nonconservative forces such as friction) does not destroy energy, but rather converts mechanical energy into heat. This process can be described quantitatively by the first law of thermodynamics.

By relating heat, internal energy, and work, the first law lays the groundwork for thermodynamics, a field that also explains the conversion of heat into mechanical energy.

Like the law of mechanical energy conservation that it generalizes, the first law of thermodynamics relates the changes in energy that occur from the beginning to the end of some process. The first law of thermodynamics involves changes in the following physical quantities:

W: work done by the system on the outside world,

Q: heat added to the system by the outside world, and

U: internal energy change of the system.

You need to look carefully at the wording used here; some other disciplines (such as chemistry) use a different sign convention for W.

Part A

Which of the following is the sign convention that results from these definitions?

W is positive when the system is compressed, and Q is positive when heat is added to the system. W is positive when the system expands, and Q is positive when heat is added to the system. W is positive when the system is compressed, and Q is positive when heat is taken from the system. W is positive when the system expands, and Q is positive when heat is taken from the system.

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Correct

Part B

Using your knowledge of energy conservation, express Q in terms of U and W.

Q =

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Explanation / Answer

Part A

W is positive when the system expands, and Q is positive when heat is added to the system.

Part B

dW = dQ - dU

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