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Reading Quiz (graded) Which of the following is NOT true of the work done on a gas as it goes from one point on a PV diagram to another? (a) It cannot be calculated without knowing n and T. (b) It depends on the path taken. (c) It equals minus the integral under the curve. (d) It has units of Joules. (e) It is one of the terms in the First Law of Thermodynamics.

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Work done by an expanding gas 1 m 3 of an ideal gas at 300 K supports a weight in a piston such that the pressure in the gas is 200,000 Pa (about 2 atm). The gas is heated up. It expands to 3 m 3. Plot the change on a graph of pressure vs. volume (a P-V diagram). How much work did the gas do as it expanded?

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Thought question (ungraded) A gas in a piston expands from point A to point B on the P-V plot, via either path 1 or path 2. Path 2 is a “combo path,” going down first, then over. The gas does the most work in: (a) path 1 (b) path 2 (c) same work

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Internal Energy E int (aka U) For ideal gas: E int = Sum of all of the microscopic kinetic energies. Equipartition Theorem: “The total kinetic energy of a system is shared equally among all of its independent parts, on the average, once the system has reached thermal equilibrium.”

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Thought question (ungraded) The process in which E int has the greatest magnitude is: (a) path 1 (b) path 2 (c) neither; it’s the same

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Isothermal Contours A gas changes its volume and pressure simultaneously to keep the temperature constant the whole time as it expands to twice the initial volume. What does this look like on a PV diagram?

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“First Law” E int = Q added + W on system What does that mean? You can add internal energy, by (a) adding heat (b) compressing the gas Possibly more intuitive version: Q added = E int + W by system When you add heat, it can either (a) increase internal energy (temperature) or (b) be used to do work (expand the gas)

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A monatomic gas (1.3 moles, 300K) expands from 0.1 m 3 to 0.2 m 3 in a constant pressure process.

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A diatomic gas (0.5 moles, 300K) has its pressure increased from 100,000 Pa to 200,000 Pa in a constant volume process.

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A diatomic gas (0.7 moles, 300K) gets compressed from 0.4 m 3 to 0.2 m 3 in a constant temperature process.

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