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First law thermodynamics lab answers for AP Physics 2 (free PDF download)

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The method and the results are on the first law thermodynamics lab. Do the analysis yourself first, then check the answers below.

Quick answer: The first law of thermodynamics states ΔU = Q + W: the internal energy of a gas changes by the heat added plus the work done on it. These answers go with the AP Physics 2 lab on first law thermodynamics: the graph, the calculations, the uncertainties and the conclusion explained, for all 7 analysis questions.

First law thermodynamics lab answers for AP Physics 2 (free PDF download)

Here are the first law thermodynamics lab answers for AP Physics 2: graph, calculations, uncertainties and conclusion. First do the lab yourself, then compare, and go back to the lesson if needed.

The volume is proportional to the kelvin temperature, so the line of best fit passes close to the origin. The work done by the gas is of the order of one joule, which is only about 29% of the heat supplied; the other 71% raises the internal energy of the air.

Level: AP Physics 2 (algebra-based) · Curriculum: AP P2 Unit 9

First law thermodynamics lab answers: the analysis explained

Question 1 Constant pressure

The plunger moves freely, so the gas pressure always equals the atmospheric pressure outside. A change at constant pressure is an isobaric process.

Question 2 The graph

The points lie close to a straight line that rises as T increases. V is proportional to T (kelvin), so the line extrapolates to the origin.

Question 3 Gradient

Gradient = (58.4 − 50.0)/(343 − 293) = 8.4/50 = 0.168 mL/K.

Question 4 Absolute zero

V = 0 when T = 293 − 50.0/0.168 = 293 − 298 = −4.62 K, which is −278 °C and close to the accepted value of −273 °C.

Question 5 Amount of air

n = (1.01 × 10⁵ × 50.0 × 10⁻⁶)/(8.31 × 293) = 2.07 × 10⁻³ mol.

Question 6 Work done

Work by the gas = pΔV = 1.01 × 10⁵ × (58.4 − 50.0) × 10⁻⁶ = 0.848 J.

Question 7 Heat needed

Q = 3.5 × 2.07 × 10⁻³ × 8.31 × 50 = 3.02 J. Fraction = 0.848/3.02 = 0.28 (the theory gives 2/7 = 0.29).

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