# T4 - Phy sics 7B WS T4 rev 3.0 Page 1 T-4 The First Law of...

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Physics 7B WS T4 (rev. 3.0) Page 1 T-4. The First Law of Thermodynamics The First Law of Thermodynamics Δ E int = Q into gas – W by gas W 1 2,by gas = pdV V 1 V 2 " Questions for discussion In the following, you may assume that all of the processes described are reversible. 1 . When an ideal gas undergoes adiabatic expansion 1 , the temperature _____ goes up _____ goes down _____ stays the same _____ may do any of these. Explain your reasoning. Would your answer differ if the gas underwent adiabatic compression ? 2 . When an ideal gas undergoes isothermal compression, the internal energy of the gas _____ increases _____ decreases _____ stays the same _____ may do any of these. Explain your reasoning. Would your answer differ if the gas underwent isothermal expansion ? 1 Note: For a derivation of the adiabatic expansion formula, see the challenge problem at the end of this worksheet.

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Physics 7B WS T4 (rev. 3.0) Page 2 3 . When an ideal gas undergoes isothermal expansion, _____ heat flows into the gas _____ heat flows out of the gas _____ there is no heat flow in or out _____ any of these is possible. Explain your reasoning. Would your answer differ if the gas underwent isothermal compression ? 4 . If heat is added to a gas while the gas is held at constant volume, then the temperature of the gas _____ must increase _____ must decrease _____ must stay the same _____ may do any of these. Explain your reasoning. Suppose instead that the same amount of heat is added to the gas while the gas is held at constant pressure and the volume is allowed to vary. In this case, it turns out that the qualitative effect on the temperature is the same: namely, the temperature increases. But will it increase by the same amount? Explain. 5. When you let air out of a tire, the air feels cool. Explain. [Hint: air is a poor thermal conductor, so this process is approximately adiabatic.]
Physics 7B WS T4 (rev. 3.0) Page 3 Problems 1 . An ideal gas of N diatomic molecules ( γ = 1 + 2/5 = 7/5) undergoes three consecutive transformations, as diagrammed below. The transformation 1 2 is isobaric .

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