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January 7 AP Physics

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IN A gas follows the cycles shown. How much work is done?

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Objective: To study cycles in thermodynamics. Success Criteria: To be able to calculate the work done in a thermodynamic cycle.

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Homework check 2, 5, 6, 10

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PV diagram

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PV diagram-holding pressure constant

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PV diagram-holding volume constant

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PV diagram-holding temperature constant

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Work and PV diagrams (quick quiz 12.1) Put these in order: most negative work to most positive work.

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First law of Thermodynamics ΔU = Q + W

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Isobaric (pressure constant) ΔU = Q + W

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Isovolumetric (volume constant) ΔU = Q

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Isothermal (temperature constant) ΔU = 0 Therefore Q = W

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Adiabatic (isolated system) ΔU = W

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Work in thermodynamic processes W=FΔd W=PAΔd W= - PΔV

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Processconditions first law Isobaricboth temp ΔU=Q+W and volume change Isovolumetricno workΔV=0 so W=0 so ΔU=Q Isothermalno change ΔU=Q+W=0 in T or U so Q= - W Adiabaticno energyQ=0 transfer asso ΔU=W heat

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PV diagrams and thermodynamic processes (quick quiz 12.2) Identify the paths

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PV diagram of a cycle

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Steam engine

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Newcome Steam engine

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Problem 4

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Otto cycle Otto Engine

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Diesel cycle Diesel Engine

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Efficiency of an engine Work out Energy in e = W Q H

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Sadi Carnot 1796-1832 1824-Reflections on the Motive Power of Fire Efficiency of a heat engine Theoretical maximum Began the understanding that became the second law of thermodynamics. –entropy

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Carnot cycle

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The Carnot Engine

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Carnot Engine No real engine operating between two energy reservoirs can be more efficient than a Carnot engine operating between the same two reservoirs. e c = T H -T C T H

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Three engines operate between reservoirs separated in temperature by 300 K. The reservoir temperatures are: a) T h =1000 K T c =700 K b) T h =800 KT c =500 K c) T h =600 KT c =300 K Rank the engines in order of their theoretical maximum efficiency.

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Second Law of Thermodynamics No heat engine operating in a cycle can absorb energy from a reservoir and use it entirely for the performance of an equal amount of work. You cant have a totally efficient engine. In any closed system, entropy always increases.

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Entropy

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Out: Which of these processes are isothermal, isovolumetric, or adiabatic: a)A tire being rapidly inflated b)A tire expanding gradually at a constant temperature c)A steel tank of gas being heated

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