15.1 The First Law of Thermodynamics

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# 15.1 The First Law of Thermodynamics - PowerPoint PPT Presentation

15.1 The First Law of Thermodynamics A system’s internal energy can be changed by doing work or by the addition/removal of heat: Δ U = Q - W W is negative if work is done on the system Compression of the gas What is the state of the system? Described by P, V, T, m, U.

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Presentation Transcript
15.1 The First Law of Thermodynamics
• A system’s internal energy can be changed by doing work or by the addition/removal of heat:

ΔU = Q - W

• W is negative if work is done on the system
• Compression of the gas
• What is the state of the system?
• Described by P, V, T, m, U

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15.2 Thermodynamic Processes and the First Law
• Isothermal: T = constant → ΔU = 0 → W = Q
• Adiabatic: Q = 0 →ΔU = -W

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15.2 Thermodynamic Processes and the First Law
• If pressure is constant then

W = Fd = PAd = P ΔV

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15.2 Thermodynamic Processes and the First Law
• The total work done during a process is equal to the area under the PV diagram

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15.4 The Second Law of Thermodynamics
• Heat can flow spontaneously only from a hot object to a cold object.
• A reversible process is one that is always in equilibrium and can return to its initial conditions along the same path
• Most natural processes are irreversible
• Sets an upper limit on efficiency of heat engines

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15.5 Heat Engines
• Heat engines convert U into other useful forms of energy – mechanical, electrical, …

ΔUcycle = 0 → QH = W + QL

Automobile engines

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15.5 Heat Engines
• The efficiency of a heat engine is
• Carnot (ideal) engine
• Reversible processes
• Too slow for real engines

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(b) The change in internal energy is calculated from the first law of thermodynamics

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26. Find the exhaust temperature from the original Carnot efficiency, and then recalculate the intake temperature for the new Carnot efficiency, using the same exhaust temperature.

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