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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

H

L

H Q

Q

Q

We 1

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15.6 Refrigerators, Air Conditioners and Heat Pumps

A heat engine in reverse.W

QCOP L

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15.6 Refrigerators, Air Conditioners and Heat Pumps

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5 64186 J1400 kcal 6.262 10 J 5.2 10 J

1 kcalU Q W

2. (a) The work done by a gas at constant pressure is found from Eq. 15-3.

(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.