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Introduction

Physics 313Professor Lee

CarknerLecture 1

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What is Thermodynamics?

• Hot and cold and how they get that way

• Classical Thermodynamics– Temperature, heat and work can be

transformed into one another

• Statistical Physics– The behavior of particles is governed

by the laws of probability

Laws of Thermodynamics

• Zeroth Law– Temperature always means the same thing

• 1st Law– Energy is conserved

• 2nd Law– Randomness always increases

• 3rd Law– Randomness decreases with decreasing

temperature

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Thermal Equilibrium

• Two objects at different temperatures placed together will exchange heat until they are at the same temperature

• Zeroth Law:– If A and B are each in thermal

equilibrium with T then they also are in thermal equilibrium with each other

Temperature• For temperature it is best to use the Kelvin

scale:TK = TC + 273.15

TF = (9/5)TC + 32

• Temperature is a measure of the random motions of the molecules of a substance:

vrms = (3RT/M)½

Kave = (3/2)kT

Eint = nCV T

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Heat• Heat is energy transferred between two

substances as a result of a temperature difference

• The amount of heat needed to change temperature is:

Q = cm(Tf - Ti)

• The amount of heat needed to change phase is:

Q = Lm

Work• Work is the energy change associated

with expansion or contractionW = integral [p dV]

• Work only occurs with a change of volume– If the volume increases, work is done by

the system and the work is positive– If the volume decreases work is done on

the system and the work is negative

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Internal Energy

• Internal energy is a measure of the energy of the molecules of a substance and depends only on temperature:

Eint = nCV T

CV = (3/2)R

R = 8.31 J/mol K

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First Law of Thermodynamics

• The first law always holds in any situation:

Eint = Q - W

• Energy is conserved– You cannot get more out of a system

than you put into it

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Heat Transfer• Conduction

H = Q/t = A(TH-TC) / (L/k)

• RadiationPr = AT4

– Net power is power received minus power emitted

• Convection– Hot fluid rises in a gravitational field

moving heat with it

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Ideal Gas• Most gasses can be approximated as ideal

gasesM = mNA

NA = 6.02 X 1023 (Avogadro’s number)

• Ideal Gas Law:pV = nRT

• n (number of moles) and R (gas constant) do not change for any given amount of gas

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First Law and Ideal Gas

• Any ideal gas must obey:pV = nRT

Eint = Q - W Eint = n CV T

Q = n C TW = integral [p dV]

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Entropy

• High entropy means high randomness• Entropy always increases

S = Sf-Si = integral [dQ/T]

• For isothermal processes:S = Q/T

• First Law: output cannot exceed input• Second Law: output cannot equal

input

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Adiabatic• Definition

– No heat• pV, TV are constant

• Heat, Work and Internal EnergyQ = 0

W = -Eint = -n CV T

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Isochoric• Definition

– Constant volume

• Heat, Work and Internal EnergyQ = Eint = n CV T

W = 0

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Isothermal• Definition

– Constant temperature

• Heat, Work and Internal EnergyQ = W = nRT ln (Vf/Vi)

Eint = 0

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Isobaric• Definition

– Constant pressure

• Heat, Work and Internal EnergyQ = n Cp T

W = p VEint = n CV T