Physics 213: Lecture 3, Pg 1

Packet 3.4Packet 3.4

ThermodynamicsThermodynamics Internal Energy Internal Energy W = PW = PΔΔVV 11stst Law of Thermodynamics: Law of Thermodynamics: ΔΔU = Q – WU = Q – W Define: Adiabatic, isothermal, isobaric & Define: Adiabatic, isothermal, isobaric &

isochoric and show these on a P-V diagramisochoric and show these on a P-V diagram Irreversibility & disorderIrreversibility & disorder Entropy is a measure of disorderEntropy is a measure of disorder State 2State 2ndnd Law of Thermodynamics Law of Thermodynamics Heat engine efficiency, Heat engine efficiency, ηη = W/Q = W/Qhh

Carnot EngineCarnot Engine Energy DegradationEnergy Degradation

Physics 213: Lecture 3, Pg 2

Internal EnergyInternal Energy

Is the total potential and kinetic energy of the molecules in a substance. Potential energy is associated with intermolecular

forces.

Kinetic energy includes both translational and rotational motion.

When we consider an ideal gas, the intermolecular forces are assumed to be zero!

Internal energy of a gas comes only from the random kinetic energy of the atom of the gas.

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Ideal Gas & Internal EnergyIdeal Gas & Internal Energy

Ek = 1/2mv2 = 3/2kT so, U = 3/2 NkT

U =3/2nRT

Δ U = 3/2 nR Δ T

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Ideal Gas & Internal EnergyIdeal Gas & Internal Energy

What is the internal energy of 30 moles of oxygen gas at room temperature?

U =3/2nRT

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Ideal Gas & Internal EnergyIdeal Gas & Internal Energy

If the room were moving at a high velocity would that mean the internal energy of the gas would be greater?

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Work done on or by a gasWork done on or by a gas Imagine compressing a gas by exerting a force on the

piston from the outside. Consider heating the piston and it expands to perform

work.

W = F x D & F = PA

W = P (A x D)

W = P Δ V

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ExampleExample

A gas is compressed at constant pressure 2.00 x 105 Pa from a volume of 2.00 m3 to a volume of 0.500 m3. What is the work done on the gas. If the temperature initially was 40˚ C what is the

final temperature of the gas?

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Thermodynamic ProcessesThermodynamic Processes CLICK HERE – TO GO OVER EACH PROCESS!!

Isochoric Isobaric Isothermal

Const. Volume Const. Pressure Const. Temp

Remember! Area under curve is Work Done!

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Adiabatic ExpansionAdiabatic Expansion

Rapid expansion or compression of a gas.

No Heat (Q) can flow in or out of the system. ΔQ = 0

Any work done equals a direct change in internal energy. ΔU = ΔW

Bottle Rockets

Diesel Engines

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Figure 18-11Figure 18-11Adiabatic HeatingAdiabatic Heating

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-- Heat and work are forms of energy transfer and energy is conserved.

The First Law of Thermodynamics (FLT)The First Law of Thermodynamics (FLT)

U = Q - Wby

work doneby the system

change intotal internal energy

heat added

to system

or

U = Q + Won

State Function Process Functions

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"Process Problems"

For which process is W the largest? smallest?

For which process is Q the largest? smallest?

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Physics JokePhysics Joke

Once all the scientists die and go to heaven. They decide to play hide-n-seek and Einstein goes first. Einstein counts up to 100 and then start searching. Everyone starts hiding except Newton. Newton just draws a square of 1 meter and stands in it, right in front of Einstein. Einsteins counting ....97,98,99,100, opens his eyes and finds Newton standing in front. Einstein says "Newtons out, Newtons out." Newton denies and says I am not out. He claims that he is not Newton. All the scientists come out and he proves that he is not Newton. How??

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His proof:His proof:

Newton says: I am standing in a square of area 1m square.. That means I am Newton per meter square.. Hence I am Pascal. Since newton per meter square = Pascal

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Conceptual Checkpoint 18-2Conceptual Checkpoint 18-2Which is the adiabatic curve?Which is the adiabatic curve?

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The second law of thermodynamicsThe second law of thermodynamics

When objects of different temperatures are brought into thermal contact, the spontaneous flow of heat that results is always from the high temperature object to the low temperature object.

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Heat EnginesHeat Engines Energy goes in Useful Work taken out Some gets wasted

Max Efficiency:

TH – Tc/ Th= x 100

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The 2The 2ndnd Law of Thermodynamics Law of Thermodynamics

The second law of thermodynamics deals with the limitations imposed on heat engines: that is on devices whose aim is to covert heat (disordered energy) into mechanical energy (ordered energy).

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The 2The 2ndnd Law of Thermodynamics Law of Thermodynamics

The Entropy of an isolated system never decreases. It is impossible for heat to (spontaneously) flow from a cold

to a hot object. It is impossible for a heat engine working in a cycle to

absorb heat and perform an equal amount of work. The most efficient heat engine operating between two

given temperatures is the Carnot Engine.

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EntropyEntropy

Entropy like Internal energy is a State Function!

Entropy Is a measure of the disorder of a system.

ΔS = ΔQ/T If ΔQ > 0 entropy increases If ΔQ < 0 entropy decreases.

Physics 213: Lecture 3, Pg 21

PhilosophyPhilosophy

Three Laws of Thermodynamics (paraphrased): First Law: You can't get anything without working for it. Second Law: The most you can accomplish by work is to break even. Third Law: You can't break even.