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Physics Foundations > The Second Law of Thermodynamics > Flashcards

The Second Law of Thermodynamics Flashcards

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1
Q

What does the second law state?

A

That work and heat cannot be interchanged without some flow of energy from or to the surroundings.

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2
Q

What is the Kelvin-Planck statement?

A

It is impossible to build a heat engine which converts thermal energy into work with 100% efficiency.

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3
Q

What is the Clausius statement?

A

Heat will not flow spontaneously from a cold body to a hot one.

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4
Q

What is meant by a cyclic process?

A

Process in which the substance ends in the same state as it started.

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5
Q

Explain the general principle of heat engines.

A

QH from hot reservoir goes to engine which exhausts QC to cold reservoir and Work.

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6
Q

What is the equation for efficiency of a heat engine.

A

Efficiency = W/QH

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7
Q

What is the Carnot Cycle?

A

A hypothetical idealised heat engine constructed from reversible processes.

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8
Q

What are the 4 stages in the p-V diagram of the Carnot cycle?

A
  • 1 to 2 - Gas expands isothermally absorbing heat QH at temperature TH from hot reservoir
  • 2 to 3 - Gas expands adiabatically, cooling to TC
  • 3 to 4 - Gas is compressed isothermally, expelling heat to QC to cold reservoir at TC
  • 4 to 1 - Gas is compressed adiabatically, heating to TH (original state)
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9
Q

What is the work done, internal energy and heat for the isothermal expansion step of the Carnot Cycle?

A
  • W = nRTH * ln(V2/V1)
  • QH = W
  • ΔU = 0
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10
Q

What is the work done, internal energy and heat for the adiabatic expansion step of the Carnot Cycle?

A
  • W = nCv ΔT = -nCV(TC-TH)
  • Q = 0
  • ΔU = -W = nCv (TC - TH)
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11
Q

What is the work done, internal energy and heat for the isothermal compression step of the Carnot Cycle?

A
  • W = nRTC * ln(V4/V3)
  • Qc = W = nRTC * ln(V4/V3)
  • ΔU = 0
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12
Q

What is the work done, internal energy and heat for the adiabatic compression step of the Carnot Cycle?

A
  • W = -nCvΔT = -nCv(TH-TC)
  • Q = 0
  • = -W = nCv(TH-TC)
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13
Q

What is the work done, internal energy and heat for the entire Carnot Cycle?

A
  • W = W12 + W23 + W34 + W41
  • Q = nR(TH-TC)*ln(V2/V1)
  • ΔU = 0
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14
Q

How do you calculate the efficiency of a Carnot engine?

A
  • E = work done per cycle/heat input per cycle = W/QH
  • W = QH - QC
  • E = 1-(QC/QH) = 1- TC/TH
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15
Q

What is the Carnot Refrigerator?

A

Run the Carnot cycle in reverse. All completely opposite to Carnot cycle.

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16
Q

What is a heat pump?

A

Heating a hot source from a cold source by doing work.

17
Q

State Carnot’s Theorem.

A

The Carnot Cycle is the most efficient of all possible engines operating between the same two temperatures.

18
Q

How can you proove Carnot’s Theorem?

A
  • Assume an irreversible engine that is more efficient than the Carnot Cycle, connect it to a Carnot engine in reverse
  • Qcc-Qcn = Qhc-Qhn > 0 - this means there is a transfer of heat with no work - not possible
19
Q

State the third law of thermodynamics.

A

It is impossible to reduce the temperature of a body to absolute zero in a finite number of steps.

20
Q

What is entropy?

A

The state variable associated with the order in a system. It is a quantitative measure of disorder.

21
Q

What is the equation for entropy S?

A

ΔS = integral between 1 and 2 of dQrev/T

22
Q

What is the entropy change for a reversible cyclic process?

A

ΔS = 0

23
Q

What are the units of entropy?

A

JK^-1

24
Q

What is the entropy for a reverisible process of the system and the surroundings?

A

ΔSsys = integral of dQ/T = ΔSsurr

25
Q

What is the total entropy of a system and its surroundings?

A

ΔStot = ΔSsys + ΔSsurr = 0

26
Q

What is the entropy for an irreversible process?

A

ΔS >= integral between 1 and 2 of dQ/T

27
Q

How do you work out the entropy change when an ideal gas expands from V to 2V, without any work or temperature change?

A

Consider a reversible process between the two states - isothermal expansion. W = nRT*ln(2)
Q = W
ΔS = nRln(2)

28
Q

What is the equation of the entropy of a macroscopic state?

A

S = kB ln(omega), where omega is the number of microstates

Physics Foundations (10 decks)

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