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Thermal Physics > Explanations and Definitions > Flashcards

Explanations and Definitions Flashcards

1
Q

thermal equilibrium

A

If there is no net flow of thermal energy between two physical systems

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2
Q

zeroth law

A

allows us to determine if two systems are in thermal equilibrium

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3
Q

exact differential

A

path independent

i.e. dU

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4
Q

inexact differential

A

path-dependent and relate to irreversible processes

i.e. δQ and δW

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5
Q

isothermal process

A

Temperature does not change

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6
Q

isobaric process

A

Pressure does not change

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7
Q

Isochoric process

A

Volume does not change

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8
Q

the link between the first law and Bernoulli’s equation

A

they are identical if there is no work, heat and no change in internal energy

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9
Q

the significance of the 2nd law in modern physics

A

the second law breaks the symmetry in Newtonian mechanics, quantum mechanics, electrodynamics and relativity.

it gives the concept of an ‘arrow of time’

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10
Q

carnot cycle PV diagram

A

see notes for PV diagram

1 -> 2 isothermal heat absorption Q(in)
2 -> 3 adiabatic expansion
3 -> 4 isothermal heat loss Q(out)
4 -> 1 adiabatic compression

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11
Q

kelvin Planck form of the 2nd law

A

it is impossible to construct a device that operating in a cycle will produce no other effect other than the conversion of heat into work

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12
Q

Clausis form of the 2nd law

A

it is impossible to construct a device that operating in a cycle produces no other effect than the transfer of heat from a colder body to a hotter body

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13
Q

kelvin planck and clausis statement of the second law

A

are equivalent

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14
Q

carnot’s theorem

A

no engine operating between two thermal reservoirs can be more efficient than a reversible engine operating between reserviors

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15
Q

consequences of carnot’s theorem

A

The efficiency is independent of the working substance and internal workings of the engine and solely dependent on the reservoir temperatures.

Providing a means to define an absolute temperature scale

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16
Q

the corollary to carnot’s theorem

A

all Carnot engines operating between the same two temperature reservoirs have the same efficiency

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17
Q

the thermodynamic temperature scale

A

is the triple point of water

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18
Q

the operation of a heat pump

A

3 loops connected by 2 heat exchangers to take heat from the ground to heat up a house.

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19
Q

the significance of the clausius inequality

A

the purpose of the Clausius inequality is to introduce entropy

for a closed system, we always need to consider the entropy of the surroundings.

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20
Q

discuss the significance of entropy

A

the entropy increases for an irreversible process

the total entropy of the universe cannot decrease

local decreases in S are allowed

it is necessary to consider the surroundings for systems that are not thermally isolated

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21
Q

carnot cycle on a TS diagram

A

see notes

it is a square

adiabatic lines are vertical lines

and isothermals are horizontal lines

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22
Q

functions of state

A

U,S and V

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23
Q

conjugate variables

A

T and S

S is extensive and T is intensive

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24
Q

four common thermodynamic potentials

A

U,H,F and G

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25
Q

the use of the four common thermodynamic potentials

A

express a given derivative into a measurable quantity

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26
Q

the need for a third law of thermodynamics in terms of absolute values of entropy

A

what is the standard point s(0) and whether is it the same for an ideal case can be provided by the third law

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27
Q

van der waals forces give rise to

A

a potential which is repulsive at short distances, but attractive at longer distances

become pronounced at small volumes and negligible at large volumes.

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28
Q

define a throttling process

A

a process to regulate or restrict the flow of a fluid by using a porous plug.

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29
Q

inversion curve

A

connects the maxima of different isenthalps on a PT diagram where µ = 0

µ < 0 heating upon expansion

µ > 0 cooling upon expansion

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30
Q

the physical process of producing liquid nitrogen and other liquid gases

A

gases can be directly liquified by throttling.

31
Q

the operation of a domestic refrigerator

A

four stages

  1. throttling
  2. evaporation
  3. compression
  4. condensation
32
Q

sketch of a typical refrigerator cycle on a TS plot

A

see notes

33
Q

sketch of a Rankine cycle on a TS plot

A

see notes

34
Q

the operation of a Rankine cycle

A

state a: pump
state b: boiler
state e: turbine
state f: condenser

35
Q

steam engines utilise

A

the Rankine cycle

36
Q

gas turbines utilise

A

the Brayton cycle

37
Q

Brayton cycle TS plot

A

see notes

38
Q

Brayton cycle PV plot

A

see notes

39
Q

brayton cycle can be maximised for

A

the output of work W - W(c)

40
Q

rocket engine sketch

A

see notes

41
Q

define a first-order phase change

A

will have a discontinuous first derivative of G.

Measurement of the specific heat capacity will show a discontinuity at the phase transition temperature

42
Q

higher order phase changes

A

third-order phase transitions occur in ferromagnetics or superfluids.

43
Q

S(T)

A

∂S/∂T)(P) = c(P)/T > 0 and c(P) is always positive so the gradient on either side of the transition is positive.

Nothing can be said of the curvature

44
Q

G(T)

A

∂G/∂T)P = -S < 0 so the gradient is negative on either side of the transition

45
Q

∂^2G/∂T^2)P =

A

-∂S/∂T)P = -c(P)/T < 0 so the plots are curved downwards on either side

46
Q

sketch of S(T) and G(T)

A

see notes

47
Q

significance of F = -k(B)TlnZ

A

the equation links macroscopic classical thermodynamic variables to statistical mechanics quantities

48
Q

PV diagram guide

A

See diagram

49
Q

TS diagram guide

A

See diagram

50
Q

why is Gibbs free energy used to describe phase changes

A

at a phase transition, two phases co-exist at the transition temperature and the Gibbs free energy for each phase is the same.

G is preferred over F because it is appropriate to the more commonly-used isobaric conditions

51
Q

why is C(P) greater than C(V) for fluids

A

most materials expand upon heating, so C(P) includes work done by the material against the surrounding atmosphere.

52
Q

why can we usually only consider either C(P) or C(V) for solids

A

for solids, the expansion is relatively small and the difference between them can often be ignored

53
Q

Einstein model of specific heat capacity

A
  1. assumes a simple cubic lattice of N atoms with interatomic potentials modelled by springs, so that atoms can be modelled as simple harmonic in 3 dimensions
  2. assumed that each oscillator has a single frequency of oscillaton
  3. assumed that the solid can be treated as 3N independent oscillators
54
Q

extensive thermodynamic variable

A

Variables that scale proportionally system size such as total mass or volume

55
Q

throttling process

A

the rapid expansion of a fluid flowing through a restriction from a high-pressure region to a low pressure region, conducted under adiabatic conditions

56
Q

an experiment that uses the Joule-Kelvin coefficient to determine whether a gas is well-described by the ideal gas law

A

porous plug experiment

57
Q

porous plug experiment

A

throttling a high-pressure gas through a restriction in a pipe. The temperature drop across the restriction is measured. As the joule-kelvin coefficient is zero, there should be no temperature drop.

58
Q

adiabatic

A

ΔQ = 0, exchanges no heat with its surroundings

59
Q

blackbody

A

an object that perfectly absorbs all radiation that falls on it regardless of wavelength

Re radiates at energies characterised by a black body distribution

60
Q

modern-day use of the Brayton cycle

A

used in jet turbines, for propulsion

61
Q

why is the Debye model an improvement on the Einsten model

A
  1. allows a spectrum of excitation frequencies rather than the single frequency assumed by Einstein
  2. uses the concept of phonons, or coupled oscillations rather than the independent oscillations of the Einstein model.
  3. correctly predicts the low temperature T^3 dependence of heat capacity that is seen experimentally for simple solids.
62
Q

Equipartition of energy

A

the thermal energy of an atomic or molecular system will be uniformly distributed between each ‘degree of freedom’ each of which accounts for energy 1/2 k(B)T

63
Q

how does the equipartition of energy lead to differences in thermodynamic calculations for monatomic and diatomic gases

A

in monatomic systems, thermal energy is distributed between three degrees of freedom.

for diatomic molecules, there are also vibrational and rotational degrees of freedom to consider.

64
Q

heat exchanger

A

enables the transfer from a hot material to cold material without the two materials physically mixing.

65
Q

Heat pumps

A

operate similarly to refrigerators, taking some work to transfer heat from a cold body to a warmer body.

66
Q

how to determine temperature from infrared intensity

A

Stefan-Boltzmann law on the formula sheet alongside the spectral density given by the Planck distribution, produces a unique curve that is only a function of T.

67
Q

isentropic

A

where ΔS = 0

adiabatic process, where there is no transfer of heat ΔQ = 0.

It is also a reversible adiabatic process.

68
Q

the kelvin temperature scale

A

is defined with respect to 273.16K

which is the triple point of pure water

which specifies a unique pressure and temperature which can be easily replicated in a lab

the scaling ensures that 1K is equivalent to 1 degree C.

69
Q

The Joule-Kelvin process

A

a throttling process that is isenthalpic ΔH = 0. The gas is forced through a restriction or throttling valve in a lower-pressure region.

70
Q

heat engine diagram

A

see notes

warm body

engine -> w

cold body

71
Q

refrigerator diagram

A

see notes

warm body at T(1)

engine <- W

cold body at T(2)

72
Q

How do we determine the one-particle partition function

A

by considering the Schrodinger equation for a particle in a box, which produces quantised energy levels that derive from standing wave solutions with nodes at the box walls.

73
Q

Gibbs paradox is solved

A

if the particles are assumed to be
indistinguishable

74
Q

The Gibbs Paradox

A

S is expected to be an extensive property but is not extensive in the above result.

Thermal Physics (10 decks)

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