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Theoretical physics year 1 > Thermal Physics > Flashcards

Thermal Physics Flashcards

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1
Q

Name 5 ways of measuring temperature

A
  • Change in volume of a gas or liquid
  • Change in pressure of a quantity of gas
  • Change in resistance of a conducting wire
  • IR detection
  • Bimetallic strip
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2
Q

Qualitatively define thermal equilibrium

A

When two substances in contact with each other exchange no heat energy (they are said to be at the same temperature)

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3
Q

Define insulator when applied to thermodynamics

A

A material that causes two objects to transfer heat energy more slowly

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4
Q

State the zeroth law of thermodynamics

A

If C is in equilibrium with B and A then A is in thermal equilibrium with B

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5
Q

What are the freezing and boiling points of pure water in the Fahrenheit scale?

A
  • Freezing = 32ºF
  • Boiling = 212ºF
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6
Q

What is the equation converting Celsius to Fahrenheit?

A
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7
Q

What is the chosen reference temperature for the kelvin scale?

A

The triple point of water

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8
Q

Define the triple point of water

A

A combination of pressure and temperature at which water can exist as a vapour, liquid and solid

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9
Q

What equation equates temperature and pressure of two points on the kelvin scale?

A
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10
Q

At what value of pressure and temperature does the triple point of water occur?

A

610Pa, 0.01 Degrees centigrade

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11
Q

What effects stop us from saying that all motion stops at zero degrees kelvin?

A

Quantum effects

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12
Q

What is the equation for linear expansion?

A
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13
Q

What does ‘alpha’ stand for in linear expansion and what is its unit?

A

Coefficient of linear expansion (K^-1)

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14
Q

Why is the linear thermal expansion equation only approximately correct?

A

Alpha varies with initial temperature and temperature interval

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15
Q

What is the equation for volume expansion?

A
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16
Q

What is beta in the volume expansion equation and what are its limits?

A

The coefficient of volume expansion (K-1)

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17
Q

When is the volume expansion equation valid?

A

Small changes in temperature

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18
Q

What is the relationship between linear and volume expansion coefficients?

A
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19
Q

Between what temperatures in °C does water contract?

A

0 → 4°C

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20
Q

Define thermal stress

A

Stress on a body or structure due to inequalities of temperature

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21
Q

What is the equation for thermal stress?

A
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22
Q

What is the equation for young’s modulus?

A

*add photo*

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23
Q

Define heat

A

Energy transfer due to a difference in temperature

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24
Q

What can cause specific heat (C) to change?

A

Changes to initial temperature and temperature change

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25
Q

Define molar mass

A

Mass of one mole

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26
Q

What is the equation for molar heat capacity?

A
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27
Q

What happens to work done by system if volume is constant?

A

No work is done

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28
Q

What is another name for measuring heat?

A

Calorimetry

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29
Q

A phase change takes place at a specific temperature if what is true?

A

Constant pressure

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30
Q

Define the ‘heat of fusion’

A

Heat required per unit mass for the change between solid and liquid (Lf)

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31
Q

What is the equation for a phase change?

A
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32
Q

What is the heat of vapourisaiton?

A

Heat required per unit mass for the phase change between liquid and gas (Lv)

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33
Q

Define sublimation

A

A substance changing from a solid directly into a gas

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34
Q

Define supercooled

A

Very pure water can be cooled several degrees below its freezing point without freezing

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35
Q

Define heat of combustion

A

Lc for complete combustion of one gram of a substance

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36
Q

Mathematicaly what is heat currend defined as?

A

Heat energy that flows past a point per unit time

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37
Q

What is the equation for heat current with respect to heat gradient?

A
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38
Q

Define thermal resistance

A

(R) Resistance of a material to the flow of heat

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39
Q

Many thermal insulators consist mainly of what?

A

dead air (trapped stationary air)

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40
Q

How are the heat currents in two different materials related when they are placed together?

A

Heat current must be the same in both

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41
Q

Define convection

A

Heat transfer due to motion of mass of a fluid form one region of space to another

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42
Q

What is factors effect heat current in natural convection and how?

A
  • Surface area of object emitting heat into the fluid directly proportional to heat current
  • Viscosity slows heat natural convection near a stationary surface
  • Temperature difference to the 5/4 is approximately proportional to heat current
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43
Q

Define radiation in terms of thermodynamics

A

Transfer of heat by electromagnetic waves

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44
Q

What is the equation for heat current of radiation

A
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45
Q

What is the equation of rate of radiation

A
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46
Q

What must a good emitter also be?

A

A good absorber

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47
Q

What is a perfect emitter/absorber?

A

Black body

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48
Q

How does the global warming work in terms of thermal output of the earth?

A

Most radiation given out by earth is infared which CO2 molecules absorb and re-emit, much of which is back at earth, this heats the earth and increases its rate of radiating energy

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49
Q

How does the van de walls equation improve upon the ideal gas equation?

A

Makes approximate corrections for omissions of dimensions of molecules and attractive forces between molecules in the ideal gas assumptions

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50
Q

What is an isotherm?

A

A curve on a P/V graph representing the behaviour at a specific temperature

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51
Q

What does the area under a P/V curve represent?

A

Work done during a volume change

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52
Q

For r<r>0 what happens to the force between molecules?</r>

A

The force is repulsive

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53
Q

What is r0 in terms of intermolecular forces?

A

The radius from a molecule past which the force is attractive (force at r0 is zero)

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54
Q

What happens to potential energy at r0?

A

It is at its lowest point

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55
Q

Define potential well

A

A region in a field of force that has a greater potential immediately either side of it (a particle will stay there unless given a energy)

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56
Q

What unit of mass does multiplying Avogadro’s number by molar mass give?

A

Grams

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57
Q

Derive the equation for the kinetic molecular modle of gas

A
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58
Q

What is the kinetic molecular modle of gas?

A

A modle representing gas as a large number of point particles bouncing around a box

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59
Q

What are the assumptions for the kinetic molecular modle of gas?

A
  • The container volume (V) contains a large number (N) of identicle particles each of mass (m)
  • The molecules behave as point particles (small compared to the size of the container and the average distance between molecules)
  • Molecules are in constant motion and occasionally collide with the walls of the container with a perfectly elastic collision
  • The container walls are rigid and infinitely massive and so do not move
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60
Q

What constants does the boltzmann constant consist of and in what mathematical relation?

A

R/NA

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61
Q

What is the equation for average speed of a molecule in a gas?

A
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62
Q

What is the equation for mean free path of a gas molecule?

A
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63
Q

Define ‘mean free path’ of a gas molecule

A

Average distance traveled between collisions

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64
Q

What is the equation for molar heat capacty at a constant volume for a monoatomic gas?

A
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65
Q

Why does molar heat capacity only work for monoatiomic gases?

A

Multiatomic gases have additional rotaional energy and vibrational as bonds expand and contract

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66
Q

Which has larger heat capacities in general? Monoatomic or polyatomic gases?

A

Polyatomic

67
Q

What is equipartition of energy?

A

Velocity of each component of a gas has on average an associated kinetic energy per molecule of 1/2KT

68
Q

Define degrees of freedom

A

The number of velocity components a gas can have

69
Q

Describe the motion of an H2 molecule under 50k

A
  • Molecule undergoes translation
  • No rotation
  • No vibration
70
Q

How many degrees of freedom does a monoatomic gas have?

A

3: Vx, Vy, Vz

71
Q

How many degrees of freedom do diatomic gasses have?

A

5: Vx, Vy, Vz and two axes of rotation

72
Q

What is the equation for molar heat capacity at constant temperature of a diatomic gas?

A
73
Q

Describe the motion of a H2 molecule between 50k and 600k

A
  • Translational motion
  • Appreciable rotational motion
  • No vibration
74
Q

Describe the motion of an H2 molecule above 600k

A
  • Translational motion
  • Rotational motion
  • Vibation
75
Q

How many degrees of freedom does a metal atom in a solid have?

A

3: Vx, Vy, Vz

76
Q

What energies does a atom with heat energy have in a solid?

A
  • Kinetic
  • Potential
77
Q

What is the equation for molecular heat capacity of a cyrstal?

A
78
Q

What is the maxwell boltzmann distribution?

A

The actual distribution of molecular speeds in a substance

79
Q

State the maxwell boltzmann equation

A
80
Q

Define phase equilibrium

A

A combination of pressure and temperatre at which a substance can be in an equilibrium of change between multiple phases

81
Q

Define fusion curve

A

A line on the pressure against temperature graph at which a substance can exist as a liquid and solid in equilibrium

82
Q

Define vapourisation curve

A

A line on the graph of pressure against temperature at which a substance can exist as a liquid and gas in equilibrium

83
Q

Define critical point

A

A pressure above which phase boundaries dissapear

84
Q

Define ‘thermodynamic system’

A

A collection of objects that can be viewed as one unit and has potential to exchange energy with its surroundings

85
Q

Define ‘thermodynamic process’

A

When a change occours in a thermodynamic system

86
Q

What integral gives work as a function of volume change?

A
87
Q

What equation gives work done as a volume change at constant pressure?

A
88
Q

Can you work out work done by a system based purely on initial states?

A

No, you must take into account the path taken

89
Q

Can you work out heat added to a system based purely on the start and finish states of a thermodynamic system?

A

No, path must be taken into account

90
Q

Define ‘free expansion’

A

Uncontrolled expansion of gas into a vacuum

91
Q

Define internal energy

A

The sum of kinetic energies of all constituent particles plus the sum of all potential energies between constituent particles

92
Q

What equation is the first law of thermodynamics?

A
93
Q

Can the change in internal energy of a system be calculated purely in terms of the start and finish points?

A

Yes, the first law of thermodynamics tells us that the change in internal energy of a system depends only on the initial and final states of a system

94
Q

Define ‘cyclic process’

A

A process that eventually returns a system to its initial state

95
Q

Define ‘isolated system’

A

A system that does no work to surroundings and has no heat flow to or from surroundings

96
Q

What is the differential form of the first law of thermodynamics?

A
97
Q

Define the ‘adiabatic’ thermodynamic process

A

A thermodynamic procss in which the heat transfer is zero

98
Q

Define an ‘isochronic’ thermodynamic process

A

A thermodynamic process in which volume is constant therfore non work is done by the system

99
Q

Define an ‘isobaric’ thermodynamic process

A

A thermodynamic process in which pressure remains constant

100
Q

Define an ‘isothermal’ thermodynamic process

A

A thermodynamic process at constant temperature (any heat flow in or out of the system must occour slowly enough that thermal equilibrium is maintained)

101
Q

Internal energy of an ideal gas depends only on what?

A

Temperature

102
Q

What do Cv and Cp mean?

A
  • Cv represents molar heat capacity at constant volume
  • Cp represents molar heat capacity at constant pressure
103
Q

If neither P nor V are constant how many possible values are there for molar heat capacity?

A

Infinite

104
Q

Which process requires more heat input? isochonic or isobaric?

A

isobaric requires more heat input than isochonic (except in rare situations where a substance decreases in volume as it is heated)

105
Q

How do Cp and Cv relate for an ideal gas?

A
106
Q

What is the equation for performance constant (K) of a heat engine?

A
107
Q

What does gamma represent?

A

Cp/Cv

108
Q

Under what conditions is a process approximately adiabatic?

A

If a system is well insulate or the process takes place so quickly that there is minimal heat transfer in that time

109
Q

Which has a steeper gradient? An adiabatic curve or an isotherm both starting at the same point?

A

Adiabatic curve is always steeper

110
Q

What string of equations relate V, P and T?

A
111
Q

What type of process is always very close to being in thermodynamic equilibrium with it’s self and surroundings?

A

A reversible process

112
Q

What relation does a reversable process have to an equilibrium process?

A

Reversable processes are a type of equilibrium processes and are therfore not precisely acheivable in the real world

113
Q

What does conservation of mechanical energy into heat reqire an increase in?

A

Randomness/disorder

114
Q

Define ‘heat engine’

A

A device that transforms heat partially into work or mechanical energy

115
Q

Define ‘working substance’

A

Matter inside a heat engine that undergoes inflow and outflow of heat, expansion and compression and sometimes change of phase

116
Q

Describe the general process that all heat engines go through

A
  • Absorb heat from a high temperature source
  • Perform mechanical work
  • Discard some heat at a lower temperature
117
Q

How are net work and net heat flow linked for a heat engine?

A

Q = W

118
Q

Defien thermal efficiency

A

The ratio of work done to heat absorbed in one cycle

119
Q

What is the general equation for thermal efficiency?

A
120
Q

List the operations of a gasoline engine in chronological order starting at the start of a cycle

A
  1. Intake stroke- Mix of air and gasoline into cylinder as piston decends increasing the volume from V to rV (where r is the compression ratio)
  2. Compression stroke - Valve closes and mixture is compressed adiabaticaly to roughly V
  3. Power stroke - Mixture ignited by spark plug, heated gas expands adiabaticaly back to rV pushing the piston
  4. Exaust stroke - Exaust valve opens and combustion products are pushed out
121
Q

Chronologically list the steps of the otto cycle from the first step

A
  1. Compression stroke - adiabatic compression
  2. Heating at constant volume
  3. Power stroke - adiabatic expansion
  4. Cooling at constant pressure
122
Q

What is the equation for efficiency of an otto cycle?

A
123
Q

How can efficiency be increased for the otto cycle?

A

Compression ratio r increased, this increases temperature which can cause an explosion during the compression stroke

124
Q

What is a typical thermal efficiency for the otto cycle?

A

35%

125
Q

What are the main differences between the diesel and gasoline cycles?

A
  • No fuel in cylinder at start of compression stroke (diesel cycle)
  • A little fuel before the power stroke (diesel cycle)
  • Fuel is injected directly into the cylinder and keeps pressure approximately constant throughout the first part of the power stroke (diesel cycle)
  • Due to the high pressure during the adibatic compression fuel ignites spontaneously (diesel cycle)
126
Q

What is a refrigerator?

A

A heat engine with a net input of mechanical work

127
Q

What properties do ‘cooling coils’ have?

A

Low temperature, Low pressure, Liquid and vapour in a phase equilibrium

128
Q

What properties do ‘condenser coils’ have?

A

High temperature (greater than that of air outside) , High pressure, Liquid and vapour in a phase equilibrium

129
Q

What does an ‘evapourator’ do?

A

Expands fluid adiabaticaly at a rate dictated by the expansion valve. Fluid given out is less dense than before and cooler than surrounding air

130
Q

What does a ‘compressor’ do?

A

Takes fluid and adiabaticaly compresses it before delivering it into the compressor coil in high pressure and higher temperature than outside air

131
Q

Do either the compressor or the expander do work on the fluid?

A

The compressor does work on the fluid but the expander does not

132
Q

State the second law of thermodynamics

A

It is impossible to have a system undergo a process that takes heat from a hot reservouir of a single temperature and convert it completely into mechanical work with the system ending in the same state as it began

133
Q

Generally what is the ‘carnot cycle’?

A

A hypothetical idealised heat engine with maximum efficiency. To acheive this it has to avoid ‘irreversible processes’ and so every porcess in it is adiabatic or isothermal, thermal and mechnical equilibrium must be maintained at all times

134
Q

List from the first step in chronological order the steps of the carnot cycle

A
  • Gas expands isothermally at temperature TH, absorbing heat QH
  • Adiabatic expansion occurs until temperature drops to TC
  • Isothermal compression, rejects heat |QC|
  • Adiabatic compression back to initial state at temperature TH
135
Q

State the eqution giving the efficiency of the carnot cycle

A

*add photo*

136
Q

What is the carnot refrigerator?

A

The carnot cycle in reverse

137
Q

State the equation giving the coefficient of perfomance (K) for the carnot refrigerator

A
138
Q

What is the heat engine?

A

The carnot cycle

139
Q

The ratio of heat absorbed and rejected in the carnot cycle is equal to what?

A

Ratio of the temperatures of the reservoirs (kelvin)

140
Q

Define entropy

A

A quantitative measure of disorder

141
Q

What is the equation for entropy?

A
142
Q

In a reversable process the entropy change of the universe equals what?

A

0

143
Q

What is the enthalpy change of one cycle of a carnot cycle?

A

0

144
Q

Is entropy change of a system dependent on path?

A

No

145
Q

The entropy change of the universe is what?

A

> 0

146
Q

For all individual systems taking part in a whole system the entropy does what?

A

Either increases or remains constant

147
Q

There can be many microscopic states corresponding to one macroscopic state, which microscopic state is the most likely?

A

The state with the greatest entropy

148
Q

What equation describes the microscopic expression of entropy?

A
149
Q

How does the second law of thermodynamics effect the change of microstates?

A

The number of microstates in a system cannot spontaneously decrease

150
Q

Young’s modulus is tensile stress over tensile strain, what is the modulus used for compressive stress and strain?

A

Young’s modulus

151
Q

What materials have are strong under compression but weak under tension?

A

Concrete or stone

152
Q

How can you minimise stress on a beam?

A
  • Top and bottom of beam are given a large cross sectional area
  • center line has a small cross section as neither compression or tension occur there (this reduces weight of the beam and reduces stress)
153
Q

If an object is immersed in a fluid what direction is the pressure to the surface?

A

Perpndicular

154
Q

What is the equation for bulk modulus?

A
155
Q

What is compressability?

A

The reciprocal of the bulk modulus

156
Q

What is the equation for shear stress?

A
157
Q

What is the equation for shear strain?

A
158
Q

Define ‘elastic hysterisis’

A

When a material follows different stress strain curves for increasing and decreasing stress

159
Q

What causes hysterisis to happen?

A

Non-conservative forces associated with internal friction

160
Q

Define the ‘breaking stress’ (also known as ultimate strength or tensile strength in the case of tension)

A

The stress required to cause fracture of a material

161
Q

Define ‘ductile’

A

A large ammount of plastic deformation occurs between the elastic limit and the fracuture point

162
Q

Define ‘brittle’

A

Fracture occours soon after elastic limit

163
Q

What is the equation for most probable speed of a molecule of an ideal gas?

A

Theoretical physics year 1 (3 decks)

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