Thermal Physics Flashcards
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
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)
3
Q
Define insulator when applied to thermodynamics
A
A material that causes two objects to transfer heat energy more slowly
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
5
Q
What are the freezing and boiling points of pure water in the Fahrenheit scale?
A
- Freezing = 32ºF
- Boiling = 212ºF
6
Q
What is the equation converting Celsius to Fahrenheit?
A
7
Q
What is the chosen reference temperature for the kelvin scale?
A
The triple point of water
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
9
Q
What equation equates temperature and pressure of two points on the kelvin scale?
A
10
Q
At what value of pressure and temperature does the triple point of water occur?
A
610Pa, 0.01 Degrees centigrade
11
Q
What effects stop us from saying that all motion stops at zero degrees kelvin?
A
Quantum effects
12
Q
What is the equation for linear expansion?
A
13
Q
What does ‘alpha’ stand for in linear expansion and what is its unit?
A
Coefficient of linear expansion (K^-1)
14
Q
Why is the linear thermal expansion equation only approximately correct?
A
Alpha varies with initial temperature and temperature interval
15
Q
What is the equation for volume expansion?
A
16
Q
What is beta in the volume expansion equation and what are its limits?
A
The coefficient of volume expansion (K-1)
17
Q
When is the volume expansion equation valid?
A
Small changes in temperature
18
Q
What is the relationship between linear and volume expansion coefficients?
A
19
Q
Between what temperatures in °C does water contract?
A
0 → 4°C
20
Q
Define thermal stress
A
Stress on a body or structure due to inequalities of temperature
21
Q
What is the equation for thermal stress?
A
22
Q
What is the equation for young’s modulus?
A
*add photo*
23
Q
Define heat
A
Energy transfer due to a difference in temperature
24
Q
What can cause specific heat (C) to change?
A
Changes to initial temperature and temperature change
25
Define molar mass
Mass of one mole
26
What is the equation for molar heat capacity?
27
What happens to work done by system if volume is constant?
No work is done
28
What is another name for measuring heat?
Calorimetry
29
A phase change takes place at a specific temperature if what is true?
Constant pressure
30
Define the 'heat of fusion'
Heat required per unit mass for the change between solid and liquid (Lf)
31
What is the equation for a phase change?
32
What is the heat of vapourisaiton?
Heat required per unit mass for the phase change between liquid and gas (Lv)
33
Define sublimation
A substance changing from a solid directly into a gas
34
Define supercooled
Very pure water can be cooled several degrees below its freezing point without freezing
35
Define heat of combustion
Lc for complete combustion of one gram of a substance
36
Mathematicaly what is heat currend defined as?
Heat energy that flows past a point per unit time
37
What is the equation for heat current with respect to heat gradient?
38
Define thermal resistance
(R) Resistance of a material to the flow of heat
39
Many thermal insulators consist mainly of what?
dead air (trapped stationary air)
40
How are the heat currents in two different materials related when they are placed together?
Heat current must be the same in both
41
Define convection
Heat transfer due to motion of mass of a fluid form one region of space to another
42
What is factors effect heat current in natural convection and how?
* 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
43
Define radiation in terms of thermodynamics
Transfer of heat by electromagnetic waves
44
What is the equation for heat current of radiation
45
What is the equation of rate of radiation
46
What must a good emitter also be?
A good absorber
47
What is a perfect emitter/absorber?
Black body
48
How does the global warming work in terms of thermal output of the earth?
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
49
How does the van de walls equation improve upon the ideal gas equation?
Makes approximate corrections for omissions of dimensions of molecules and attractive forces between molecules in the ideal gas assumptions
50
What is an isotherm?
A curve on a P/V graph representing the behaviour at a specific temperature
51
What does the area under a P/V curve represent?
Work done during a volume change
52
For r0 what happens to the force between molecules?
The force is repulsive
53
What is r0 in terms of intermolecular forces?
The radius from a molecule past which the force is attractive (force at r0 is zero)
54
What happens to potential energy at r0?
It is at its lowest point
55
Define potential well
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)
56
What unit of mass does multiplying Avogadro's number by molar mass give?
Grams
57
Derive the equation for the kinetic molecular modle of gas
58
What is the kinetic molecular modle of gas?
A modle representing gas as a large number of point particles bouncing around a box
59
What are the assumptions for the kinetic molecular modle of gas?
* 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
60
What constants does the boltzmann constant consist of and in what mathematical relation?
R/NA
61
What is the equation for average speed of a molecule in a gas?
62
What is the equation for mean free path of a gas molecule?
63
Define 'mean free path' of a gas molecule
Average distance traveled between collisions
64
What is the equation for molar heat capacty at a constant volume for a monoatomic gas?
65
Why does molar heat capacity only work for monoatiomic gases?
Multiatomic gases have additional rotaional energy and vibrational as bonds expand and contract
66
Which has larger heat capacities in general? Monoatomic or polyatomic gases?
Polyatomic
67
What is equipartition of energy?
Velocity of each component of a gas has on average an associated kinetic energy per molecule of 1/2KT
68
Define degrees of freedom
The number of velocity components a gas can have
69
Describe the motion of an H2 molecule under 50k
* Molecule undergoes translation
* No rotation
* No vibration
70
How many degrees of freedom does a monoatomic gas have?
3: Vx, Vy, Vz
71
How many degrees of freedom do diatomic gasses have?
5: Vx, Vy, Vz and two axes of rotation
72
What is the equation for molar heat capacity at constant temperature of a diatomic gas?
73
Describe the motion of a H2 molecule between 50k and 600k
* Translational motion
* Appreciable rotational motion
* No vibration
74
Describe the motion of an H2 molecule above 600k
* Translational motion
* Rotational motion
* Vibation
75
How many degrees of freedom does a metal atom in a solid have?
3: Vx, Vy, Vz
76
What energies does a atom with heat energy have in a solid?
* Kinetic
* Potential
77
What is the equation for molecular heat capacity of a cyrstal?
78
What is the maxwell boltzmann distribution?
The actual distribution of molecular speeds in a substance
79
State the maxwell boltzmann equation
80
Define phase equilibrium
A combination of pressure and temperatre at which a substance can be in an equilibrium of change between multiple phases
81
Define fusion curve
A line on the pressure against temperature graph at which a substance can exist as a liquid and solid in equilibrium
82
Define vapourisation curve
A line on the graph of pressure against temperature at which a substance can exist as a liquid and gas in equilibrium
83
Define critical point
A pressure above which phase boundaries dissapear
84
Define 'thermodynamic system'
A collection of objects that can be viewed as one unit and has potential to exchange energy with its surroundings
85
Define 'thermodynamic process'
When a change occours in a thermodynamic system
86
What integral gives work as a function of volume change?
87
What equation gives work done as a volume change at constant pressure?
88
Can you work out work done by a system based purely on initial states?
No, you must take into account the path taken
89
Can you work out heat added to a system based purely on the start and finish states of a thermodynamic system?
No, path must be taken into account
90
Define 'free expansion'
Uncontrolled expansion of gas into a vacuum
91
Define internal energy
The sum of kinetic energies of all constituent particles plus the sum of all potential energies between constituent particles
92
What equation is the first law of thermodynamics?
93
Can the change in internal energy of a system be calculated purely in terms of the start and finish points?
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
Define 'cyclic process'
A process that eventually returns a system to its initial state
95
Define 'isolated system'
A system that does no work to surroundings and has no heat flow to or from surroundings
96
What is the differential form of the first law of thermodynamics?
97
Define the 'adiabatic' thermodynamic process
A thermodynamic procss in which the heat transfer is zero
98
Define an 'isochronic' thermodynamic process
A thermodynamic process in which volume is constant therfore non work is done by the system
99
Define an 'isobaric' thermodynamic process
A thermodynamic process in which pressure remains constant
100
Define an 'isothermal' thermodynamic process
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
Internal energy of an ideal gas depends only on what?
Temperature
102
What do Cv and Cp mean?
* Cv represents molar heat capacity at constant volume
* Cp represents molar heat capacity at constant pressure
103
If neither P nor V are constant how many possible values are there for molar heat capacity?
Infinite
104
Which process requires more heat input? isochonic or isobaric?
isobaric requires more heat input than isochonic (except in rare situations where a substance decreases in volume as it is heated)
105
How do Cp and Cv relate for an ideal gas?
106
What is the equation for performance constant (K) of a heat engine?
107
What does gamma represent?
Cp/Cv
108
Under what conditions is a process approximately adiabatic?
If a system is well insulate or the process takes place so quickly that there is minimal heat transfer in that time
109
Which has a steeper gradient? An adiabatic curve or an isotherm both starting at the same point?
Adiabatic curve is always steeper
110
What string of equations relate V, P and T?
111
What type of process is always very close to being in thermodynamic equilibrium with it's self and surroundings?
A reversible process
112
What relation does a reversable process have to an equilibrium process?
Reversable processes are a type of equilibrium processes and are therfore not precisely acheivable in the real world
113
What does conservation of mechanical energy into heat reqire an increase in?
Randomness/disorder
114
Define 'heat engine'
A device that transforms heat partially into work or mechanical energy
115
Define 'working substance'
Matter inside a heat engine that undergoes inflow and outflow of heat, expansion and compression and sometimes change of phase
116
Describe the general process that all heat engines go through
* Absorb heat from a high temperature source
* Perform mechanical work
* Discard some heat at a lower temperature
117
How are net work and net heat flow linked for a heat engine?
Q = W
118
Defien thermal efficiency
The ratio of work done to heat absorbed in one cycle
119
What is the general equation for thermal efficiency?
120
List the operations of a gasoline engine in chronological order starting at the start of a cycle
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
Chronologically list the steps of the otto cycle from the first step
1. Compression stroke - adiabatic compression
2. Heating at constant volume
3. Power stroke - adiabatic expansion
4. Cooling at constant pressure
122
What is the equation for efficiency of an otto cycle?
123
How can efficiency be increased for the otto cycle?
Compression ratio r increased, this increases temperature which can cause an explosion during the compression stroke
124
What is a typical thermal efficiency for the otto cycle?
35%
125
What are the main differences between the diesel and gasoline cycles?
* 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
What is a refrigerator?
A heat engine with a net input of mechanical work
127
What properties do 'cooling coils' have?
Low temperature, Low pressure, Liquid and vapour in a phase equilibrium
128
What properties do 'condenser coils' have?
High temperature (greater than that of air outside) , High pressure, Liquid and vapour in a phase equilibrium
129
What does an 'evapourator' do?
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
What does a 'compressor' do?
Takes fluid and adiabaticaly compresses it before delivering it into the compressor coil in high pressure and higher temperature than outside air
131
Do either the compressor or the expander do work on the fluid?
The compressor does work on the fluid but the expander does not
132
State the second law of thermodynamics
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
Generally what is the 'carnot cycle'?
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
List from the first step in chronological order the steps of the carnot cycle
* 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
State the eqution giving the efficiency of the carnot cycle
\*add photo\*
136
What is the carnot refrigerator?
The carnot cycle in reverse
137
State the equation giving the coefficient of perfomance (K) for the carnot refrigerator
138
What is the heat engine?
The carnot cycle
139
The ratio of heat absorbed and rejected in the carnot cycle is equal to what?
Ratio of the temperatures of the reservoirs (kelvin)
140
Define entropy
A quantitative measure of disorder
141
What is the equation for entropy?
142
In a reversable process the entropy change of the universe equals what?
0
143
What is the enthalpy change of one cycle of a carnot cycle?
0
144
Is entropy change of a system dependent on path?
No
145
The entropy change of the universe is what?
\> 0
146
For all individual systems taking part in a whole system the entropy does what?
Either increases or remains constant
147
There can be many microscopic states corresponding to one macroscopic state, which microscopic state is the most likely?
The state with the greatest entropy
148
What equation describes the microscopic expression of entropy?
149
How does the second law of thermodynamics effect the change of microstates?
The number of microstates in a system cannot spontaneously decrease
150
Young's modulus is tensile stress over tensile strain, what is the modulus used for compressive stress and strain?
Young's modulus
151
What materials have are strong under compression but weak under tension?
Concrete or stone
152
How can you minimise stress on a beam?
* 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
If an object is immersed in a fluid what direction is the pressure to the surface?
Perpndicular
154
What is the equation for bulk modulus?
155
What is compressability?
The reciprocal of the bulk modulus
156
What is the equation for shear stress?
157
What is the equation for shear strain?
158
Define 'elastic hysterisis'
When a material follows different stress strain curves for increasing and decreasing stress
159
What causes hysterisis to happen?
Non-conservative forces associated with internal friction
160
Define the 'breaking stress' (also known as ultimate strength or tensile strength in the case of tension)
The stress required to cause fracture of a material
161
Define 'ductile'
A large ammount of plastic deformation occurs between the elastic limit and the fracuture point
162
Define 'brittle'
Fracture occours soon after elastic limit
163
What is the equation for most probable speed of a molecule of an ideal gas?
