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PHYSICS 160 > Section 3: Thermal Physics > Flashcards

Section 3: Thermal Physics Flashcards

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1
Q

Temperature

A

The measure of how ‘cold’ or ‘hot’ a substance is
The common property that two bodies possess when they are in thermal equilibrium with each other
A measure of a body’s ability to transfer heat to other bodies

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2
Q

Human blood temp

A

37°C

98.6°F

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3
Q

Volume and temperature

A

V ∝ T

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4
Q

When T is below fusion point…

A

Gas becomes liquid

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5
Q

Triple point of water

A

A specific combination of pressure and temp at which all three phases of water co-exist simultaneously
Defined as 0.01°C

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6
Q

Celsius and Kelvin

A

Granularity are same (difference of 1°C is same as difference of 1K)
If problem involves a difference in temp, doesn’t matter if you use °C or K, as long its in the same units
If problem only involves a single temp, must use K

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7
Q

Absolute scale of temperature

A

Kelvin scale (K)

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8
Q

Using a thermometer

A

Essentially only gives a reading of its own temp
Thus, it’s critical to wait until the temp of the thermometer becomes the same as the substance you are measuring
i.e. must wait until thermometer is in thermal equilibrium with the substance

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9
Q

Thermal equilibrium

A

When objects are in thermal contact, their temperatures eventually equalise –> thermal equilibrium
Heat flows from hot to cold objects

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10
Q

Human body - thermal equilibrium?

A

Human body is NOT at thermal equilibrium with its surroundings

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11
Q

Zero-th law of thermodynamics

A

If A and B are separately in thermal equilibrium with C, then A and B are in thermal equilibrium with each other
i.e. they are all the same temp

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12
Q

Thermal expansion

A

Where the size of an object changes with temperature, typically increasing with increasing temp

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13
Q

Length and temperature

A

L ∝ T

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14
Q

α

A

Linear expansion coefficient - quantifies change in linear dimensions of an object
Fractional change in length per unit of temp change
Unit K^-1 or °C^-1

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15
Q

β

A

Volumetric expansion coefficient
Fractional change in volume per unit of temp change
Unit K^-1 or °C^-1

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16
Q

α and β

A

β = 3α
Solids usually use α (linear)
Liquids and gas usually use β (volume)

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17
Q

Hollows and cavities - heating

A

Hollows and cavities in solids expand on heating as do the solid parts of the object

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18
Q

Gases - coefficient

A

All gases have same coefficient, irrespective of their nature
β = 3.4 x 10^-3 K

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19
Q

Solids, liquids and gases - β

A

Generally:
β(gas)&raquo_space; β(liquid)&raquo_space; β(solid)

Solids: β < 10^-4 K^-1
Liquids: β < 10^-3 K^-1
Gases: β > 10^-3 K-1

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20
Q

Density (ρ) and temperature

A

ρ ∝ T

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21
Q

Thermal stress

A

Stress created in objects constrained to a precise, fixed dimension when temp changes occur

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22
Q

Gas

A

A state of matter where the diff atoms/molecules constituting the material have no bonds betwen them, are v far apart, and are moving around randomly in all directions at high speed
Compressible and deformable

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23
Q

Pressure

A

The average force (F) per unit of surface area (A)

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24
Q

Pressure - units

A

Standard unit of pressure is N/m^2 = 1 Pa (pascal)

1 bar = 10^5 Pa

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25
Q

Standard atmospheric pressure

A

The mean pressure exerted by our atmosphere at mean sea level at the latitude of Paris

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26
Q

1 atm is roughly equal to…

A

1 bar to within 1%

i.e. 10^5 Pa

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27
Q

Gauge pressure

A

Refers to pressure measured wrt atmospheric pressure
e.g. if you inflate a tyre to x bar of gauge pressure, that means the absolute (total) pressure inside the tyre is x bar + the current atmospheric pressure (i.e. about 1.5 bar)

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28
Q

1 atm (or 1 bar) pressure = ? of water

A

Roughly 10m of water

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29
Q

Ideal gas law - points to remember

A

P is the absolute pressure (not gauge pressure)

T is the absolute temp (in K)

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30
Q

Boltzmann constant

A

Proportionality constant in gas law, k

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31
Q

Avogadro number

A

N(A)

Unit is /mol (mol^-1)

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32
Q

Atomic masses

A

Atomic masses given in periodic table = masses in grams of one mole of that element

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33
Q

Kinetic theory

A

Describes the behaviour of gases as that obtained by the averaged effect of many microscopic particles (atoms or molecules) moving and colliding rapidly

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34
Q

Volume and pressure

A

Decreased volume –> increase no of collisions per unit of surface –> increased pressure

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35
Q

Number of particles and pressure

A

Increased no of particles –> more collisions –> increased pressure

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36
Q

Velocity of particles and pressure

A

Increased velocity of particles –> stronger collisions –> increased pressure

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37
Q

Kinetic theory - assumptions

A

Collisions of molecules with other molecules and the container walls are perfectly elastic and of zero time duration
Molecules occupy a negligible volume compared to size of container
Molecules obey Newton’s law of motion (F = ma)
Molecules exert no forces on each other except elastic forces during instantaneous collisions

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38
Q

Average kinetic energy

A

AKA thermal energy

Shows temp is a measure of the average energy of the particles in the gas

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39
Q

Existence of absolute zero of temp

A

Can be inferred from average kinetic energy
To lower the temp of a body, one needs to remove energy from its constitutive particles
Since energy is finite, once we have removed all the energy they have, one can’t lower the temp anymore - this is the absolute zero

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40
Q

RMS velocity

A

The square root of the mean of the square of the velocities of all particles

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41
Q

Molar mass and velocities of particles

A

Increased molar mass = heavier particles, which are harder to put in motion (have more inertia) –> decreased speed

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42
Q

Distribution of velocities

A

A graph showing the percentage of particles in a gas having such and such velocity
Bell-shaped curve that depends on temp
Starts from origin and tends towards zero for very large velocities
Tail extends to large velocities for increasing temp

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43
Q

Distribution of velocities - curve max and RMS

A

Curve maximum (‘most probable’ velocity) is slightly diff that RMS (RMS is slightly lower)

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44
Q

Solid

A

Can be seen as a collection of bound particles, each of them vibrating due to their averaged kinetic energy

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45
Q

Heating a material - particles

A

Particles will vibrate more strongly –> increase in average distances –> thermal expansion

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46
Q

Expansion of water

A

Water density decreases with increasing temp across majority of its liquid range, but does the opp in range of 0 to 4°C

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47
Q

Freezing a cell

A

Will burst because of the water expanding as it freezes

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48
Q

To determine which phase a substance is, must specify..

A

Temperature (T)
Pressure (P)
Volume (V)

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49
Q

Phase diagram

A

Represents which phase a given substance will be found in as a function of T, P and V

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50
Q

Types of phase diagrams (2D)

A

V-T diagram
P-V diagram
P-T diagram

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51
Q

V-T diagram

A

P is constant
V varies linearly with T - straight line
Below the bpt, the gas becomes liquid, and as molecules are bound together, volume shrinks significantly - gives discontinuity in V-T curve
Same occurs for liquid to solid, but less obvious

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52
Q

P-V diagram

A

Hyperbola on right part (vapour phase) of diagrams

Liquid phase curves pretty much verticle

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53
Q

Liquids - compression

A

Most liquids are largely incompressible - V barely changes when applying pressure because molecules are v close and intermolecular forces resist the applied pressure

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54
Q

P-V diagram - grey zone

A

Liquid-vapour region
Where liquid and vapour can coexist
Pressure stays constant while volume varies

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55
Q

P-V diagram - critical pressure

A

The pressure where the co-existence zone of liquid and vapour disappears
This is because the pressure is so large that molecules of a gas would be brought so close tgt that it starts to behave like a liquid

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56
Q

P-V diagram - above the critical point

A

There is no more distinction between liquid and gas phase

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57
Q

P-T diagram

A

Volume is constant

Typically contains 3 curves separating the solid, liquid and gas phase

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58
Q

P-T diagram - melting/fusion curve

A

Line separating solid from liquid

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59
Q

P-T diagram - boiling curve

A

Line separating liquid from gas

Does not extend indefinitely - it ends at the critical point, above which liquid and gas becomes indistinguishable

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60
Q

P-T diagram - sublimation curve

A

Line separating solid from gas (lower part)

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61
Q

Sublimation

A

The process where a solid becomes directly a gas without transition to a liquid

62
Q

P-T diagram - triple point

A

Where the 3 curves of the P-T diagram meet

Where all 3 phases coexist

63
Q

What happens to the molecules that escape the liquid - open container

A

Infinite space –> escaping molecule will never come back
Evaporation occurs irremediably
THus a liquid can’t stay liquid in a vacuum and is bound to disappear

64
Q

What happens to the molecules that escape the liquid - closed container

A

It’s possible for molecules to come back and for evaporation to effectively cease
Called the liquid-vapour equilibrium

65
Q

Evaporation rate depends on…

A

Temperature

66
Q

Evaporation - surface of liquid

A

Liquid molecules accumulate at surface, resulting in presence of a vapour phase co-existing with the liquid above the surface

67
Q

Closed container: Condensation rate

A

Once there is enough molecules in the vapour phase, it may happen that collisions between molecules in the vapour phase may push back some molecules into the liquid

68
Q

Condensation rate depends on…

A

Pressure in gas phase

Higher pressure = more molecules = larger the no of suitable collisions

69
Q

Closed container: Liquid-vapour equilibrium

A

As more molecules evaporate, the pressure in vapour phase will build up
Condensation rate increases until it’s equal to evaporation rate –> equilibrium is reached

70
Q

Closed container: Saturated vapour pressure

A

The pressure in the gas phase where the container is in liquid-vapour equilibrium

71
Q

How to determine saturated vapour pressure

A

Can simply be read (for a given temp) on the boiling curve on the P-T diagram
Saturated vapour pressure at a given temp = pressure required to make a liquid boil at that temp

72
Q

Closed container: Smaller overall volume, but same amount of liquid

A

Evaporation is unchanged –> same pressure in gas phase required to reach equilibrium
Explains why the curve on the P-V diagram is horizontal in these conditions; pressure is same for diff volumes

73
Q

Closed container: Larger overall volume, but same amount of liquid

A

You will reach a point where even when all the molecules are in the gas phase, the pressure will still be too low to overcome the evaporation rate –> substance will be purely in gas phase and have totally evaporated

74
Q

Closed container: Increased temperature

A

Evaporation rate increases (more Ek available to kick out molecules in gas phase) –> equilibrium will be reached for a larger pressure (with less liquid and more vapour)

75
Q

How does boiling occur

A

Through the presence of impurities or through surface roughness of container
These act as nucleation sites for bubbles of the heated substance to form inside the liquid

76
Q

Boiling: Saturated vapour pressure

A

The pressure inside the bubbles in liquid

77
Q

Boiling: If pressure of surrounding atmosphere (above surface of liquid) is larger than saturated vapour pressure…

A

The bubbles can’t survive - may rise in lqiuid a bit (through buoyancy forces) but will eventually be crushed by the external pressure

78
Q

Boiling: As temp rises..

A

Bubbles grow bigger because of increased evaporation rate
Saturated vapour pressure increases, which eventually becomes = to surrounding atmosphere –> bubbles can withstand crushing pressure of atmosphere –> float and rise to top of liquid –> vapour content leaves liquid at once

79
Q

The boiling curve also represents…

A

The saturated vapour pressure

80
Q

Where does boiling vs evaporation occur

A

Boiling: bubbles can escape the liquid from anywhere in the volume
Evaporation: molecules can only escape form the surface

Therefore a liquid boils much faster than it evaporates

81
Q

Dalton’s law

A

States the total pressure exerted by the mixture is the sum of all the partial pressures

82
Q

RH = 100%

A

Liquid and vapour phases are in equilibrium
Atmosphere can’t absorb any more water vapour (if there was, condensation rate would become larger than evaporation rate –> water vapour returns to liquid phase)
We say the air is saturated with water vapour - saturated vapour pressure

83
Q

Preventing equilibrium in the atmosphere

A

Currents, winds and inhomogeneities in the atmosphere can prevent equilibrium between the gas and liquid phases of water to be reached locally

84
Q

Numerator and denominator of RH

A

Numerator: how much water there is in the air
Denominator: how much space there is for water in the air

85
Q

Set quantity of water vapour in air, but changing temp - effect on RH

A

As temp drops, saturated vapour pressure decreases –> RH increases because there is less space for water in the air (saturated vapour pressure is lower)

86
Q

Dew point

A

RH = 100%

Depends on partial pressure of water vapour - the more there is, the quicker the dew appears as the temp drops

87
Q

Determining the dew point for a given level of RH and initial temperature

A

Use saturated vapour pressure at initial temp and RH to solve for partial pressure of water vapour
Then look at which temp the saturated vapour pressure is = to that partial pressure
This temp = dew point

88
Q

Heat

A

The energy transferred from a warm object to a cold object due to their temp difference

89
Q

Heat vs temperature

A

Heat is not the same as temp
Temp = energy in an object
Heat = energy in and out an object

90
Q

What prompts our brains to identify an object as warm or cold

A

Heat flow - not temperature

91
Q

Heat units

A

Joule

92
Q

Heat symbol

A

Q

93
Q

Mechanical and thermal energy

A 
Same thing
Mechanical energy (e.g. Ep) can be converted into thermal energy
94
Q

Calorie

A

Conversion of calorie to joule is known as the mechanical equivalent of heat

95
Q

If heat is transferred to an object…

A

It’s temp increases

96
Q

When is Q > 0 and Q < 0

A

Q > 0 when the object temp increases and energy is brought into the object
Q < 0 when the object temp decreases and energy is removed from the object

97
Q

Specific heat capacity

A

Coefficient c
Depends on the substance the material is made of and the phase of the substance
Represents how much heat is needed to increase the temp of 1 kg of a substance by 1°C

98
Q

Water - specific heat capacity

A

Comparatively large wrt other substances

99
Q

Molar heat capacity

A

Coefficient C

Represents the amount of heat needed to increase the temp of 1 mol of substance by 1°C

100
Q

Thermal inertia and size of object

A

If one object is much larger than the other, it’s the smallest one that experiences the largest temp change
The larger object essentially acts as a reservoir of constant temp - has larger thermal inertia - harder to change its temp

101
Q

Latent heat

A

The transfer of heat due to a phase change

102
Q

Phase change - temperature

A

During a phase change, the temp remains constant

e.g. only when all ice has melted (only liquid water left), will the temp rise again

103
Q

Vapourising water vs melting ice

A

More energy required to vapourise water than to melt ice

104
Q

3 diff ways to transfer heat

A

Convection
Conduction
Radiation

105
Q

Convection

A

The physical, macroscopic displacement of matter

106
Q

Conduction

A

The physical contact between hot and warm objects, but apart from thermal vibrations, no matter actually changes place

107
Q

Radiation

A

The heat transferred by electromagnetic radiation

Heat transport method doesn’t require any matter

108
Q

Types of convection

A

Natural/free convection

Forced convection

109
Q

What type of convection creates convection currents

A

Natural convection

110
Q

Conduction vs convection - speed

A

Conduction is a much slower process than convection - relies on random collisions and a gradual diffuse transfer of energy

111
Q

Convection - state

A

If substances in contact are liquid or gas, the temp gradient established will lead to natural convection
When solids are in contact, convection isn’t possible

112
Q

Conduction: Calculating Q vs Q/t

A 
Q = heat transferred by conduction (in J)
Q/t = rate of conductive heat transfer (power, in W)
113
Q

Thermal conductivity

A

Constant k in conduction equation

Metals generally have large thermal conductivity

114
Q

Diffusion

A

The mass flow due to a difference in conc between diff parts of a fluid

115
Q

Diffusion: D

A

Diffusion coefficient

116
Q

Osmotic pressure

A

The pressure due to the difference in levels on both sides of the membrane, i.e. the pressure you need to apply on high conc side to maintain the original conc difference

117
Q

EM radiation/waves

A

Waves made up of oscillating electric and magnetic fields

118
Q

How do different types of EM waves differ

A

By their frequency at which the fields osscilate, or equivalently by their wavelength

119
Q

Wavelength

A

The distance between 2 peaks of oscillating fields

120
Q

Relationship between wavelength and frequency

A

Inverse

Smaller wavelength = larger frequency

121
Q

EM waves - energy

A

When they hit a material, some (or all) or their energy is absorbed –> material heats up

122
Q

Temperature and EM radiation

A

Higher temp of object = more Ek its molecules have = vibrate faster = higher f of emitted EM radiation

123
Q

Emission spectrum

A

The distribution of the proportion of radiation emitted at each wavelength

124
Q

Cold vs warm objects

A

Long wavelength radiation = cold objects (e.g. red)

Short wavelength radiation = hot objects (e.g. blue)

125
Q

EM radiation - thermal equilibrium

A

Object absorbs as much EM radiation energy as it emits

126
Q

Perfect absorber of EM radiation

A

A body that can absorb all the incoming radiation falling upon it (all wavelengths) with nothing being reflected
Known as a black body
Also the best emitter

127
Q

Emissivity (e) - values

A

Blackbody has emissivity of 1 = good emitter
Totally reflecting body (reflects all radiation at all wavelengths) has emissivity of 0 = poor emitter
0 < e < 1 = grey body

128
Q

What does it mean if mechanical energy is thermalised

A

Converted into air’s internal energy

129
Q

Internal energy (U)

A

The sum of all kinetic energy (and all potential energy) of all particles of the system

130
Q

First law of thermodynamics

A

If a system performs some work on the surroundings while consuming some heat, energy conversation imposes that the difference must appear as a change of internal energy

131
Q

W vs Q when positive

A

W > 0:
Work is done by the system on the surroundings
Loss of energy for system

Q > 0:
Heat is entering system from surroundings
Gain of energy for system

132
Q

State variable

A

e.g. U

Characterises how the system is in its present state without having to know how the system has been put into that state

133
Q

Transformation ssytem

A

Changing the state of the system

134
Q

Heat engine

A

A device using heat to produce work
Only work if surrounded by 2 thermal reservoirs - a hot one and a cold one
Takes heat (Qin) from hot reservoir, then produces work while releasing some exhaust gas to cold reservoir (Qout)

135
Q

PV diagram and work

A

Can find the work produced during transformation using area beneath the PV curve

136
Q

Isobaric transformation

A

A transformation where pressure is constant

Curve is horizontal, so area = rectangle

137
Q

PV diagrams - sign convention

A

W > 0: transformation goes left to right; machine generates work; it is an engine
W < 0: transformation goes right to left; machine consumes work

138
Q

Engines work in ____ cycles

A

Closed

Periodically come back to same original state and repeatedly perform the same transformation

139
Q

PV diagram - cycle

A

The SA encircled by the cycle

140
Q

Isochoric process

A

A transformation where volume is constant
Curve is a vertical line –> no area –> W = 0
ΔU = Q

141
Q

What does internal energy depend on

A

Essentially only depends on temperature

142
Q

Mono-atomic vs di-atomic gases

A

Mono-atomic: have 3 degrees of freedom
Di-atomic: have 5 degrees of freedom

where f = degrees of freedom

143
Q

Isothermal process

A

Temperature is constant

Slow process

144
Q

Adiabatic process

A

No heat exchange is involved

Fast process

145
Q

Producing work is associated with a temperature ___

A

Drop

146
Q

An adiabatic compression is associated with a temperature ____

A

Increase

147
Q

Second law of thermodynamics

A

States it is impossible for irreversible processes to occur

148
Q

Irreversible processes

A

Only occur spontaneously in one direction

149
Q

Thermodynamic temperature scale

A

Matches with absolute Kelvin scale

150
Q

Ways to increase efficiency of heat engine

A

Increase temp difference between hot and cold reservoirs

i.e. increasing hot temp or decreasing cold temp

151
Q

3rd law of thermodynamics

A

It’s impossible to lower the temp of any system to the absolute zero of temperature
(because Q(C) can’t be reached as it would amount to a single reservoir engine)

PHYSICS 160 (5 decks)

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