Thermal Physics Flashcards
zeroth law of thermodynamics
if C is initially in thermal equilibrium with both A and B, then A and B are also in thermal equilibrium with each other
two systems are in thermal equilibrium if and only if
they have the same temperature
absolute temperature is linearly proportional to
the gas pressure
T2/T1=p2/p1
converting celsius to kelvin
Tk=Tc+273.15
for a gas at constant volume, pressure is proportional to
temperature
this gives a temperature scale
most materials will
expand when heated and contract when cooled
units of the coefficient of linear expansion
per kelvin (or per degree Celsius)
what also increases when temperature increases
the average distance between atoms
what happens to material with hole in it when heated
hole expands too
comparing linear and volume expansion
beta = 3 alpha
thermal expansion of water
unusual variation with temperature
water is most dense at 4 degrees celsius
thermal stress will develop if
the ends of a rod are rigidly clamped and then the rod heated or cooled
fractional change if the rod were not clamped
Δl/l0 = alpha ΔT
what to use to calculate tension
young’s modulus
Y=(F/A)/(ΔL/L0)
Young’s modulus of material describes
how easy it is to stretch or compress
heat flow/ heat transfer
energy transfer that takes place solely because of a temperature difference
energy transferred in this way - heat
both work and heat change the
internal energy of a body
ΔU=Q-W
calorie
the amount of heat required to raise the temperature of 1g of water from 14.5 degrees celsius to 15.5 degrees celsius
unit of heat
joule
amount of heat needed to raise the temperature of a mass of material is proportional to
mass
temperature change
constant of proportionality is the specific heat capacity
mole
SI unit of amount of substance
one mole contains 6.02*10^23 elementary entities (avogadro constant)
molar mass M
mass per mole
so total mass m=nM
molar heat capacity
sometimes more convenient to describe quantity of material in terms of moles rather than mass
elemental solids tend to have the same molar heat capacity of around
25 J/mole/kelvin
dulong-petit law
heat capacity for solids depends on number of particles, related to energy one vibrating atom has
Molar heat capacity is C=3R =(approx) 25J/mole/kelvin
R is the molar gas constant
phase
a specific state of matter such as solid, liquid, gas
for a given pressure, a phase change takes place at
a definite temperature
phase changes usually involve
change in volume and pressure
absorption/emission of heat - latent heat
latent heat of fusion
to change 1kg of ice at 0 degrees C to 1kg of liquid water at 0 degrees C at normal atmospheric pressure, you need 3.34*10^5J of heat
in general, heat for a phase transition is
Q=+/-mL
L -latent heat
condensation
gas to liquid
vaporisation
liquid to gas
freezing
liquid to solid
melting
solid to liquid
sublimation
solid to gas
deposition
gas to solid
gas-liquid latent heat
latent heat of Vaporisation
liquid-solid latent heat
latent heat of fusion
solid-gas latent heat
latent heat of sublimation
compare latent heat of vaporisation with latent heat of fusion
Lv>Lf
calorimetry
measurement of heat
heat needed to raise temp and enable phase transitions
temp constant during transitions
supercooling
very pure water can be cooled below 0 without freezing
resulting state is unstable and known as supercooled
disturbance/impurity to condense around can trigger phase transition
for heat to flow between regions, there must be
a temperature difference
heat always flows from
hot to cold
heat current
dQ/dt
how is heat transferred in conduction
through transfer of energy between atoms
kinetic energy transfer
vibrations in a material carry energy
vibrations propagate through materials
some energy also carried by free electrons
heat current proportional to
ΔT
A
1/l
constant of proportionality in H=-kAΔT/l
thermal conductivity
units W/m/k
if the heat flow along the length of the rod is not constant
heat current depends on temperature gradient
H=-kAdT/dx
-ve indicates heat flows in direction of decreasing temp
thermal resistance
defined so that the heat current through the slab is
H=-AΔT/Rt
Rt=l/k
units km^2W^-1
a larger thermal resistance means
a better insulator
convection
transfer of heat by mass motion of a fluid from one region to another
natural convection arises through
differences in density due to thermal expansion
heat current due to convection is
directly proportional to surface area
forced flow
eg hair dryer
free flow
driven by changes in density, eg radiator
convection heat current
H=-hAΔT
heat traansfer coefficient also depends on temp difference depending on nature of flow
free laminar flow
h proportional to |ΔT|^1/4
turbulent flow
h proportional to |ΔT|^1/3
what slows natural convection near a stationary surface
viscosity of fluids
gives an insulating surface film, thinner for forced convection
radiation
transfer of heat by EM waves
dominant mechanism when feel sun on skin
every object emits energy
at room temp, this is mostly
infrared
as temp rises, wavelength
shifts to shorter values
radiation heat current depends on
surface area, surface temp and emissivity
emissivity
ratio of emission to that of an ideal emitter
if in thermal equilibrium, rates of absorption and emission
must be equal
black body
ideal radiator (and absorber) would have emissivity 1
ideal reflector
emissivity 0
absorb/emit no radiation at all (why vacuum flasks are silvered)
state variables
quantities such as pressure, volume, temp, mass
variables that can be used to describe the conditions in which a material can exist
equation of state
relationship between state variables
eg: ideal gas law
ideal gases
volume proportional to no of moles
volume inversely prop. to pressure
pressure prop. to absolute temp
ideal gas equation
pV=nRT
for real gases, ideal gas equation holds best at…
low number densities
ideal gas equation applications
breathing (diaphragm contracts; lungs expand, vice versa for exhalation)
constant volume gas thermometer
van der waals equation
tries to take into account the finite size of molecules and interactions between them
a accounts for attractive forces
b for finite size
van der waals - attractive forces
pull gas molecules together, reducing pressure exerted on walls by has
van der waals - finite size
this reduces the total volume in which molecules can move around
molecules and intermolecular forces
molecules not point-like charges so interaction is more complicated
intermolecular force at large distance
attractive
intermolecular force at small distance
repulsive
force and potential related by
Fr=-dU/dr
isotherms
lines of constant temperature
(on pressure-volume diagram)
when gas molecules are forced close together, attractive forces can get stronger which leads to
phase change
(formation of liquid or solid)
raising temperature of molecules
more energy so their motion can more easily break the bonds
