THERMAL PHYSICS Flashcards
(51 cards)
1st law of thermodynamics
Change in internal energy of object = total energy transfer due to work done and heating
Fun fact about graduations of thermometer
Interval distance isn’t the same at the middle of the scale vs ends (measure w travelling microscope) - expansion of liquid isn’t directly proportional to temperature
When does energy transfer occur?
When one object exerts a force on another object, causing it to move (does work), or when one object is hotter than another, so energy is transferred via convection, conduction or radiation
Conditions for constant internal energy
No work done/energy transfer by heating or energy transfer by heating and work done “balance each other out”
What is internal energy?
The sum of the random distribution of the kinetic and potential energies of its molecules
Boyle’s law
pV = constant for constant temperature + n
Charles’ Law
V/T = constant for constant pressure + n
isobaric meaning
At a constant pressure
Pressure and temperature relationship
P/T is constant
Work done in terms of p &V
E=pΔV
Avocado constant definition
Number of atoms in 12g of carbon12
Atomic mass unit defintion
1/12 the mass of a carbon12 isotope
1 Mole definiton
Molar mass definition
Quantity of a substance (identical particles) that contains Nₐ particles
= (Mass/μ)
Molar mass - Mass in 1 mole = Molxμ
mew is atomic mass
Ideal gas eq
pV = nRT
graph of pV against T is straight line through absolute zero
Manipulating ideal gas law (in terms of density and number of molecules)
n=pV/RT Mₛ=Mn (mass=molarmassxn)
Mₛ = MpV/RT → ρ=Mp/RT for molar mass M
or nM/V
n=N/Nₐ where N is number of molecules
pV = NkT where k = R/Nₐ (boltzmann constant)
Assumptions made in kinetic theory
- Molecules are point molecules - volume of
each molecule negligable wrt volume of gas - Molecules don’t attract each other (would
reduce force on impact with container) - Move around in continual random motion
- Collisions with other molecules and
container are elastic - Each collision with the container is of
much shorter duration than the time
during impacts
Getting p = NM/3V
Pₓ = (NMvₓ^2)/V and etc for Pᵧ and P₂
so 3P = NM/V (Pₓ^2+Pᵧ ^2+ P₂ ^2)
P = NM/3V (c^2) where c = rms
also u^2 = 1/N(u1^2+u2^2…un^2)
as c²=x²+y²+z²
crms = c²+c²+c²…cn²/n
= x²+y²+z² … xn²+yn²+zn²/n
= xrms²+yrms²+zrms²
What happens to arrangement of atoms during state change specifically
Energy transferred reduced number of nearest atomic neighbours
(crystalline to amorphous from solid to liquid)
atoms move to centre of vibration
also avoid using velocity instead of speed when talking abt motion
Deductions from experiment where Brownian motion is demonstrated using smoke particles in air
The motion is caused by collisions between air molecules
and smoke particles
Ideal gas law + kinetic theory interpretations of absolute zero
Ideal gas - where pressure/volume =/(extrapolates to) 0
Kinetic- random motion stops or Ek of particles = 0
Compare mean Ek of two types of particles enclosed in a volume at the same temp
System is at equilibrium so same mean Ek
Explain using kinetic theory model why a gas exerts a force on a piston
Particles collide with piston and change momentum
Force = rate of change of momentum
Pressure = force/area
Using the kinetic theory model, two changes that can be made independently to
reduce the pressure exerted by a gas
change
the volume could be increased
explanation
which increases the time between collisions OR results in less frequent collisions
(with the piston/wall so reducing the rate of change of momentum)
OR
which increases the area of the piston/wall (and so reduces the pressure)
change
the temperature could be reduced
explanation
which reduces the momentum (change at the wall)
Which of the following is not an assumption of the kinetic model of ideal gases?
A. All particles in the gas have the same mass.
B. All particles in the gas have the same speed.
C. The duration of collisions between particles is very short.
D. Collisions with the walls of the container are elastic.
C - The particles have a distribution of speeds with a mean (temp)