3.6.2 Thermal Physics Flashcards
Define internal energy
Sum of the randomly distributed KE and PE of the particles in a body.
What is absolute zero?
Lowest possible temperate, no kinetic energy. 0K.
What is the triple point of water?
273K, the temperature at which pure water exists in thermal equilibrium with ice and water vapour.
Define latent heat.
Energy required to change the state of a substance eg. melting or boiling.
Define specific latent heat.
Energy required to change the state of a substance without change of temperature.
What is the equation for specific latent heat?
Q = mL
Q = energy, J
L = specific latent heat, Jkg-1
m = mass, kg
What is the latent heat of fusion?
Melting or freezing.
What is the latent heat of vapourisation?
Boiling or condensing.
Define pressure.
The force per unit area that it exerts normally (at 90°) to the surface. Pa or Nm-2.
What is pressure affected by?
Temperature, volume of the gas particles, mass of the gas particles.
What is Boyle’s law?
Pressure is inversely proportional to volume.
What is the equation for Boyle’s law?
P1 x V1 = P2 x V2
What is Charles’s law?
For a fixed mass of gas at constant pressure, volume is proportional to temperature.
What is Gay-Lussac’s law?
For a fixed mass of a gas at constant volume, P/T = constant.
How do we increase the internal energy of a gas?
Increase the KE (increase temp)
Increase the PE (increase work done).
What is the equation for work done?
W = P∆V
W = work done, J
P = pressure, Pa
V = volume, m3
Which ideal gas equation do we use for moles?
PV = nRT
Which ideal gas equation do we use for molecules?
PV = NkT
What are the general steps to derive the kinetic theory equation?
1) calculate force exerted by one particle
2) many particles
3) convert to pressure
4) 3 dimensions
5) RMS
What is the actual derivation for the kinetic theory formula?
ρ1 = mu
ρ2 = -mu (after elastic collision)
∆ρ = -mu - mu = -2mu
Distance between collisions = 2L
Number of collisions per second = u/2L
t = s/u = 2L/u
∆ρ/∆t = -2mu^2/2L
= -mu^2/L
(N’s 2nd L) F = ∆ρ/∆t
Force on particle = -mu^2/L
(N’s 3rd L) Force on wall = mu^2/L
Lots of particles = N
Particles don’t all move at same speeds so use mean squared speed (bar)u^2
F = Nm(bar)u^2/L
P = F/A = Nm(bar)u^2/AL
= Nm(bar)u^2/V
Overall mean squared speed= (bar)L^2 = (bar)u^2 + (bar)w^2 + (bar)v^2
Because on average they will be at the same speed
(bar)u^2 = (bar)v^2 = (bar)w^2
(bar)c^2 = 3(bar)u^2
(bar)u^2 = (bar)c^2/3
(bar)c^2 = root mean speed
P = 1/3 Nm(bar)c^2/V
P = 1/3 Nm(crms)^2/V
What is step one for the derivation of the kinetic theory formula? (Calculate force exerted by one particle.)
ρ1 = mu
ρ2 = -mu (after elastic collision)
∆ρ = -mu - mu = -2mu
Distance between collisions = 2L
Number of collisions per second = u/2L
t = s/u = 2L/u
∆ρ/∆t = -2mu^2/2L
= -mu^2/L
(N’s 2nd L) F = ∆ρ/∆t
Force on particle = -mu^2/L
(N’s 3rd L) Force on wall = mu^2/L
What is step two of the kinetic theory equation? (Many particles)
Lots of particles = N
Particles don’t all move at same speeds so use mean squared speed (bar)u^2
F = Nm(bar)u^2/L
What is step 3 of the kinetic theory derivation? (Convert to pressure)
P = F/A = Nm(bar)u^2/AL
= Nm(bar)u^2/V
What is step 4 of the kinetic theory equation? (3 dimensions)
Overall mean squared speed= (bar)L^2 = (bar)u^2 + (bar)w^2 + (bar)v^2
Because on average they will be at the same speed
(bar)u^2 = (bar)v^2 = (bar)w^2
(bar)c^2 = 3(bar)u^2
(bar)u^2 = (bar)c^2/3
What is step 5 of the kinetic theory equation? (RMS)
(bar)c^2 = root mean speed
P = 1/3 Nm(bar)c^2/V
P = 1/3 Nm(crms)^2/V
What assumptions do we make in the kinetic theory model?
- All particles are identical
- Gas has a large number of particles
- Particles have negligible volume compared with volume of the container
- Particles move at random
- Particles obey Newtonian mechanics
- All collisions are perfectly elastic
- Particles move in straight lines between collisions
- Forces that act during collisions last much sorter times than the times between collisions
What is Brownian motion?
Pollen grains on water move randomly, resulting from collisions with the fast, randomly moving particles in the water.
What is Brownian motion evidence for?
Existence of atoms.
