Thermal physics Flashcards
Why do particles have random kinetic energies
They all have random speeds
Why do particles have randomly distributed potential energies
Potential energy based on the particle’s relative positions to each other, which is random
Define internal energy
Sum of the randomly distributed kinetic and potential energies of all the particles
Which factor determines the speed and kinetic energy of particles
Temperature as it is a measure of the average kinetic energy of the particles in the system
Define a closed system
No matter is transferred in and out so the internal energy is constant unless work is done
How can you change the internal energy within a closed system
- Increase by heating or doing work such as changing shape
- Decrease by cooling
Since internal energy has changed, average KE and PE has also changed
How does energy change during a change of state
Energy is going into breaking bonds and not increasing temperature so KE stays constant but PE rises and internal energy rises
Define specific heat capacity
Energy required to raise temperature by 1°C or 1 K for a 1 Kg substance
Q = mcΔθ
Define specific latent heat
Thermal energy required to change the state of a 1 Kg substance where l of vapourisation is for boiling and condensing and l of fusion is for melting or freezing
Q = ml
What is absolute 0
- Lowest temperature possible (0 K or -273 °C)
- Means particles have minimum KE
Explain Boyle’s law
P ∝ 1/V at a constant T
Because decreased volume means particles are closer together so collide more with each other and container so exert more force on each other and container so more pressure
How does a change in temperature affect the graph of pressure against volume
- Normally looks like a graph of y = k/x
- Increasing temperature causes shift out of the graph
- A gas that obeys Boyle’s law at all temperatures is called an ideal gas
Explain Charles’ law
V ∝ T at a constant P
Because as T increases average KE is higher so speed increases meaning particles spread further apart from each other
Describe the graph of V against T for an ideal gas
Straight line which passes through origin if in K or touches x axis at -273 if in °C (i.e. absolute 0)
Explain The Pressure Law
P ∝ T at a constant V
Because as T increases average KE is higher so speed increases, meaning they collide and exert force on the walls more frequently with more force, meaning an increase in pressure
Describe the graph of V against T for an ideal gas
Straight line which passes through origin if in K or touches x axis at -273 if in °C (i.e. absolute 0)
What is the ideal gas equation
PV = nRT
or
PV = NkT
where N = n x 6.02x10^23
and k = R / (6.02x10^23)
How can you calculate the work done in expanding or contracting a gas
Work done = P x ΔV
Area under a pressure volume graph
What are the assumptions in the kinetic theory model
Remember VIRCE
- Volume of container is much larger than the volume of gas molecules
- Intermolecular forces between molecules are negligible
- Random motion of gas molecules
- Collision duration is negligible compared to the time between collisions
- Elastic collisions between gas molecules and container
Derive PV = 1/3 Nm(crms)^2
- Change in momentum during collision = mu - (-mu) = 2mu
- Time between collisions = 2l/u
- F = 2mu /(2l/u) = mu^2 /l
- P = (mu^2 / l) / (l^2) = (mu^2) / l^3 = mu^2 / V
- For N particles, x by N and use the average speed (rms to ignore (-)s)
so P = Nm(urms)^2 /V - u is only 1 of 3 directions which make up resultant velocity c so urms^2 = 1/3 crms^2 (with Pythagoras)
P = (Nm (crms^2 / 3)) / V
so PV = 1/3 Nm crms^2
Derive the average kinetic energy equation u
- nRT = 1/3 Nm crms^2
- (x 3/2) so
3/2 nRT = 1/2 Nmcrms^2 - 3/2 NkT = 1/2 Nm crms^2
- 3/2 kT = 1/2 m crms^2
and 1/2 m crms^2 is the average KE of 1 molecule
Explain Brownian motion
Where large particles in a fluid collide with the small fast moving fluid particles, causing random zig-zag motion i.e. Brownian motion
How does Brownian motion help prove the kinetic theory
Kinetic theory assumes that particles have random motion so assumption would be wrong if not for Brownian motion
