Thermal Physics Flashcards
What units are used for absolute temperature?
Kelvin (K)
How do you convert from degrees Celsius to kelvin?
Add 273
What do you know about molecules at absolute zero?
All molecules have zero kinetic energy
Define internal energy.
Sum of the randomly distributed, kinetic and potential energies, of all the particles in a body
What does the kinetic energy of the particles depend on?
Depends on the speed of the particles
What does the potential energy of the particles depend on?
Depends on position of the particles and bonds between them
What is a closed system?
A body (or group of bodies), which do not allow any transfer of matter in or out
What do you know about the internal energy of a closed system?
Internal energy is constant, if not heated/cooled and no work done
How can the internal energy of a system be increased?
Heating or doing work to transfer energy to the system
How can the internal energy of a system be decreased?
Cooling or doing work to remove energy from the system
What two changes can happen as a result of a change in internal energy?
Change in temperature and change in state
How do the particle energies change when temperature increases/decreases?
Mean kinetic energy increases/decreases; mean potential energy is constant
Define specific heat capacity.
Amount of energy required, to raise the temperature, of 1 kg of a substance by 1K / 1°C
Give units for specific heat capacity.
J kg-1 K-1 or J kg-1 °C-1
What happens to the bonds when a substance melts?
Some bonds break as energy is supplied
What happens to the bonds when a substance boils?
All bonds break as energy is supplied
How do the particle energies change when a substance melts/boils?
Mean potential energy increases; mean kinetic energy is constant
What happens to the bonds when a substance condenses?
Bonds partially reform as energy is removed
What happens to the bonds when a substance freezes?
Bonds fully reform as energy is removed
How do the particle energies change when a substance condenses/freezes?
Mean potential energy decreases; mean kinetic energy is constant
Define specific latent heat of fusion.
Amount of energy required, to melt 1 kg of a substance, at its melting point
Define specific latent heat of vaporisation.
Amount of energy required, to boil 1 kg of a substance, at its boiling point
Give units for specific latent heat.
J kg-1
What are the conditions for the gas laws?
They apply to a fixed mass of gas and ideal gases
Give Boyle’s law in words.
At a constant temperature, pressure and volume are inversely proportional
Give the equation for using Boyle’s law in calculations.
p1V1 = p2V2
Give Charles’ law in words.
At a constant pressure, volume and absolute temperature are directly proportional
Give the equation for using Charles’ law in calculations.
V1 / V2 = T1 / T2
Give the pressure law in words.
At constant volume, pressure and absolute temperature are directly proportional
Give the equation for using the pressure law in calculations.
P1 / P2 = T1 / T2
Give the combined equation for using the three gas laws in calculations.
P1V1 / P2V2 = T1 / T2
How do you work out the relative molecular mass of a molecule?
Add up the relative atomic masses
What is Avogadro’s constant (NA)?
Number of particles/molecules in one mole of a substance, NA = 6.02 x 10^23
What equation links number of particles/molecules (N) and number of moles (n)?
no. of particles (N) = no. of moles (n) x Avogadro constant (NA)
What is molar mass (M)?
Mass of one mole of a substance, equal to the relative molecular mass in grams
What equation links mass (m), number of moles (n) and molar mass (M)?
mass in g (m) = no. of moles (n) x molar mass in g mol-1 (M)
Give the ideal gas equation in terms of number of moles (n).
pV = nRT
Name R and give the value.
Molar gas constant, R = 8.31 J K-1 mol-1
Give the ideal gas equation in terms of number of particles/molecules (N).
pV = NkT
Name k and give the value.
Boltzmann’s constant, k = 1.38 x 10^-23 J K-1
How do you work out the work done in changing the volume of a gas at constant pressure?
Work done = p ΔV = p (V2 – V1)
When a gas expands, is work being done on or by the gas?
Work is being done by the gas
When a gas contracts, is work being done on or by the gas?
Work is being done on the gas
How was the kinetic theory of gases developed?
Using mathematics and theories based on assumptions and derivations
Give the kinetic theory of gases equation and define each term.
p = pressure, V = volume, N = number of particles/molecules, m = mass of one particle/molecule, crms = root mean square speed
List 8 assumptions in kinetic theory.
- The gas contains a very large number of particles
- All of particles of the gas are identical
- Particles continually move about randomly
- Particles have negligible volume compared with volume of container
- Particle motion follows Newton’s laws of motion
- Collisions between particles are perfectly elastic
- Collision time is negligible compared to time between collisions
- Negligible forces between particles, except during collisions
What type of collisions do we assume the particles have with the walls?
Elastic collisions
Derive an expression for the change in momentum for a particle with velocity u when it collides perpendicularly with wall W.
Change in momentum = 2mu
Derive an expression for the time between collisions with wall W.
Time = 2l/u
Derive an expression for the number of collisions with wall W per second.
Frequency = 2u/l
State the alternative form of Newton’s second law.
Resultant force = rate of change of momentum
Derive an expression for the pressure on the wall.
p = (N * m * (urms)^2) / V
Describe the relationship between mean kinetic energy and absolute temperature.
Mean kinetic energy is directly proportional to absolute temperature
How can you calculate the total kinetic energy of N particles?
N × 1/2 m (crms)^2
For an ideal gas, explain why the total kinetic energy is equal to the total internal energy.
Ideal gas particles have zero potential energy
What is Brownian motion?
The random motion of particles suspended in fluid
How does Brownian motion support kinetic theory?
Provided evidence that gases are made up of particles
