Thermal Physics Flashcards
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.
As particle motion is random, particles have a randomly distributed range of speeds.
Absolute temperature ∝ mean kinetic energy.
What does the potential energy of the particles depend on?
Depends on position of the particles and bonds between them.
If energy is supplied to break bonds – potential energy increases.
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.
(Though energy is still transferred between particles in collisions.)
How can the internal energy of a system be increased?
Heating.
Doing work to transfer energy to the system.
How can the internal energy of a system be decreased?
Cooling.
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.
Change in state.
How do the particle energies change when temperature increases/decreases?
Mean kinetic energy increases/decreases.
Mean potential energy is constant.
Give the equation for the energy required for a change in state.
Q = mc∆θ
Define specific heat capacity.
Amount of energy required,
to raise the temperature,
of 1 kg of a substance by 1K / 1oC.
How can the energy supplied by the heating element in continuous flow calorimetry be calculated?
P = IV and E = Pt
So Q = IVt
Write an expression for the energy transfers in continuous flow calorimetry.
- Energy supplied = Energy gained by fluid + Energy lost - Q = IVt = mcΔθ + Elost
How can the specific heat capacity of a fluid be determined using continuous flow calorimetry?
Repeat the experiment at a new flow rate.
Keep current and time the same.
Adjust the p.d. to give the same change in temperature.
Assume energy lost to the surroundings is the same.
Combine equations (see booklet).
What happens to the bonds when a substance melts? Why?
Some bonds break.
As energy is supplied.
What happens to the bonds when a substance boils? Why?
All bonds break.
As energy is supplied.
How do the particle energies change when a substance melts/boils?
Mean potential energy increases (as positions change).
Mean kinetic energy is constant (as no change in temperature).
What happens to the bonds when a substance condenses? Why?
Bonds partially reform.
As energy is removed.
What happens to the bonds when a substance freezes? Why?
Bonds fully reform.
As energy is removed.
How do the particle energies change when a substance condenses/freezes?
Mean potential energy decreases (as positions change).
Mean kinetic energy is constant (as no change in temperature).
Give the equation for the energy required for a change in state.
Q = ml
Define specific latent heat of fusion.
Amount of energy required,
to melt 1 kg of a substance,
at its melting point (i.e. with no change in temperature).
Define specific latent heat of vaporisation
Amount of energy required,
to boil 1 kg of a substance,
at its boiling point (i.e. with no change in temperature).
How were the gas laws determined?
By experiment
What are the conditions for the gas laws?
They apply to a fixed mass of gas.
They apply to 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 / T1 = V2 / 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 / T1 = p2 / T2
Give the combined equation for using the three gas laws in calculations.
p1V1 / T1 = p2V2 / T2
How do you work out the relative molecular mass of a molecule?
Add up the relative atomic masses.
No units as it is relative.
(e.g. C = 12.0, O = 16.0, so CO2 = 44.0)
What is Avogadro’s constant (NA)?
Number of particles/molecules in one mole of a substance.
NA = 6.02 x 1023
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.
Units = g mol-1. - (e.g. CO2 = 44.0 g mol-1)
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
Give the ideal gas equation in terms of number of particles/molecules (N).
pV = NkT
How do you work out the work done in changing the volume of a gas at constant pressure?
Work done = pΔV = p x (V2 – V1)
Equal to area unde a pressure-volume graph.
When a gas expands, is work being done on or by the gas?
Work is being done by the gas.
Gas loses energy.
When a gas contracts, is work being done on or by the gas?
Work is being done on the gas (by an external force).
Gas gains energy.
Give the kinetic theory of gases equation and define each term.
1/3 x Nm(crms)2
p = pressure
V = volume
N = number of particles/molecules
m = mass of one particle/molecule
crms = root mean square speed
Give the equation for calculating root mean square speed (crms) of N particles.
crms = √((c12 + c22 + c32 + … + cN2) / N)
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 (i.e. range of speeds and no preferred direction).
- Particles have negligible volume compared with volume of container.
- Particle motion follows Newton’s laws of motion.
- Collisions between particles themselves or at the walls of a container 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? What does this mean?
Elastic collisions.
Kinetic energy is conserved.
Particles rebound with same speed.
Derive an expression for the change in momentum for a particle with velocity u when it collides perpendicularly with wall W.
Change in momentum = final momentum – initial momentum
-mu – mu = - 2mu
Derive an experession for the time between collisions with wall W.
Particle travels to the opposite wall and back (2l).
Speed = distance/time - Time = 2l / u
Derive an experession for the number of collisions with wall W per second (i.e. the frequency).
Frequency = 1 / time period
Frequency = u / 2l
State the alternative form of Newton’s second law (i.e. not F=ma)
Resultant force = rate of change of momentum.
(i.e. change in momentum per second.)
Derive an expression for the force exerted by the wall on the particle
F = momentum change per collision x no. of collisions per sec
F = -2mu x (u / 2l) = -mu2 / l
Derive an expression for the force exerted by the particle on the wall.
Newton’s third law – every force has an equal and opposite reaction.
This force is in the opposite direction (i.e. + direction)
F = +mu2 / l
Derive an expression for the pressure on the wall.
Pressure = force / area
Area = l2
p = Nm(urms)2 / l x l2 = Nm(urms)2 / V
Write an expression for (crms)2 in terms of the three components of velocity.
(crms)2 = (urms)2 + (vrms)2 + (wrms)2
Derive an expression for (crms)2 in terms of (urms)2.
Random motion means no preferred direction
(urms)2 = (vrms)2 = (wrms)2
(crms)2 = 3(urms)2
(urms)2 = 1/3 x (crms)2
Describe the relationship between mean kinetic energy and absolute temperature.
Mean kinetic energy is directly proportional to absolute temperature.
For an ideal gas, explain why the total kinetic energy is equal to the total internal energy.
Ideal gas particles have zero potential energy.
Due to the assumption that there are no forces between particles (except during collisions).
How were the gas laws developed?
Empirically (i.e. through experiments).
How was kinetic theory developed?
Using mathematics and theories.
Based on assumptions and derivations.
Why did it take a long time for kinetic theory to become widely accepted?
Required evidence that gases were made up of particles.
This required multiple experiments that must also be verified.
What is Brownian motion?
The random motion of particles suspended in fluid. - (e.g. pollen grains in water, smoke particles in air.)
This is caused by the particles colliding with fast, randomly moving fluid particles.
How does Brownian motion support kinetic theory?
Provided evidence that gases are made up of particles.
