Thermal Physics

Question 1

How do you convert from degrees Celsius to kelvin?

Answer 1

Add 273

Question 2

What do you know about molecules at absolute zero?

Answer 2

All molecules have zero kinetic energy.

Question 3

Define internal energy.

Answer 3

Sum of the randomly distributed,
kinetic and potential energies,
of all the particles in a body.

Question 4

What does the kinetic energy of the particles depend on?

Answer 4

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.

Question 5

What does the potential energy of the particles depend on?

Answer 5

Depends on position of the particles and bonds between them.
If energy is supplied to break bonds – potential energy increases.

Question 6

What is a closed system?

Answer 6

A body (or group of bodies),
which do not allow any transfer of matter in or out.

Question 7

What do you know about the internal energy of a closed system?

Answer 7

Internal energy is constant,
if not heated/cooled and no work done.
(Though energy is still transferred between particles in collisions.)

Question 8

How can the internal energy of a system be increased?

Answer 8

Heating.
Doing work to transfer energy to the system.

Question 9

How can the internal energy of a system be decreased?

Answer 9

Cooling.
Doing work to remove energy from the system.

Question 10

What two changes can happen as a result of a change in internal energy?

Answer 10

Change in temperature.
Change in state.

Question 11

How do the particle energies change when temperature increases/decreases?

Answer 11

Mean kinetic energy increases/decreases.
Mean potential energy is constant.

Question 12

Give the equation for the energy required for a change in state.

Answer 12

Q = mc∆θ

Question 13

Define specific heat capacity.

Answer 13

Amount of energy required,
to raise the temperature,
of 1 kg of a substance by 1K / 1^oC.

Question 14

How can the energy supplied by the heating element in continuous flow calorimetry be calculated?

Answer 14

P = IV and E = Pt
So Q = IVt

Question 15

Write an expression for the energy transfers in continuous flow calorimetry.

Answer 15

• Energy supplied = Energy gained by fluid + Energy lost - Q = IVt = mcΔθ + E_lost

Question 16

How can the specific heat capacity of a fluid be determined using continuous flow calorimetry?

Answer 16

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).

Question 17

What happens to the bonds when a substance melts? Why?

Answer 17

Some bonds break.
As energy is supplied.

Question 18

What happens to the bonds when a substance boils? Why?

Answer 18

All bonds break.
As energy is supplied.

Question 19

How do the particle energies change when a substance melts/boils?

Answer 19

Mean potential energy increases (as positions change).
Mean kinetic energy is constant (as no change in temperature).

Question 20

What happens to the bonds when a substance condenses? Why?

Answer 20

Bonds partially reform.
As energy is removed.

Question 21

What happens to the bonds when a substance freezes? Why?

Answer 21

Bonds fully reform.
As energy is removed.

Question 22

How do the particle energies change when a substance condenses/freezes?

Answer 22

Mean potential energy decreases (as positions change).
Mean kinetic energy is constant (as no change in temperature).

Question 23

Give the equation for the energy required for a change in state.

Answer 23

Q = ml

Question 24

Define specific latent heat of fusion.

Answer 24

Amount of energy required,
to melt 1 kg of a substance,
at its melting point (i.e. with no change in temperature).

Question 25

Define specific latent heat of vaporisation

Answer 25

Amount of energy required, to boil 1 kg of a substance, at its boiling point (i.e. with no change in temperature).

Question 26

How were the gas laws determined?

Answer 26

By experiment

Question 27

What are the conditions for the gas laws?

Answer 27

They apply to a fixed mass of gas. They apply to ideal gases.

Question 28

Give Boyle’s law in words.

Answer 28

At a constant temperature, pressure and volume are inversely proportional.

Question 29

Give the equation for using Boyle’s law in calculations.

Answer 29

p₁V₁ = p₂V₂

Question 30

Give Charles’ law in words.

Answer 30

At a constant pressure, volume and absolute temperature are directly proportional.

Question 31

Give the equation for using Charles’ law in calculations.

Answer 31

V₁ / T₁ = V₂ / T₂

Question 32

Give the pressure law in words.

Answer 32

At constant volume, pressure and absolute temperature are directly proportional.

Question 33

Give the equation for using the pressure law in calculations.

Answer 33

p₁ / T₁ = p₂ / T₂

Question 34

Give the combined equation for using the three gas laws in calculations.

Answer 34

p₁V₁ / T₁ = p₂V₂ / T₂

Question 35

How do you work out the relative molecular mass of a molecule?

Answer 35

Add up the relative atomic masses. No units as it is relative. (e.g. C = 12.0, O = 16.0, so CO2 = 44.0)

Question 36

What is Avogadro’s constant (N_A)?

Answer 36

Number of particles/molecules in one mole of a substance. N_A = 6.02 x 10²³

Question 37

What equation links number of particles/molecules (N) and number of moles (n)?

Answer 37

no. of particles (N) = no. of moles (n) x Avogadro constant (N_A)

Question 38

What is molar mass (M)?

Answer 38

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)

Question 39

What equation links mass (m), number of moles (n) and molar mass (M)?

Answer 39

mass in g (m) = no. of moles (n) x molar mass in g mol^-1 (M)

Question 40

Give the ideal gas equation in terms of number of moles (n).

Answer 40

pV = nRT

Question 41

Give the ideal gas equation in terms of number of particles/molecules (N).

Answer 41

pV = NkT

Question 42

How do you work out the work done in changing the volume of a gas at constant pressure?

Answer 42

Work done = pΔV = p x (V₂ – V₁) Equal to area unde a pressure-volume graph.

Question 43

When a gas expands, is work being done on or by the gas?

Answer 43

Work is being done by the gas. Gas loses energy.

Question 44

When a gas contracts, is work being done on or by the gas?

Answer 44

Work is being done on the gas (by an external force). Gas gains energy.

Question 45

Give the kinetic theory of gases equation and define each term.

Answer 45

1/3 x Nm(c_rms)² p = pressure V = volume N = number of particles/molecules m = mass of one particle/molecule c_rms = root mean square speed

Question 46

Give the equation for calculating root mean square speed (c_rms) of N particles.

Answer 46

c_rms = √((c₁² + c₂² + c₃² + ... + c_N²) / N)

Question 47

List 8 assumptions in kinetic theory.

Answer 47

1. The gas contains a very large number of particles. 2. All of particles of the gas are identical. 3. Particles continually move about randomly (i.e. range of speeds and no preferred direction). 4. Particles have negligible volume compared with volume of container. 5. Particle motion follows Newton’s laws of motion. 6. Collisions between particles themselves or at the walls of a container are perfectly elastic. 7. Collision time is negligible compared to time between collisions. 8. Negligible forces between particles, except during collisions.

Question 48

What type of collisions do we assume the particles have with the walls? What does this mean?

Answer 48

Elastic collisions. Kinetic energy is conserved. Particles rebound with same speed.

Question 49

Derive an expression for the change in momentum for a particle with velocity u when it collides perpendicularly with wall W.

Answer 49

Change in momentum = final momentum – initial momentum -mu – mu = - 2mu

Question 50

Derive an experession for the time between collisions with wall W.

Answer 50

Particle travels to the opposite wall and back (2l). Speed = distance/time - Time = 2l / u

Question 51

Derive an experession for the number of collisions with wall W per second (i.e. the frequency).

Answer 51

Frequency = 1 / time period Frequency = u / 2l

Question 52

State the alternative form of Newton’s second law (i.e. not F=ma)

Answer 52

Resultant force = rate of change of momentum. (i.e. change in momentum per second.)

Question 53

Derive an expression for the force exerted by the wall on the particle

Answer 53

F = momentum change per collision x no. of collisions per sec F = -2mu x (u / 2l) = -mu² / l

Question 54

Derive an expression for the force exerted by the particle on the wall.

Answer 54

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

Question 55

Derive an expression for the pressure on the wall.

Answer 55

Pressure = force / area Area = l² p = Nm(u_rms)² / l x l² = Nm(u_rms)² / V

Question 56

Write an expression for (c_rms)² in terms of the three components of velocity.

Answer 56

(c_rms)² = (u_rms)² + (v_rms)² + (w_rms)²

Question 57

Derive an expression for (c_rms)² in terms of (u_rms)².

Answer 57

Random motion means no preferred direction (u_rms)² = (v_rms)² = (w_rms)² (c_rms)² = 3(u_rms)² (u_rms)² = 1/3 x (c_rms)²

Question 58

Describe the relationship between mean kinetic energy and absolute temperature.

Answer 58

Mean kinetic energy is directly proportional to absolute temperature.

Question 59

For an ideal gas, explain why the total kinetic energy is equal to the total internal energy.

Answer 59

Ideal gas particles have zero potential energy. Due to the assumption that there are no forces between particles (except during collisions).

Question 60

How were the gas laws developed?

Answer 60

Empirically (i.e. through experiments).

Question 61

How was kinetic theory developed?

Answer 61

Using mathematics and theories. Based on assumptions and derivations.

Question 62

Why did it take a long time for kinetic theory to become widely accepted?

Answer 62

Required evidence that gases were made up of particles. This required multiple experiments that must also be verified.

Question 63

What is Brownian motion?

Answer 63

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.

Question 64

How does Brownian motion support kinetic theory?

Answer 64

Provided evidence that gases are made up of particles.