Thermal Physics (M5)

Question 1

r.m.s.

Answer 1

Root mean square speed. The square root of the mean square speed of all the particles in a gas.

Question 2

Pressure equation including density

Answer 2

P = 1/3 p (rms)^2

Question 3

Avogadro’s number

Answer 3

6.02x10^23
The number of atoms in 0.012kg of carbon-12
Symbol: Na
Na = N/n
Na = M/m
N = (m/M)Na
n = m/M

Question 4

Pressure equation for moles

Answer 4

PV = nRT

Question 5

Pressure equation for molecules

Answer 5

PV = NkT

Question 6

Pressure speed equation

Answer 6

PV = 1/3 Nm (rms)^2
Average KE = 1/2 m(rms) = 3/2 kT

Question 7

Specific heat capacity

Answer 7

The energy required per unit mass to change the temperature by 1k (or 1°C)
E = mc dtheta
Unit: Jkg^-1k^-1

Question 8

Specific latent heat

Answer 8

The energy required to change the phase per unit mass while at a constant temperature.
E = mL
Symbol: L
Unit: Jkg^-1

Question 9

Internal energy

Answer 9

The sum of the kinetic energy and potential energy of the molecules of a substance.

Question 10

Specific latent heat of vaporisation experiment

Answer 10

Vertical tube with heater in a gas at the top and flask at the bottom to collect liquid. Cold water in at the bottom, water out at the top. Tube condenses the liquid.

Lv = IVt/m
Measure time and mass of liquid in glass
Have ammeter and voltmeter for I and V

Question 11

Specific latent heat of fusion experiment

Answer 11

Conical flask of ice with tube coming out of bottom and heater in the middle.
Melted ice collected in flask under the tube.
Measure mass of melted ice and time taken
Lf = IVt/m
Use a second without header on as a control and take away this mass.
Use a stirrer to fill air gaps.

Question 12

Assumptions of the kinetic model

Answer 12

~Large number of molecules with random directions and speeds
~Molecules occupy negligible volume compared to the gas
~Collisions are perfectly elastic
~Time of collisions is negligible
~Electrostatic forces are negligible except during collisions

Question 13

Derive KE = 3/2 kT

Answer 13

1/3 Nmc^2 = NkT
1/3 mc^2 = kT
2/3 (1/2 mc^2) = kT
1/2 mc^2 = 3/2 kT
Ek = 3/2 kT

Question 14

Avogadro’s number equations

Answer 14

n = N/Na
m = nM
N = (m/M)Na

Question 15

Boyle’s law

Answer 15

The pressure of an ideal gas is inversely proportional to its volume, provided that the mass of gas and the temperature do not vary

Question 16

Boltzmann constant

Answer 16

The molar gas constant R divided by Avogadro’s number Na.
A constant that relates the mean kinetic energy of the molecules in a gas to the gas temperature.
Symbol k

Question 17

Brownian motion

Answer 17

Smoke particles in air collide elastically with particles of air and the transfer of momentum from air to smoke causes smoke to move in haphazard random ways.
Can’t see air particles so they must be much smaller.