Thermal Physics

Question 1

What is thermal equilibrium?

Answer 1

Two objects are in thermal equilibrium if the temperature difference between the two is zero. Therefore there is no net transfer of energy via heating between the two objects.

Question 2

What is absolute zero?

Answer 2

• The minimum possible temperature where the kinetic energy is zero
• At this temperature, the volume and pressure is zero
• At this temperature, the internal energy is a minimum
• The temperature is -273.15 C

Question 3

Two findings as a result of the Brownian motion experiment:

Answer 3

• The smaller molecules are moving randomly and colliding with the larger molecules to cause its random motion
• The smaller molecules must be smaller than the larger molecules (e.g. when the experiment was done with pollen grains and water, we can conclude the water molecules must be much smaller then the pollen grains as they were unobservable)

Question 4

Define internal energy:

Answer 4

The sum of the randomly distributed kinetic and potential energies of the atoms or molecules within a substance.

Question 5

During a state change what changes, potential or kinetic energy?

Answer 5

Potential energy, as the temperature DOES NOT change during a state change, therefore the kinetic energy cannot change

Question 6

Define specific heat capacity:

Answer 6

The specific heat capacity of a substance is the energy required per unit mass to raise the temperature of a substance by 1K

Question 7

E=mc▲θ

Answer 7

E = energy supplied in Joules, J
m = mass in kilograms, kg
▲θ = change in temperature, Kelvin, K
c = specific heat capacity (Jkg-1K-1)

Question 8

Define specific latent heat of fusion:

Answer 8

The energy required per unit mass to change it from a solid into liquid at a constant temperature

Question 9

Define the specific latent heat of vapourisation:

Answer 9

The energy required per unit mass to change it from a liquid into a gas at constant temperature

Question 10

E=mL

Answer 10

E = Energy (J)
L = Specific latent heat (Jkg^-1)
m = Mass (kg)

Question 11

Define Boyle’s Law

Answer 11

pV = constant
- The pressure of a gas is inversely proportional to its volume at a constant temperature.

Question 12

Define the pressure law:

Answer 12

p/T = constant
- The pressure of a gas is directly proportional to the temperature of a gas at constant volume

Question 13

Define Charles’ Law

Answer 13

V/T = constant
-The volume of a gas is directly proportional to the temperature of the gas at a constant pressure

Question 14

Define m = Mn

Answer 14

m = mass
M = molar mass per g
n = number of moles

Question 15

Define N = nN_A

Answer 15

N = number of particles
n = number of moles
N_A = Avogadro’s constant

Question 16

Kinetic theory of gases assumptions:

Answer 16

• The gas contains a huge number of particles moving in random directions with random speeds
• The molecules of the gas occupy negligible volume compared with the volume of the container
• The collisions between particles, and between particles and the wall is completely elastic
• Electrostatic forces between particles is negligible except during collisions
• The time of a collision between the particles is negligible compared to the time between collisions

Question 17

Define pV=nRT

Answer 17

p = Pressure (Nm^-2)
V = Volume (m^3)
n = Number of moles (mol)
R = Molar gas constant (Jmol^-1K^-1)
T = Temperature (K)

Question 18

pV = NkT

Answer 18

p = Pressure (Nm^-2)
V = Volume (m^3)
N = Number of particles
k = Boltzmann constant (JK^-1)
T = Temperature (K)

Question 19

Find the root mean square speed of 5,7,8,9:

Answer 19

1) Square each number
2) Add them together and divide by the number of values:
(25+49+64+81)/4 = 54.75
3) Take the square root of the mean: squroot(54.75) = 7.4
= 7.4