Module 5 Chapter 1 - Thermal Physics

Question 1

What is the internal energy of a gas?

Answer 1

Internal energy is the sum of the kinetic and potential energies of all the particles in a gas.

Question 2

What is the specific heat capacity of a mateiral?

Answer 2

The specific heat capacity is the amount of energy required to increase 1 kg of the material by 1 degree. From this it’s easy to derive Q = mcΔθ.

Question 3

What are the units of specific heat capacity?

Answer 3

J kg-1 K-1.

Question 4

What is the specific latent heat of a material?

Answer 4

The specific latent heat is the energy required to change the state of 1 kg of the material.

Question 5

What are the units of specific latent heat?

Answer 5

Specific latent heat has the units J kg-1.

Question 6

Converting from degrees to Kelvin:

Answer 6

Add 273.15 to convert to kelvin.

Question 7

What is the ideal gas law (equation)?

Answer 7

The equation is pV = nRT, where R = 8.31 (ideal gas constant). This is equivalent to pV = kNT.

Question 8

What is the Boltzmann constant?

Answer 8

The Boltzmann constant is 2/3 of the average kinetic energy of a molecule in a gas per kelvin.

Question 9

What equation links number of molecules in a gas and the number of moles?

Answer 9

N = Na n, where Na is Avogadro’s constant.

Question 10

What equation links Avogadro’s constant, the ideal gas constant and the Boltzmann constant?

Answer 10

k Nₐ = R

Question 11

Equation for work done on an ideal gas:

Answer 11

W = P ΔV

Work done = pressure * change in volume

Question 12

What is the molar mass of a molecule?

Answer 12

The mass of a mole of a substance, equal (in grams) to the relative molecular mass of the molecule.

Question 13

What is Boyle’s law?

Answer 13

Boyle’s law states that the volume of an ideal gas of fixed mass is inversely proportional to its pressure exerted at a fixed temperature.
PV ∝ T, where T is constant.

Question 14

What is Charles’ law?

Answer 14

Charles’ law states that the volume of an ideal gas of fixed mass is directly proportional to its temperature at a fixed pressure.
PV ∝ T, where P is constant.

Question 15

What is Gay-Lussac’s Law?

Answer 15

Gay-Lussac’s Law states that the pressure of an ideal gas of fixed mass is directly proportional to its temperature at a fixed volume.
PV ∝ T, where V is constant.

Question 16

What was Brownian motion direct evidence of?

Answer 16

Brownian motion was evidence of atoms existing.

Question 17

How did Robert Brown first observe Brownian motion?

Answer 17

Robert Brown noticed that the paths of pollen in water were irregular.

Question 18

What equation can be used to calculate the pressure a gas exerts on its container?

Answer 18

P = 1/3 ρ v^2. Note that v is the root mean square velocity of the molecules in the container.

Question 19

Assumptions for the kinetic model of a gas:

Answer 19

• Large number of molecules in random, rapid motion particles.
• Atoms (or molecules) occupy negligible volume compared to the volume of gas.
• All collisions are perfectly elastic.
• Time of the collisions is negligible compared to the time between collisions .
• Negligible forces between particles except during collision.

Question 20

Derive Eₖ = 3/2 kT from P = 1/3 pv².

Answer 20

P = 1/3 pv²
P = 1/3 (Nm / V) v²
3PV = Nmv²
Using PV = NKT:
3NkT = Nmv²
3kT = mv²
3/2 kT = 1/2 mv²
Eₖ = 3/2 kT

Question 21

What is the value of Avogadro’s constant?

Answer 21

Nₐ = 6.02 x 10^23 mol^-1

Question 22

What is the value of the ideal gas constant?

Answer 22

R = 8.31 J mol^-1 K^-1

Question 23

What is the value of the Boltzmann constant?

Answer 23

k = 1.38x10^-23 J K^-1 (derived from R / Nₐ)