5.1: Thermal Physics, F

Question 1

Define: Absolute zero

Answer 1

The temperature in which a substance has minimal internal energy

Question 2

What is the kinetic theory

Answer 2

The idea that solids, liquids and gases are made up of tiny moving/vibrating particles

Question 3

Give an example of how brownian motion can be observed in a lab

Answer 3

Place smoke within a brightly illuminated glass jar, then observe particles using a microscope

Question 4

Define: Brownian motion

Answer 4

Random movement of small visible particles suspended in a fluid due to collisions with much smaller randomly moving molecules

Question 5

Define: Internal energy

Answer 5

Sum of randomly distributed kinetic and potential energies of all atoms or molecules within a system

Question 6

Explain what and why an absolute scale of temperature was created

Answer 6

It is independent of the properties of any specific substance, measured in Kelvins(K)

Question 7

What is thermal equilibrium

Answer 7

Thermal energy is always transferred from regions of higher to lower regions of temperature, until there is no net flow between them

Question 8

Define: Specific heat capacity

Answer 8

Spc of a substance is the amount of energy required to raise the temperature of 1kg of the substance by 1K

Question 9

Define: Latent heat of vaporisation

Answer 9

Amount of energy required to change the phase of 1kg of a substance from a liquid to a gas

Question 10

Define: Latent heat of fusion

Answer 10

Amount of energy required to change the phase of 1kg of a substance from a solid to a liquid

Question 11

State Boyle’s law

Answer 11

At a constant temperature, the pressure(p) and the volume(V) of a fixed mass of gas are inversely proportional
-pV = constant

Question 12

State Charle’s law

Answer 12

At constant pressure, the volume(V) of a gas is directly proportional to it absolute temperature(T)
-V/T = constant

Question 13

State the pressure-temperature law

Answer 13

At constant volume, the pressure(p) of a gas is directly proportionals to its absolute temperature(T)
-P/T = constant

Question 14

Given an equation you can use to find the number of particles in a substance

Answer 14

-N = n Na
N = no. of particles
n = no. of moles
Na = Avogadros constant

Question 15

What is the boltzmann constant(k)

Answer 15

Gas constant for one particle of gas, k = R/Na

R = Gas constant for 1mol of gas

Question 16

How are Newton’s law used in the explanation of the pressure of an ideal gas

Answer 16

1st - No forces of attraction ∴ inertia until collision
3rd - Collision with wall, exerts equal n opposite force
2nd - Opposite force is equal to rate of change of momentum
∴ change in momentum = 2mu as mu - -mu

Question 17

What happens to the pressure(p) of an ideal gas, if you increase the volume(V)

Answer 17

The frequency of collisions decreases bc particles have to travel further in between collisions ∴ decreasing the pressure

Question 18

What happens to the pressure(p) of an ideal gas, if you increase the number of particles(N)

Answer 18

Frequency of collisions increases between the particles and the container ∴ increases total force exerted by all collisions meaning pressure increases

Question 19

What happens to the pressure(p) of an ideal gas, if you increase the mass(m) of the particles

Answer 19

Following newton’s second law, Force is proportional to mass ∴ heavier particles will exert a greater force meaning pressure increases

Question 20

What happens to the pressure(p) of an ideal gas, if you increase the speed(c) of the particles

Answer 20

The faster the particles are moving when they collide with the walls, the greater the change in momentum and force exerted ∴ pressure increases

Question 21

List any 3 of the 6 assumptions involved when trying to model the behaviour of ideal gases

Answer 21

• Gas contains a large number of particles
• Particles move randomly and rapidly
• Volume of particles is negligible, compared to the volume of the gas
• All collisions are perfectly elastic(Ek is conserved)
• Duration of collisions is negligible compared to the time between collisions
• Negligible attractive forces between particles except during collisions

Question 22

List another 3 different assumptions involved when trying to model the behaviour of ideal gases

Answer 22

• Gas contains a large number of particles
• Particles move randomly and rapidly
• Volume of particles is negligible, compared to the volume of the gas
• All collisions are perfectly elastic(Ek is conserved)
• Duration of collisions is negligible compared to the time between collisions
• Negligible attractive forces between particles except during collisions

Question 23

What is mean square speed and what are its units

Answer 23

Its the average of the squared speeds of all the particles (m^2/s^2) ∴ square root gives the typical speed(r.m.s.speed)

Question 24

Show how you can derive an equation for average kinetic energy of particles in an ideal gas

Answer 24

-We know pV=NkT and pressure of ideal gas given by kinetic theory is pV=1/3NmC^2
- ∴ NkT=1/3NmC^2
- cancel N and rearrange
- 3kT = mC^2, we know 1/2mC^2 is Ek of an individual particle ∴
- E = 1/2mC^2 = 3/2kT ∴
-Average kinetic energy, E of one gas particle is given by
E = 3/2kT

Question 25

What happens to the average kinetic energy of a particle if the temperature of a gas doubles

Answer 25

As Ek and internal energy is directly proportional to absolute temperature. The avg Ek will also double.