5.1 Thermal Physics

Question 1

What two fixed points does the thermodynamic scale of temperature use?

Answer 1

The triple point of pure water, 273.16K, and absolute 0.

Question 2

Define absolute temperature.

Answer 2

A temperature value relative to absolute zero.

Question 3

How can celsius be converted to kelvin?

Answer 3

Temp In Kelvin = Temp in Celsius + 273.16

Question 4

What is the triple point of water?

Answer 4

The right temperature and pressure is achieved where water, ice and water vapour exist simultaneously. There is only one specific temperature and pressure where this can occur.

Question 5

What is thermal equillibrium?

Answer 5

A stable state in which there is no net transfer of thermal energy between two regions, due to the two regions being the same temperature.

Question 6

Two subtances are in contact. One is hotter than the other. What occurs?

Answer 6

There is a net flow of thermal energy from the hotter object to the cooler object.

The hotter object cools down, and the cooler object warms up, until they are the same temperature, where there is no energy transfer.

Question 7

What is brownian motion?

Answer 7

The random motion of particles.

Question 8

How can brownian motion be observed?

Answer 8

By looking at smoke particles in air, under a microscope.

Question 9

Why do smoke particles suspended in air exhibit random motion?

Answer 9

Because of their collisions with the molecules in air, resulting in a transfer of momentum in random ways.

Question 10

Define internal energy

Answer 10

The sum of the randomly distributed kinetic and potential energies associated with the atoms or molecules which make up the substance.

Question 11

A substance is heated, but remains in the same state. How has the kinetic and potential energy of the molecules changed?

Answer 11

The kinetic energy of the molecules increases, but the potential energy remains the same.

Question 12

When a substance changes state (from solid -> liquid -> gas), how does the potential and kinetic energy change?

Answer 12

The potential energy increases, and the kinetic energy remains the same.

Question 13

How does temperature change while a substance changes phase?
Why is this the case?

Answer 13

It doesn’t change. This is because the thermal energy is being used to overcome electrostatic bonds between molecules.

Question 14

What is absolute zero?

Answer 14

The minimum possible temperature. At absolute zero, the internal energy is minimum
(Additional Details: KE=0, p=0)

Question 15

Why do molecules still have internal energy at 0K?

Answer 15

Because the molecules still have electrostatic potential energy.

Question 16

Define the specific heat capacity of a substance, c.

Answer 16

The energy required per unit mass to increase the temperature by 1K.
E = mc∆θ

E: Energy supplied to substance
m: Mass of substance
∆θ: Change in temperature of substance
Units: Jkg-1K-1

Question 17

What is temperature?

Answer 17

A quantifiable measure of how hot or cold an object is. It is related to the average kinetic energy of particles in a substance.

Question 18

What is the specific latent heat of fusion?

Answer 18

The energy required per unit mass to change the phase of a substance from solid to liquid.

Question 19

What is the specific latent heat of vaporisation?

Answer 19

The energy required per unit mass to change the phase of a substance from liquid to gas.

Question 20

What is the formula for specific latent heat?

Answer 20

E = mL

E is the Energy required to change the phase of a substance of Mass m.
Unit for L is Jkg-1

Question 21

State the formula which relates the number of particles to the number of moles.

Answer 21

N = nA

N: Number of particles
n: Moles
A: Avogadro

Question 22

What is the Avogadro Constant?

Answer 22

The number of particles in one mole of any substance.
6.02x10^23

Question 23

What quantity is used to represent the amount of a substance?

Answer 23

Mole.

Note: Remember this is an SI base unit.

Question 24

State the formula which relates the moles of a substance to its mass.

Answer 24

n = m / M

n: Moles
m: Mass
M: Molar mass (in grams, which is the same as the nucleon number for the atom/molecule) of the particles that make up the substance.

Question 25

What is the kinetic theory of gases?

Answer 25

A model which can be used to describe how particles within an ideal gas behave.

Question 26

Why do gases exert a pressure on the container they’re in?

Answer 26

Gas particles collide with the surfaces of the container
The container exerts a force on the particles to change their direction. The particles exert an equal and opposite force on the container.
Pressure is force applied (in total, by all particles) per unit area.

Question 27

What does Boyle’s law state?

Answer 27

For a fixed mass of gas at constant temperature, the pressure is inversely proportional to the volume.

Question 28

What does Charles’ law state?

Answer 28

For a fixed mass of gas at a constant volume, the pressure is proportional to the temperature.

Question 29

What is the ideal gas equation?

Answer 29

pV = nRT

p: Pressure of gas (Pa)
V: Volume the gas is contained in (m3)
n: Number of moles (mol)
R: Molar gas constant (8.31 Jmol-1K-1)
T: Temperature of gas (Kelvin)

Question 30

What is an ideal gas?

Answer 30

Particles have negligible volume compared to that of the container.
All collisions are elastic and there are no interactions between particles (except during collisions).

Question 31

Use the kinetic theory of gases to explain why a temperature increase leads to an increase in pressure.

Answer 31

• A temperature increase means the particles have more kinetic energy.
• More kinetic energy means a greater change in momentum during collisions with the container. There are also more frequent collisions.
• Change in momentum is proportional to force applied, and therefore to pressure as well.

Question 32

What is meant by the root mean square speed?

Answer 32

The square root of the mean of the squares of the speeds of the molecules.

Question 33

What equation involves using root mean square speed to find the pressure of a gas?

Answer 33

pV = 1/3 Nmc^2

p: Pressure
V: Volume
N: Number of molecules
c^2: r.m.s

Question 34

What does the maxwell boltzmann distribution show?

Answer 34

The number of molecules with each speed.

Question 35

What does the area under the maxwell boltzmann graph reprsent?

Answer 35

The total number of molecules.

Question 36

The temperature of a gas has increased. How does the curve on the maxwell boltzmann graph change to represent this?

Answer 36

The peak of the graph shifts right (to higher speeds), and the distribution becomes more spread out.

Question 37

What can pV=nRT also be written as?

Answer 37

pV = NkT

N: Number of molecules
k: Boltzmann’s Constant

Question 38

Kinetic energy is proportional to temperature. State the equation for this.

Answer 38

EK = 3/2 kT

EK: Mean kinetic energy of gas molecules
k: Boltzmann’s Constant
T: Temperature

Question 39

What is the Boltzmann constant (k) equal to?

Answer 39

The molar gas constant (R) divided by Avogadro’s Constant (Na).
1.38x10^-23JK-1

(Note: This is used for derivation of pV = NkT from pV = nRT)

Question 40

Why is it that the total internal energy of an ideal gas is equal to only the kinetic energy?

Answer 40

Since we assume there are no electrostatic forces between molecules, there is no potential energy. Internal energy is the sum of kinetic and potential energies. Therefore, the kinetic energy is equal to the total internal energy.

Question 41

Solids

Answer 41

• Atoms are closely packed together
• Atoms are in a fixed regular structure/arrangement.
• Atoms vibrate
• Atoms have strong electrostatic forces of attraction between molecules
• Molecules have negative electrostatic potential energy.

Question 42

Liquids

Answer 42

• No regular structure.
• Particles are slightly further apart compared to solids, but still close together.
• More kinetic energy than solids, so they can move around.

Question 43

Gas

Answer 43

• Molecules have the highest average kinetic energy
• Molecules move freely and rapidly
• Molecules collide elastically with each other.

Question 44

In an ideal gas, is internal energy proportional to temperature?

Answer 44

Yes.

Question 45

State the 5 kinetic theory assumptions

Answer 45

The gas contains a large number of atoms which move with random, rapid motion.

The volume of the gas atoms is negligible when compared to the total volume of the gas.

All collisions between atoms, with other atoms and with the walls of the container they are in, are perfectly elastic.

The time taken for atoms to collide is negligible compared to the time between collisions.

The electrostatic forces between atoms are negligible, except for when the atoms are colliding.