Module 5.1 - Thermal Physics

Question 1

What is 0K in degrees C?

Answer 1

0K = -273^C

Question 2

What is an advantage of the thermodynamic scale?

Answer 2

(Kelvin scale) It does not depend on the properties of any particular substance

Question 3

How do you observe Brownian motion in the lab?

Answer 3

1) Put some smoke in a brightly illuminated glass jar (sealed) and set up a microscope above
2) The smoke particles should appear as bright specks moving haphazardly around when viewed through the microscope

Question 4

What is Brownian motion?

Answer 4

Brownian motion is the random zigzag motion of any particle suspended in a fluid.

Question 5

How was Brownian motion explained?

Answer 5

Einstein showed this provided evidence for the existence of randomly moving atoms/molecules in the air. The randomly moving particles hit the smoke particles unevenly causing the random motion.

Question 6

What is internal energy?

Answer 6

Internal energy is the sum of the kinetic and potential energy of the particles within a system - the amount of energy contained within a system

Question 7

What affects the kinetic energy of a particle?

Answer 7

The kinetic energy of a particle depends on its mass and its speed. Average kinetic energy is proportional to temperature

Question 8

What affects the potential energy of a particle?

Answer 8

Potential energy is based on the relative positions of particles. Solids have the least whilst gases have the most potential energy.

Question 9

What state of matter has the most potential energy?

Answer 9

Gas particles have the most potential energy whereas solids have the least. Gasses actually have 0J potential energy whereas solids have -ive potential energy

Question 10

Why does temperature stop increasing during a phase change?

Answer 10

1) The temperature of a substance depends on the kinetic energy of its particles.
2) During a phase change the kinetic energy of the particles of a substance does not change
3) All input energy goes towards breaking bonds between particles and therefore changing their potential energy

Question 11

How does internal energy change as a substance is heated (through multiple state changes)?

Answer 11

Internal energy increases linearly as a substance is heated. If there is no phase change occurring it is increasing due to the kinetic energy of the particles increasing. If a phase change is occurring it is due to the potential energies of the particles increasing.

Question 12

Describe solids in terms of particles?

Answer 12

Solid - Particles vibrate in fixed positions in a regular lattice. They are held together by strong forces of attraction

Question 13

Describe liquids in terms of particles?

Answer 13

Liquid - Particles are constantly moving and free to move past one another but forces of attraction still keep them grouped together

Question 14

Describe gases in terms of particles?

Answer 14

Gas - Particles are free to move around with constant random motion. Negligible forces of attraction act between them.

Question 15

What is the kinetic model of matter?

Answer 15

Kinetic model of matter - the model of matter where solids liquids and gasses are made up of tiny moving or vibrating particles

Question 16

Why is potential energy negative for solids?

Answer 16

Potential energy is negative for solids as energy must be supplied to break molecular bonds

Question 17

What is the magic thermal equilibrium law?

Answer 17

If body A and body B are both in thermal equilibrium with body C, then body A and B must be in thermal equilibrium with each other

Question 18

Define specific heat capacity?

Answer 18

The specific heat capacity (c) of a substance is the amount of energy needed to raise the temperature of 1kg of the substance by 1 K

Question 19

State the specific heat capacity formula?

Answer 19

E = mcΔθ

Question 20

Define specific latent heat?

Answer 20

The specific latent heat (of fusion/vaporisation) is the quantity of thermal energy required to change the state (liquid/gas) of 1kg of a substance

Question 21

State the equation for specific latent heat?

Answer 21

L = E/m
E > Energy change in J
m > mass

Question 22

What is the latent heat of fusion?

Answer 22

The latent heat of fusion is the energy needed to change between solid and liquid states, breaking the bonds that hold the particles in place.

not to be confused with SPECIFIC latent heat of fusion

Question 23

What is the latent heat of vaporisation?

Answer 23

The latent heat of vaporization is the energy needed to pull the particles of a liquid apart completely, changing between liquid and gas states.

not to be confused with SPECIFIC latent heat of vaporisation

Question 24

What are the three gas laws?

Answer 24

Boyle’s Law: pV = constant
Pressure Law: p / T = constant
Charles’s Law: V = kT

Question 25

State Boyle’s Law

Answer 25

At a constant temperature the pressure and volume of a gas are inversely proportional. pV = k

Question 26

State Pressure Law

Answer 26

At a constant volume the pressure p of a gas is directly proportional to its absolute temperature. For an ideal gas p = 0 at T = -273.15 deg C

Question 27

State Charles’s Law

Answer 27

At a constant pressure the volume of a gas is directly proportional to its temperature

Question 28

How do you investigate Boyle’s Law? (Equipment already set up)

Answer 28

1) Measure the volume of air and pressure when the system is at atmospheric pressure
2) Gradually increase the pressure in regular intervals, writing down pressure and volume of the air
3) Multiplying p and V at any point together should give a constant

Question 29

How do you investigate Pressure Law? (Equipment already set up)

Answer 29

1) Record temperature of the water and pressure on the gauge
2) Heat the water for a few minutes, stir to ensure uniform temperature. Leave for a bit so air heats up. Record temperature and pressure on gauge
3) Repeat until water boils
4) Repeat twice with fresh cool water
5) Plot graph of p against T and draw line of best fit
6) Can use line of best fit to estimate absolute zero

Question 30

State the equation of state of an ideal gas

Answer 30

pV = nRT
R - Molar gas constant

pV = NkT

Question 31

When does the equation of state work best for real gases?

Answer 31

Low pressures and high temperatures

Question 32

What is the setup for the Boyle’s law investigation?

Answer 32

Sealed measuring tube with air in it and oil at the bottom, a pump and pressure gauge used to increase pressure in the tube

Question 33

What is the setup for the Determining absolute Zero experiment?

Answer 33

• Sealed flask of air immersed in a water bath.
• Flask connected to a pressure gauge
• Thermometer in water bath
• Heater under water bath

Question 34

How do you calculate the Boltzmann constant?

Answer 34

k = R/Nₐ

Question 35

What is the equation linking moles of a substance and particle count

Answer 35

N = nNₐ

Question 36

Give 4 assumptions obeyed by an ideal gas

Answer 36

1) The gas contains a large number of particles
2) The particles move rapidly and randomly
3) Collisions between particles or particle and wall are perfectly elastic
4) No forces act between particles except in collisions
5) The volume of the particles is negligible compared to the volume of the gas
6) The duration of each collision is negligible compared to the duration between collisions

Question 37

What four things does the pressure exerted by a gas depend on and why?

Answer 37

Volume - smaller volume, less distance to travel, more frequent collisions
Number of particles - More particles, more collisions per second
mass - Force is proportional to mass, more force = more pressure
speed of particles - faster particles have more momentum so exert more force

Question 38

Explain pressure

Answer 38

Particles of a gas move around with constant random motion. When a particle hits a wall it exerts a tiny force on the wall and rebounds in the opposite direction. However a gas has millions of particles so a significant number will be colliding with each wall of the container and the collisions will be spread over the surface of the wall. This results in a large combined force - the pressure.

Question 39

How does the speed distribution of gas particles change as the temperature of the gas increases?

Answer 39

• Average particle speed increases
• The maximum particle speed increases
• The distribution curve becomes more spread out with a less sharp peak

Question 40

Derive the equation for kinetic energy of a gas particle?

Answer 40

1) pV = NkT
2) From kinetic theory pV = 1/3 Nmc²
3) Cancel N to get kT = 1/3mc²
mc² = 3kT
4) 1/2 mc² is the average kinetic energy of an individual particle so multiply by 1/2 to get:
E = 3/2 kT