Module 5: Chapter 14 - Thermal Physics

Question 1

What is the triple point of a substance?

Answer 1

The specific temperature and pressure where the 3 phases of matter of that substance can exist in thermal equilibrium

Question 2

What is thermal equilibrium?

Answer 2

Thermal equilibrium is a state in which here is no net flow of thermal energy between the objects involved, that is, objects in thermal equilibrium must be at the same temperature. There would be no exchange of energy when in thermal contact

Question 3

What is a phase of matter?

Answer 3

Its state (solid, liquid, or gas)

Question 4

What is thermal contact?

Answer 4

Two objects are in thermal contact with each other if energy can be exchanged between them due to a temperature difference

Question 5

What is the zeroth law of thermodynamics?

Answer 5

If objects A and B are separately in thermal equilibrium with a third object C, then A and B are in thermal equilibrium with each other

Question 6

What is the first law of thermodynamics?

Answer 6

Energy cannot be created or destroyed; it can only be redistributed or cahnged from one form to another

Question 7

What is the second law of thermodynamics?

Answer 7

The entropy of any isolated system not in thermal equilibrium almost always increases

Question 8

What is the third law of thermodynamics?

Answer 8

The entropy of a system approaches a constant value as the temperature approaches zero

Question 9

What is temperature?

Answer 9

Temperature is the property that determines whether an object is in thermal equilibrium with other objects. It is related to the average kinetic energy of the molecules in a substance

Question 10

What is a thermometer?

Answer 10

A device used to measure temperatures

Question 11

What is the basic operation principle thermometers are based on?

Answer 11

Some physical property of a system changes as the system’s temperature changes, such as:
* The volume of a liquid
* The dimensions of a solid
* The pressure of a gas at constant volume
* The volume of a gas at constant pressure
* The electrical resistance of a conductor

Question 12

Why may mercury and alcohol thermometers be used in different conditions?

Answer 12

• A mercury thermometer cannot be used below the freezing point of mercury (-39 C)
• An alcohol thermometer cannot be used above the boiling point of alcohol (85 C)

Question 13

What is the density of water?

Answer 13

1000 kg m⁻³

Question 14

What is 0°C defined as?

Answer 14

A mixture of water and ice at standard atmospheric pressure is defined to have a temperature of 0°C, it is also known as the ice point of water

Question 15

What is 100°C defined as?

Answer 15

A mixture of steam and water at standard atmospheric pressure is defined to have a temperature of 100°C, it is also known as the steam point of water

Question 16

What is absolute 0?

Answer 16

absolute 0 = 0K
This is the lowest possible temperature, the temperature at which substances have minimum internal energy

Question 17

What is 0K in °C?

Answer 17

-273.15°C

Question 18

What is the degree size for kelvin compared to celcius?

Answer 18

It is the same, therefore ΔT is the same if measured in celcius or kelvin

Question 19

What is internal energy?

Answer 19

Internal energy is the sum of the randomly distributed kinetic and potential energies of all the molecules in the substance. Internal energy therefore depends on the temperature and the amount of substance present.

Question 20

What is heat?

Answer 20

The internal energy of a substance

Question 21

What is specific heat capacity?

Answer 21

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

Question 22

What are the units of specific heat capacity?

Answer 22

• J kg⁻¹ °C⁻¹
• J kg⁻¹ K⁻¹

Question 23

What is the specific heat capacity equation?

Answer 23

• ΔE = mc ΔT
• ΔE = mc Δθ

ΔE = Energy change in J, m = mass in kg, c = specific heat capacity, ΔT/Δθ = change in temperature (in K or °C)

Question 24

What is the method of mixtures?

Answer 24

If 2 objects are placed in thermal contact with each other, they will end up in thermal equilibrium at some intermediate temperature between the 2 initial starting temperatures of the objects (assuming no significant heat loss to any other objects/surroundings)

Question 25

What is the equation for the method of mixtures?

Answer 25

As energy gained by cooler object = energy lost by hotter object:
* m₁c₁Δθ₁ = m₂c₂Δθ₂

Question 26

Explain why there is no increase of temperature as a substance changes state:

Answer 26

As there is no increase in temperature, there is no change in the kinetic energy of the molecules in the subtance. However, the internal energy must be increasing as energy is being supplied in the form of heat. The extra energy must be changing the potential energy of the molecules in the substance. This increase in potential energy is due to the work done in breaking the forces of attraction between molecules and in moving them apart. There is an increase in the electrostatic potential energy of the molecules

Question 27

What is specific latent heat?

Answer 27

The quantity of thermal energy required to change the phase of 1kg of a substance

Question 28

What is specific latent heat of fusion?

Answer 28

The quantity of thermal energy required to change 1kg of a substance from a liquid to a solid or from a solid to a liquid

Question 29

What is the specific latent heat of vaporisation?

Answer 29

The quantity of thermal energy required to change 1kg of a substance from a liquid to a gas or from a gas to a liquid

Question 30

What is the equation for specific latent heat?

Answer 30

E = mL

E = energy change, m = mass, L = specifc latent heat

Question 31

What is Lf?

Answer 31

The specific latent heat of fusion

Question 32

What is Lᵥ?

Answer 32

The specific latent heat of vaporisation

Question 33

What are the units for specific latent heat?

Answer 33

J kg⁻¹

Question 34

A container of water at 18°C containing 1500g of water is heated until it boils dry. How much energy is required to make this happen?

Answer 34

3,891,600 J

Question 35

What is the specific heat capacity of water?

Answer 35

4200 J kg⁻¹ K⁻¹

Question 36

What is the specific latent heat of fusion for water?

Answer 36

330,000 J kg⁻¹

Question 37

What is the specific latent heat of vaporisation for water?

Answer 37

2.26x10⁶ J kg⁻¹

Question 38

Draw a typical heating curve for a substance being heated:

Answer 38

Question 39

What is the absolute temperature scale?

Answer 39

A scale for measuring temperature based on absolute zero and the triple point of pure water, with graduations equal in size to those of the celcius scale; unit kelvin

Question 40

What is the thermodynamic temperature scale?

Answer 40

The absolute temperature scale

Question 41

What is the triple point of water?

Answer 41

• Temperature = 0.01°C / 273.16K
• Pressure = 0.61kPa

Question 42

What is the kinetic model?

Answer 42

The kinetic model describes all substances as made of atoms, ions, or molecules, arranged differntly depending on the phase of the substance

Question 43

How does the kinetic model describe solids?

Answer 43

A solid is made up of particles arranged in a regular 3-dimensional structure. There are strong electrostatic forces of attraction between the particles. Although the particles can vibrate, they cannot move out of their positions in the structure. When a solid is heated, the particles gain energy and vibrate more and more vigorously. Eventually, they may break away form the solid structure and become free to more around, however when this happens the solid has turned into a liquid; it has melted.

Question 44

How does the kinetic model describe liquids?

Answer 44

In a liquid the particles are free to move around. A liquid therefore flows easly and has no fixed shape. There are still forces of attraction between the particles. When a liquid is heated, some of the particles gain enough energy to break away from the other particles. The particles which escape from the body of the liquid become a gas

Question 45

How does the kinetic model describe gases?

Answer 45

In a gas, the particles are far apart. There are almost no forces of attraction between them. The particles move about at high speed. Because the particles are so far apart, a gas occupies a very much larger volume than the same mass of liquid. The molecules collide with the container. These collisions are responsible for the pressure which a gas exerts on its container

Question 46

What is brownian motion?

Answer 46

Brownian motion is the continuous random motion of small particles suspended in a fluid, visible under a microscope

Question 47

Explain brownian motion:

Answer 47

Brownian motion is the result of collisions between the suspended particle and the particles of the fluid. These collisions are elastic and results in a transfer of momentum from the fluid molecules to the particle, causing it to continuously move in a random motion

Question 48

How does the state of a substance affect its density?

Answer 48

The spacing between the particles in a substance in different phases affects the density of the substance. In general, a substance is most dense in its solid state and least dense in its gaseous state.

Water is an exception, being most dense as a liquid due to the crystalline patten the solid is held in

Question 49

Compare the electrostatic potential energy (potential energy) in different states of matter:

Answer 49

The electrostatic potential energy is lowest in solids, higher in liquids, and at its highest (0J) in gases:
* In a solid the electrostatic forces between atoms or molecules are very large, so the electrostatic potential energy has a large negative value (It is negative to indicate energy must be supplied to break the atomic or molecular bonds)
* In a liquid the forces are not quite as strong and therefore the electrostatic potential energy is not as negative
* In a gas the electrostatic forces between particles is negligible and therefore the electrostatic potential energy is zero

Question 50

Explain how you can determine specific heat capacity using this equipment:

Answer 50

• The energy transferred to the substance is given by E = IVt. Therefore the specific heat capacity can be determined using the equation: c = IVt/mΔθ
• You can also plot a graph of temperature against time: