Topic 3: Thermal physics

Question 1

Define: temperature (2)

Answer 1

1. A measure of the average kinetic energy of the particles within a substance.
2. If two substances have the same temperature, then their molecules have the same average kinetic energy.

Question 2

Define: thermal contact

Answer 2

Two objects are in thermal contact if it is possible for thermal energy to be transferred directly from one object to the other as a result of the temperature difference.

Question 3

Define: heat/thermal energy

Answer 3

1. The energy transfer that results when two objects are in thermal contact with each other
2. Thermal energy always flows from hot to cold
3. The temperature difference between the two objects will determine the direction of the natural transfer of thermal energy
4. Heat is the non-mechanical transfer of energy between a system and its surroundings.

Question 4

Explain the concept of thermal equilibrium.

Answer 4

1. The net transfer of thermal energy is always from the object with the highest temperature to the object with the lowest temperature
2. Therefore, the hottest object will cool down and the coolest object will warm up until they both reach the same temperature
3. At this point the two objects are said to be in thermal equilibrium
4. If two objects are in thermal contact and are both at the same temperature, it does not mean that no thermal energy is being transferred from one object to the other, but that there is no overall transfer of energy
5. For this to be the situation, the objects must be in thermal equilibrium.

Question 5

State the relationship between the Kelvin and Celsius scales of temperature

Answer 5

temperature in Kelvin = temperature in ºC +273 temperature in ºC = temperature in Kelvin -273

Question 6

Define: internal energy

Answer 6

The total potential energy and kinetic energy of the molecules/atoms/particles OR the amount of energy stored in a substance.

Question 7

Macroscopic concept of temperature

Answer 7

Ttemperature is the measure of the average kinetic energy of molecules in a substance.

Question 8

Macroscopic concept of internal energy.

Answer 8

Sum of its molecular kinetic and potential energies. Molecules have kinetic energy due to their motion, both vibrational and translational. Molecules have potential energy due to the intermolecular forces.

Question 9

Macroscopic concept of heat/thermal energy.

Answer 9

The transfer or change in thermal energy that is due only to a difference of temperature

Question 10

What can adding energy to a substance result in?

Answer 10

1. increase in temperature (EK increases)
2. change in state (EP increases)
3. energy loss by the substance at an equal rate (thermal equilibrium)
4. chemical change (burn, decompose, etc.)

Question 11

What does the rate of temperature change depend on? (3)

Answer 11

1. the rate at which net energy is added
2. the mass of a substance
3. the specific heat capacity of a substance

Question 12

What does the rate at which state can change depend on? (2)

Answer 12

1. the rate at which net energy is added
2. the specific latent heat of fusion/vaporisation of the substance

Question 13

Define: kinetic theory

Answer 13

Molecules are arranged in different ways depending on the phase of the substance (solid/liquid/gas)

Question 14

Define: conduction (4)

Answer 14

1. Thermal energy is passed on by molecules colliding with their neighbouring molecules – neighbouring molecules therefore move more, increasing their thermal energy and so on
2. Is the only type of heat transfer that can occur in solids and also occurs in fluids
3. Requires matter
4. Metals are good conductors of heat as they contain free electrons that assist the passage of heat through the substance.

Question 15

Define: convection (3)

Answer 15

1. Thermal energy is moved by molecules containing thermal energy moving
2. Requires matter
3. Is usually in the upwards direction, since hotter particles move more, take up more space and make that part of the fluid less dense.

Question 16

Define: radiation (3)

Answer 16

1. Thermal energy is transferred from a hot body by infrared radiation
2. This radiation can then be absorbed by another body, whose internal energy would then increase. Is most effective in a vacuum
3. All objects radiate heat.

Question 17

Define: mole

Answer 17

The amount of a substance that contains as many elementary entities as the number of atoms in 12 g of the isotope carbon-12.

Question 18

Define: molar mass

Answer 18

The mass of one mole of a substance. If an element has a certain mass number, A, then the molar mass will be A grams.

Question 19

Define: Avogadro constant

Answer 19

The number of atoms in 0.012 kg of carbon-12 (¹²C).

6.02 X 10²³mol^-1

Question 20

Define: specific heat capacity

Answer 20

The energy required to increase 1 kg of a substance by 1 K.

Question 21

What is the defining equation for specific heat capacity?

Question 22

Why do different substances have different specific heat capacities?

Answer 22

1. They contain different numbers of molecules per kilogram
2. The chemical (bonding) properties are different for different substances

Question 23

Define: thermal capacity

Answer 23

Energy required to raise an object’s temperature by 1 K.

Question 24

Symbol for thermal capacity

Answer 24

mc

Question 25

Standard index measurement for thermal capacity

Answer 25

J K^-1

Question 26

Standard index measurement for specific heat capacity

Answer 26

J kg^-1 K^-1

Question 27

Explain the physical characteristics of a solid. (2)

Answer 27

1. Fixed volume and fixed shape because molecules are held in position by bonds. However, these bonds are not absolutely rigid 2. The molecules vibrate around a mean (average) position; the higher the temperature, the greater the vibration.

Question 28

Explain the physical characteristics of a liquid.

Answer 28

1. Fixed volume but not a fixed shape because molecules are vibrating but are not completely fixed in position 2. There are strong forces between the molecules, which keeps them close to one another; however, they are free to move around each other.

Question 29

Explain the physical characteristics of a gas.

Answer 29

1. will expand to fill its container because the molecules are not fixed in position 2. Forces between the molecules are very weak.

Question 30

Describe and explain the process of phase changes in terms of molecular behaviour

Answer 30

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Question 31

Explain what happens when a substance is being heated whilst in a solid state.

Answer 31

1. energy needed = m c Δt 2. Increased energy results in an increase in the kinetic energy of the particles 3. The greater average separation of the particles means that there is also a slight increase in their potential energy.

Question 32

Explain what happens when a substance changes phase from a solid to a liquid

Answer 32

1. energy required = mL 2. Results in an increase in the potential energy of the particles to a point where the attractive forces are weak enough to allow translational movement of the particles

Question 33

Explain what happens when a substance is being heated whilst in a liquid state.

Answer 33

1. energy needed = m c Δ t 2. Increased energy results in an increase in the kinetic energy of the particles 3. The greater average separation of the particles means that there is also a slight increase in their potential energy.

Question 34

Explain what happens when a substance changes phase form a liquid to a gas.

Answer 34

1. energy needed = m L 2. Results in an increase in the potential energy of the particles to a point where there is no longer attractive forces between them. The potential energy of the particles is now zero. If the gas is heated further, the particles move faster, their kinetic energies are increased and the temperature continues to increase.

Question 35

What happens to the temperature of a substance when it changes phase?

Answer 35

The temperature remains constant even though thermal energy is still being transferred.

Question 36

Distinguish between evaporation and boiling

Answer 36

**Evaporation:** * A process in which a substance changes state from liquid to gas without boiling * Slow * No bubbles formed * Takes place only at the exposed surface * Occurs at all temperatures * Energy supplied by surroundings * A liquid evaporates continuously **Boiling** * A process in which a substance changes state from liquid to gas * Fast * Bubbles are formed * Occurs throughout the liquid * Occurs at a definite temperature; the boiling point * Energy supplied by a source

Question 37

What does the rate of evaporation depend on? (5)

Answer 37

1. the temperature of the liquid 2. the temperature of the surroundings 3. the amount of vapour already in the surroundings (if the gas above the liquid is saturated with vapour, no further net evaporation can take place – equilibrium is reached – rate of evaporation = rate of condensation) 4. the surface area of the liquid 5. the nature of the liquid

Question 38

Define: specific latent heat

Answer 38

The amount of energy associated with the phase change.

Question 39

Define: specific latent heat of fusion.

Answer 39

The energy required to change the phase of a substance from a solid to a liquid/the energy that is released when a substance changes from a liquid to a solid. It is the energy needed to melt 1kg of a substance/the energy released when 1 kg of the same substance, in its liquid phase, freezes at constant temperature.

Question 40

What is the defining equation for specific latent heat?

Answer 40

 (heat change = energy needed to melt/boil OR energy released when freezing/condensing)

Question 41

Define: specific latent heat of vaporisation

Answer 41

The energy required to change the phase of a substance from a liquid to a gas/the energy released when a substance changes from a gas to a liquid. It is the energy needed to vaporise 1 kg of a substance/the energy released when 1 kg of the same substance, in its gas phase, condenses.

Question 42

Define: pressure

Answer 42

Force per unit area.

Question 43

What is the defining equation for pressure?

Answer 43

P = F / A Where: P is pressure in Pa F is force in N A is area in m s^-2

Question 44

Define: pressure law (macroscopically)

Answer 44

At a constant volume, the pressure of a gas is proportional to its temperature in K.

Question 45

Define: pressure law (microscopically) (6)

Answer 45

1. if the temperature of a gas increases, the molecules have more average kinetic energy 2. fast moving molecules will have a greater change of momentum when they hit the walls of the container 3. thus the microscopic force from each molecule will be greater 4. the molecules are moving faster so they hit the walls more often 5. the total force on the wall goes up 6. the pressure increases

Question 46

How is pressure increased (3)

Answer 46

1. increasing the speed of the particles 2. increasing the mass of the particles 3. decreasing the area of the collision surface, by decreasing the volume of the gas (whilst maintaining the number of particles)

Question 47

State the assumptions of the kinetic model of an ideal gas.

Answer 47

1. Newton's laws apply to molecular behaviour 2. there are no intermolecular forces 3. the molecules are treated as points – they occupy no space 4. the molecules are in random motion – they tend to spread out evenly in a container 5. the collisions between the molecules are elastic (no energy is lost) – total kinetic energy remains constant 6. there is no time spent on these collisions 7. the intermolecular potential energy of the molecules of the gas is constant

Question 48

Define: temperature (ideal gas)

Answer 48

Measure of the average kinetic energy of the molecules of an ideal gas. Therefore, if gas molecules are made to increase in speed, the gas will increase in temperature. As the temperature of the gas is lowered, the molecules will move slower. At absolute zero, the molecules will have zero kinetic energy.

Question 49

Explain the macroscopic behaviour of an ideal gas in terms of a molecular model. (6)

Answer 49

1. When a molecule bounces off the walls of a container its momentum changes (due to the change in direction – momentum is a vector) 2. There must have been a force on the molecule from the wall (Newton II) 3. There must have been an equal and opposite force on the wall from the molecule (Newton III) 4. Each time there is a collision between a molecule and the wall, a force is exerted on the wall 5. The average of all the microscopic forces on the wall over a period of time means that there is effectively a constant force on the wall from the gas 6. This force per unit area of the wall is pressure

Question 50

Define: Charles' Law (macroscopically)

Answer 50

At a constant pressure, the volume of a gas is proportional to its temperature in Kelvin.

Question 51

Define: Charles' Law (microscopically) (5)

Answer 51

1. a higher temperature = faster moving molecules 2. faster moving molecules hit the walls with a greater microscopic force 3. if the volume of the gas increases, then the rate at which these collisions take place on a unit area of the wall must decrease 4. the average force on a unit area of the wall can thus be the same 5. thus the pressure remains the same

Question 52

Define: Boyles' Law (macroscopically)

Answer 52

At a constant temperature, the pressure of a gas is inversely proportional to its volume.

Question 53

Define: Boyles' Law (microscopically) (4)

Answer 53

1. the constant temperature of gas means that the molecules have a constant average speed 2. the microscopic force that each molecule exerts on the wall will remain constant 3. increasing the volume of the container decreases the rate with which the molecules hit the wall – average total force decreases 4. if the average total force decreases the pressure decreases