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IB Physics > Topic 3: Thermal physics > Flashcards

Topic 3: Thermal physics Flashcards

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1
Q

Define: temperature (2)

A
  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.
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2
Q

Define: thermal contact

A

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.

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3
Q

Define: heat/thermal energy

A
  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.
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4
Q

Explain the concept of thermal equilibrium.

A
  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.
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5
Q

State the relationship between the Kelvin and Celsius scales of temperature

A

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

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6
Q

Define: internal energy

A

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

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7
Q

Macroscopic concept of temperature

A

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

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8
Q

Macroscopic concept of internal energy.

A

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.

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9
Q

Macroscopic concept of heat/thermal energy.

A

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

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10
Q

What can adding energy to a substance result in?

A
  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.)
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11
Q

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

A
  1. the rate at which net energy is added
  2. the mass of a substance
  3. the specific heat capacity of a substance
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12
Q

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

A
  1. the rate at which net energy is added
  2. the specific latent heat of fusion/vaporisation of the substance
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13
Q

Define: kinetic theory

A

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

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14
Q

Define: conduction (4)

A
  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.
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15
Q

Define: convection (3)

A
  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.
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16
Q

Define: radiation (3)

A
  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.
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17
Q

Define: mole

A

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

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18
Q

Define: molar mass

A

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.

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19
Q

Define: Avogadro constant

A

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

6.02 X 1023mol-1

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20
Q

Define: specific heat capacity

A

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

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21
Q

What is the defining equation for specific heat capacity?

A
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22
Q

Why do different substances have different specific heat capacities?

A
  1. They contain different numbers of molecules per kilogram
  2. The chemical (bonding) properties are different for different substances
23
Q

Define: thermal capacity

A

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

24
Q

Symbol for thermal capacity

A

mc

25
Q

Standard index measurement for thermal capacity

A

J K-1

26
Q

Standard index measurement for specific heat capacity

A

J kg-1K-1

27
Q

Explain the physical characteristics of a solid. (2)

A
  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.
28
Q

Explain the physical characteristics of a liquid.

A
  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.
29
Q

Explain the physical characteristics of a gas.

A
  1. will expand to fill its container because the molecules are not fixed in position
  2. Forces between the molecules are very weak.
30
Q

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

A
31
Q

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

A
  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.
32
Q

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

A
  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
33
Q

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

A
  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.
34
Q

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

A
  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.
35
Q

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

A

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

36
Q

Distinguish between evaporation and boiling

A

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
37
Q

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

A
  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
38
Q

Define: specific latent heat

A

The amount of energy associated with the phase change.

39
Q

Define: specific latent heat of fusion.

A

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.

40
Q

What is the defining equation for specific latent heat?

A

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

41
Q

Define: specific latent heat of vaporisation

A

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.

42
Q

Define: pressure

A

Force per unit area.

43
Q

What is the defining equation for pressure?

A

P = F / A

Where:

P is pressure in Pa

F is force in N

A is area in m s<span>-2</span>

44
Q

Define: pressure law (macroscopically)

A

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

45
Q

Define: pressure law (microscopically) (6)

A
  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
46
Q

How is pressure increased (3)

A
  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)
47
Q

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

A
  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
48
Q

Define: temperature (ideal gas)

A

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.

49
Q

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

A
  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
50
Q

Define: Charles’ Law (macroscopically)

A

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

51
Q

Define: Charles’ Law (microscopically) (5)

A
  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
52
Q

Define: Boyles’ Law (macroscopically)

A

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

53
Q

Define: Boyles’ Law (microscopically) (4)

A
  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

IB Physics (9 decks)

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