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IB Physics HL Core > Topic 3: Thermal physics > Flashcards

Topic 3: Thermal physics Flashcards

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1
Q

Define: temperature (2)

A

A measure of the average kinetic energy of the particles within a substance of an object.

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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; flows from hot to cold; temperature difference between the two objects will determine the direction of the natural transfer of thermal energy
  2. 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. Therefore, the hottest object will cool down and the coolest object will warm up until they both reach the same temperature. At this point, the two objects are said to be in thermal equilibrium
    * 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. 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 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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8
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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9
Q

What can adding energy to a substance result in?

A
  1. increase in temperature (KE increases)
  2. change in state (PE increases)
  3. energy loss by the substance at an equal rate (thermal equilibrium)
  4. chemical change (burn, decompose, etc.)
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10
Q

What does the rate of temperature change depend on?

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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11
Q

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

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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12
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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13
Q

Define: conduction

A
  1. Two solids of different temperatures come in contact with one another, thermal
    energy is transferred from the hotter object to the cooler object *sometimes fluid; requires matter; metals are good conductors of heat as they contain free electrons that assist the passage of heat through the substance.
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14
Q

Define: convection

A

A fluid is heated causing the movement of groups of atoms or molecules due to
variations in density

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15
Q

Define: radiation

A

All bodies (objects), no matter what temperature, emit a spectrum of thermal radiation in the form of electromagnetic waves
*This radiation can then be absorbed by another body, whose internal energy would then increase. Is most effective in a vacuum

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16
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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17
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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18
Q

Define: Avogadro constant

A

The number of atoms/particles in 12g of carbon-12

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19
Q

Define: specific heat capacity

A

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

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20
Q

What is the defining equation for specific heat capacity?

A

c = Q/(mΔT)

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21
Q

Why do different substances have different specific heat capacities?

A
  1. Different # of molecules per kilogram
  2. Different chemical bonding properties
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22
Q

Define: thermal capacity

A

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

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23
Q

Symbol for thermal capacity

A

mc

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24
Q

Standard index measurement for thermal capacity

A

J K-1

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25
Q

Standard index measurement for specific heat capacity

A

J kg-1 K-1

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26
Q

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

A

During a change,thermal energy is transferred to or from a substance
This energy transfer does not change the temperature of the substance undergoing the phase change

The thermal energy provided (or removed) does not affect the kinetic energy of the molecules within the substance, only the potential energy (i.e.the spacing between the atoms or molecules)

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27
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.

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28
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
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29
Q

What does the rate of evaporation depend on?

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
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30
Q

Define: specific latent heat

A

The amount of energy associated with the phase change.

31
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.

32
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.

33
Q

Define: pressure

A

Force per unit area.

34
Q

Define: pressure law (macroscopically)

A

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

35
Q

Define: pressure law (microscopically)

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
36
Q

How is pressure increased

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)
37
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
38
Q

Define: temperature (ideal gas)

A

Measure of the average random kinetic energy of the molecules of an ideal gas.

39
Q

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

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
40
Q

Define: Charles’ Law (macroscopically)

A

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

41
Q

Define: Charles’ Law (microscopically)

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
42
Q

Define: Boyles’ Law (macroscopically)

A

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

43
Q

Define: Boyles’ Law (microscopically)

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
44
Q

Define: thermal equilibrium

A

Thermal equilibrium is when an object is at the same temperature as its surroundings (i.e. there is no heat transfer)

45
Q

How can thermal energy be transferred?

A
  • Conduction
  • Convection
  • Radiation
46
Q

What is a thermometric property?

A

A property that varies with temperature e.g. resistance in a wire

47
Q

What is absolute zero in Kelvin and Celsius?

A

0 K

-273.16 ºC

48
Q

Why can you not have negative Kelvin?

A

As 0 K is absolute zero, theoretically there is no zero kinetic energy in the particles.

You cannot go lower than zero energy, so therefore 0 K is the minimum.

49
Q

What is the triple point of water and what is the temperature of it?

A

The triple point of water is a unique temperature and pressure which all water can exist as liquid water, ice and water vapour).

It is at 273.16 K

50
Q

How is internal energy weighted in a solid?

A

The total internal energy generally consists of roughly equal amounts of potential and kinetic energy

51
Q

How is internal energy weighted in a gas?

A

The total internal energy generally consists mainly of kinetic energy as the forces between the particles are so small.

52
Q

What is an ideal gas?

A

A gas that obeys the gas laws under all conditions.

53
Q

Do real gases always obey gas laws?

A

Real gases do not always obey gas laws, but they approximate well to an ideal gas so long as the pressure is little more than normal atmospheric pressure.

54
Q

What conditions are there for Boyle’s law?

A
  • A fixed mass of gas
  • Constant temperature/isothermal
55
Q

What does Boyle’s law curve look like?

A

An inverse graph (1/x) of pressure vs. volume.

By increasing the constant temperature, the graph moves “out” and has a less “pointy” shape.

56
Q

At what temperature is the volume of a gas zero?

A

At -273°C

(from Charles’ law)

57
Q

What is Charles’ law written mathematically?

A
  • V* ∝ T
  • V/T* = constant
58
Q

What conditions are there for Charles’ law?

A
  • A fixed mass of gas
  • Constant pressure
59
Q

What does Charles’ law curve look like?

A

A positive linear curve that intercepts absolute zero between volume (y) and temperature (x)

60
Q

What is the pressure law written mathematically?

A
  • p* ∝ T
  • p/T* = constant
61
Q

What conditions are there for the pressure law?

A
  • A fixed mass of gas
  • Constant volume
62
Q

What does the pressure law curve look like?

A

A positive linear curve that intercepts absolute zero.

63
Q

What is the relationship between the Avogadro’s constant and the volume of the gas?

A

The number of particles in a gas, at constant temperature and pressure, is directly proportional to the volume of the gas.

64
Q

How can the NA and volume relationship be written mathematically?

A
  • n* ∝ V
  • n/V* = constant
65
Q

What is the ideal gas formula in both its forms?

A

(pV)/(nT) = R

or

pV = nRT

where p is pressure in Pascals, V is the volume in cubic metres, T is the temperature in Kelvin, n is number of moles, R is the ideal gas constant in J K-1 mol-1

66
Q

What is molar mass?

A

The mass of one mole of some molecules.

So one mol of nitrogen atoms is 14.01 g.

But one mol of nitrogen molecules, which are diatomic, will be 28.02 g.

67
Q

What is diffusion?

A

The dispersion of gas molecules down the concentration gradient i.e. from high concentration to low concentration.

68
Q

For the kinetic theory of gases, what are the 9 key assumptions it makes?

A
  • A gas consists of a large number of identical tiny particles called molecules which are in constant random motion.
  • The number is large enough for statistical averages to be made.
  • Each molecule has negligible volume when compared to the gas’ volume as a whole.
  • At any instant, as may molecules moving in one direction are moving in any other direction.
  • The molecules undergo perfectly elastic collisions between themselves and the walls of the container, during each collision, the momentum of each molecule is reversed.
  • There are no intermolecular forces between collisions (energy is entirely kinetic).
  • The duration of the collisions is negligible compared the time between collisions.
  • Each molecule produces a force on the wall of the container.
  • The forces of individual molecules will average out to produce a uniform pressure throughout the gas (ignoring the effect of gravity).
69
Q

An alternative way of writing the ideal gas formula

A

pV = NkBT

as the ideal gap formula is: pV = nRT

and nR = NkB

we can substitute it in, where N is number of molecules and KB is the Boltzmann constant

70
Q

A gas with low temperature and low pressure has the same volume as a gas with … temperature and … pressure.

What law?

A

high temperature and high pressure

Pressure law

71
Q

A gas with low temperature and low volume has the same pressure as a gas with … temperature and … volume.

What law?

A

high temperature and high volume.

Charles’ law

72
Q

A gas with low volume and high pressure has the same temperature as a gas with … volume and … pressure.

What law?

A

high volume and low pressure

Boyle’s law

73
Q

Energy changes in phase changes

A

IB Physics HL Core (13 decks)

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