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Paper 3 2022 AQA Physics > Thermal (RP 8) > Flashcards

Thermal (RP 8) Flashcards

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1
Q

What are the three gas laws and their equations?

A
  • Boyle’s Law -> pV = constant
  • Charles’ Law -> V/T = constant
  • Pressure Law -> p/T = constant
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2
Q

What is an assumption of the 3 gas laws?

A

The mass of the gas is constant.

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3
Q

What is Boyle’s Law?

A
  • pV = Constant

* At a constant temperature, the pressure and volume of a gas are inversely proportional

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4
Q

Describe the graph for Boyle’s Law.

A
  • Pressure against volume plotted
  • Like a 1/x curve, depending on the temperature
  • The higher the temperature, the further the curve is from the origin.
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5
Q

How does temperature affect the graph for Boyle’s Law (p-V)?

A

The higher the temperature, the further the curve is from the origin.

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6
Q

What is Charles’ Law?

A
  • V/T = Constant

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

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7
Q

Describe the graph for Charles’ Law.

A
  • Volume against temperature plotted
  • Straight line with positive gradient
  • x-intercept is at -273°C or 0K
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8
Q

What is the Pressure Law?

A
  • p/T = Constant

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

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9
Q

Describe the graph for the Pressure Law.

A
  • Pressure against temperature is plotted
  • Straight line with positive gradient
  • x-intercept is at -273°C or 0K
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10
Q

What is an ideal gas?

A

One that obeys all 3 gas laws.

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11
Q

Describe an experiment to investigate Boyle’s Law.

A

1) Set up a marked sealed tube with air at the top and oil at the bottom.
2) Connect the tube to a Bourdon gauge (pressure gauge) and a pump to pump more oil in.
3) Increase the pressure from atmospheric pressure using the pump. Make sure to keep the temperature constant.
4) At each pressure, record the pressure (off the Bourdon gauge) and the volume of air (off the sealed tube) by using the radius and length to find v).
5) Repeat 2 more times and average.
6) Plot a graph of p against 1/V. This should give a straight line.

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12
Q

Describe an experiment to investigate Charles’ Law.

A

1) Set up a capillary tube that’s sealed at the bottom and that has a drop of sulphuric acid trapped halfway up the tube. This traps a column of air between the drop and bottom of the tube.
2) Place the tube next to a ruler in a beaker of near-boiling water. Also place a thermometer in the beaker.
3) As the water cools, record the height of the air and temperature at several temperatures.
4) Repeat 2 more times and average.
5) Plot a graph of height against temperature. This should give a straight line. Since height is proportional to volume, this proves Charles’ Law.

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13
Q

Remember to practise drawing out the setup for the gas law experiments.

A

Pg 111 of revision guide

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14
Q

What is relative molecular mass?

A

The sum of the mass of all the atoms that make up a molecule, relative to 1/12th the mass of a carbon-12 atom.

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15
Q

What is the relative mass of carbon-12?

A

12

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16
Q

What is the relative molecular mass of carbon dioxide? (1 carbon-12 molecule and 2 oxygen-16 molecules)

A

12 + 16 + 16 = 44

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17
Q

What is the molar mass of a gas?

A

The mass of one mole of that gas, usually in grams

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18
Q

What is Avogadro’s constant?

A
  • 6.02 x 10^23 mol^-1
  • It is the number of molecules in a mole

(It is the number of atoms in 12g of Carbon 12 - 6)

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19
Q

What is the symbol for Avogadro’s constant?

A

NA (where A is in subscript)

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20
Q

What can be said about the molar mass and the relative molecular mass?

A

They are the same value.

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21
Q

What is the symbol for the number of moles?

A

n

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22
Q

What is the equation for the number of molecules in a gas?

A

N = n x NA

Where:
• N = Number of molecules
• n = Moles
• NA = Avogadro’s constant

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23
Q

What is the ideal gas equation?

A

pV = nRT

Where:
• p = Pressure (Pa)
• V = Volume (m³)
• n = No. of moles
• R = Molar gas constant = 8.31J/mol/K
• T = Temperature (K)
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24
Q

How is the ideal gas equation formed?

A

By combining the 3 gas laws.

25
Q

What units for pressure, volume and temperature are used in the ideal gas equation?

A
  • Pressure -> Pa
  • Volume -> m³
  • Temperature -> K
26
Q

When does the ideal gas equation work best?

A

At low pressures and fairly high temperatures.

27
Q

What is Boltzmann’s constant?

A
  • Equal to R/NA

* It is the gas constant for one particle of gas (as opposed to R, which is the gas constant for one mole of gas)

28
Q

What is the symbol for Boltzmann’s constant?

A

k

29
Q

What is the difference between R and k?

A
  • R = The gas constant for one mole of a gas

* k = The gas constant for one particle of a gas

30
Q

What is the value of Boltzmann’s constant?

A

1.38 x 10⁻²³ J/K

31
Q

What is the equation of state?

A

pV = NkT

Where:
• p = Pressure (Pa)
• V = Volume (m³)
• N = No. of molecules of gas
• k = Boltzmann’s constant = 1.38 x 10⁻²³ J/K
• T = Temperature (K)
32
Q

What are the two equations for gases?

A
  • Ideal gas equation -> pV = nRT

* Equation of state -> pV = NkT

33
Q

What must happen in order for a gas to expand or contract at constant pressure?

A

Work must be done, either by the gas or on the gas.

34
Q

What type of energy transfer most commonly occurs when a gas expands or contracts?

A

Heat transfer - e.g. heating a gas filled balloon makes it expand

35
Q

What is the equation for the work done to expand a gas?

A

W = p x ΔV

Where:
• W = Work done (J)
• p = Pressure (Pa)
• ΔV = Change in volume (m³)

(NOTE: This only applies when pressure is constant.)

36
Q

How can the work done to expand a gas be found using a p-V graph?

A

It is the area under the graph.

37
Q

What is the equation for the pressure of a gas in a box in 3 directions?

A

pV = 1/3 Nmc²(bar)

Where:
• p = Pressure (Pa)
• V = Volume (m³)
• N = Number of molecules
• m = Mass of particle (kg)
• c²(bar) = Mean square speed (m²/s²)
38
Q

What does c²(bar) symbolise?

A
  • Mean square speed

* It is the average of the square speeds of all of the particles

39
Q

What is the root mean square speed?

A
  • The root of c²(bar)

* It is a measure of the typical speed of a particle

40
Q

What is the symbol for the root mean square speed?

A

c(rms)

Where rms is subscript

41
Q

Give the equation that links the rms speed and the mean square speed.

A

r.m.s. speed = √(mean square speed)

√ c²(bar) = c(rms)

42
Q

What are some of the defining features of an ideal gas?

A
  • Obeys the 5 simplifying assumptions of kinetic theory
  • Follow the 3 gas laws
  • Internal energy dependent only on the kinetic energy of the particles
43
Q

Why is the potential energy of an ideal gas 0?

A

There are no forces between particles except when they are colliding.

44
Q

When do real gases behave like ideal gases?

A

When the pressure is low and the temperature is high.

45
Q

How is the average kinetic energy of a gas related to the absolute temperature?

A

It is proportional.

46
Q

Which equation demonstrates the relationship between average kinetic energy of a gas and the absolute temperature?

A
  • 1/2 m(c(rms))² = 3/2 kT

* This shows that the kinetic energy is directly proportional to T.

47
Q

Why does Charles law happen (particles)?

A

Heat gas = particles gain KE = move more quickly = (at a constant pressure) they move further apart = volume of gas increases

48
Q

Why does Boyles Law happen (particles)?

A

If you reduce the volume, the particles will be closer together and collide more often = pressure increases

49
Q

What happens in the pressure law (particles)?

A

Heat gas = particles gain KE = Moves faster = (If volume is constant) particles collide with each other more often and at higher speeds = increasing pressure

50
Q

If the the molar mass and the relative molecular mass are the same then why is the molar mass of oxygen 32g and relative molecular mass 16.0?

A

Because there are two molecules in oxygen (it’s diatomic) so the molecular mass doubles to get the molar mass.

51
Q

If temperature is increased and volume is fixed, why does pressure increase (collisions and force)?

A

More collisions between molecule and the walls in a given amount of time.

On average, a collision will result in a larger change in momentum (higher average speed), and so exert a larger force on the walls on the container.

52
Q

If temperature is increased and pressure is fixed, why does volume increase? Why does pressure stay constant?

A

If volume is larger, there will be a longer time between molecule-wall collisions, so rate of change of momentum and therefore the force on the walls of the container will be reduced.

As volume increases, the surface area of the walls increases. Pressure is defined as force per unit area, increasing area stops the pressure from increasing.

53
Q

With the number of molecules and the average kinetic energy of the molecules, how do you find the total kinetic energy of the molecules in a gas?

A

Average kinetic energy (of a molecule) x total number of of molecules

54
Q

How do you derive the equation of the pressure of a single particle in an ideal gas?

A
55
Q

How do you derive the equation of the pressure of an ideal gas, from the equation of pressure of a particle in the gas?
What other equations can you get from this?

A
56
Q

Why aren’t perfect gases perfect? (which law don’t they follow perfectly and why?)

A

Perfect gases aren’t quite perfect as they don’t follows Boyles law perfectly:

Boyles law assumes that the particles don’t have any size or (more importantly) volume of their own.

57
Q

What are the simplified kinetic theory assumptions and how do you remember it?

A
58
Q

Fill in the blanks

A

Paper 3 2022 AQA Physics (3 decks)

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