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Henry Physics RSFC > Thermal > Flashcards

Thermal Flashcards

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1
Q

What is 1 mole?

A

A collection of 6.02×1023 molecules

(Avogadro’s constant)

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2
Q

What is the molar mass of a substance?

A

The mass of each mole (every 6.02×1023 molecules) (M)

Eg for He each mole has a mass of 4g

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3
Q

How do you calculate the molar mass of a compound eg NO2

A

Add up the nucleon numbers

(14+16+16=46gmol-1)

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4
Q

How do you calculate the number of molecules in a substance?

A

N = n × NA

(Number of molecules = moles × Avogadro’s constant)

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5
Q

What is the molecular mass and how is it calculated?

A

The mass of each molecule of the substance

m = M/N

(molecular mass = total mass / number of molecules)

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6
Q

How is the total mass of a substance calculated?

A

M = n × mr

(Total mass = moles × molar mass)

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7
Q

How do you convert a temperature from °C to K?

A

T(K) = T(°C) + 273

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8
Q

Define absolute zero

A

The point at which an ideal gas exerts no pressure

(0K, -273°C, molecules have no kinetic energy)

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9
Q

What is Boyle’s Law?

A

The pressure in a gas is inversely proportional to the volume it occupies

at a fixed temperature

and a fixed mass of gas

(P ∝ 1/V)

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10
Q

What does the P-V graph look like for an ideal gas?

A
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11
Q

How do you prove Boyle’s law graphically?

A

Plot a graph of P against 1/V

Should be a straight line passing through the origin

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12
Q

What is Charles’ Law?

A

Volume a gas occupies is directly proportional to the temperature of the gas

at a fixed pressure

and a fixed mass of gas

(V ∝ T)

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13
Q

How do you prove Charles’ law by graph?

A

Plot a graph of V against T

Should be a straight line passing through the origin

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14
Q

For an ideal gas, what does a graph of V against T(°C) look like?

A

Note: x-intercept represents absolute zero

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15
Q

What is the Pressure law?

A

The pressure of a gas is directly proportional to the temperature of the gas

at a fixed volume

and a fixed mass of gas

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16
Q

How do you prove the pressure law graphically?

A

Plot a graph of P against T

Should be a straight line passing through the origin

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17
Q

For an ideal gas, what does a graph of P against T(°C) look like?

A

Note: x-intercept is absolute zero

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18
Q

What is the ideal gas relationship?

A
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19
Q

When can you use the ideal gas relationship?

A

If the mass of the gas is constant

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20
Q

How do you calculate the work done compressing or expanding a gas?

A

Calculate the area under the curve

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21
Q

What is the general equation for pressure?

A

P = F / A

(Pressure = Force / Area)

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22
Q

How does a gas exert a pressure on a container?

A
  • The gas molecules collide with the container walls changing their momentum.
  • This creates a force on the molecule and the wall
  • Exerting a pressure
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23
Q

What are the 5 conditions for an ideal gas?

A
  1. Volume of the molecules must be much smaller than the volume of the gas itself
  2. The intermolecular forces are negligible
  3. The collision time of molecules with each other and the walls is much less than the time between them
  4. The collisions are elastic (no loss in KE)
  5. The molecules’ motion is random
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24
Q

How does Brownian motion explain the random motion of smoke?

A
  • Air molecules are moving randomly
  • They collide with the smoke changing momentum and exerting a force on the smoke particles
  • If at one moment there are more collisions on one side than the other
  • The smoke particle has a resultant force so accelerates in that direction
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25
Q

Explain Boyle’s Law using the molecular Kinetic Theory

A
  • When volume of container is decreased
  • More collisions per second
  • So total momentum change bigger (▲p)
  • So force exerted bigger
  • So pressure bigger (From P = F/A)
26
Q

Explain Charles’ Law using the molecular kinetic theory

A
  • When temperature is increased
  • Volume increases to increase the distance travelled between collisions
  • Molecules have greater kinetic energy but travel further so collision frequency stays same
  • Change in momentum (▲p) stays constant
  • So pressure is constant (P = F/A)
27
Q

Explain the Pressure law using the molecular kinetic theory

A
  • As temperature increases
  • The average kinetic energy of the molecules increases
  • Increasing the number of collisions per second with container walls
  • So greater change in momentum
  • Greater force and pressure exerted (P = F/A)
28
Q

How would you use this equation to work out the density of a gas?

A
29
Q

How do you calculate crms from a list of speeds?

A
  1. Square the speeds and add up
  2. Take a mean of the squares
  3. Square root the value
30
Q

How is cms calculated?

A

cms = (crms)2

31
Q

What are the units of cms?

A

[m2s-2]

32
Q

What does the maxwell-boltzmann distribution tell us about gases?

A

Molecules have a range of kinetic energies.

So temperature of the gas is a measure of the average kinetic energy.

33
Q

For these equations how do you calculate the internal energy of the gas?

A

Multiply each by the number of molecules of the gas.

34
Q

How do two objects brought into contact reach thermal equilibrium?

A
  • There is a net flow of thermal energy from the hotter object to the colder object
  • Until both objects are at the same temperature
  • And there is now no net flow of thermal energy
35
Q

Define specific heat capacity

A

The energy required to increase 1kg of a substance by 1K [Jkg-1K-1]

36
Q

Why does the temperature of a substance changing state not increase?

A

The thermal energy is used to break some of the intermolecular bonds (solid → liquid) or the rest of the intermolecular bonds (liquid → gas)

37
Q

Define specific latent heat of fusion

A

The energy required to change the state of 1kg of a solid to a liquid at its melting point.

38
Q

Define specific latent heat of vaporization

A

The energy required to change the state of 1kg of a liquid to a gas at its boiling point.

39
Q

What is wrong with this?

A

Haven’t considered the change of states. Need to break it into 3 equations:

40
Q

What is the approximate value for atmospheric pressure?

A

101kPa

41
Q

What is the approximate value for room temperature?

A

293K

42
Q

What must be the case when using Boyle’s, Charles’ & the Pressure law?

A

Temperature must be in Kelvin and the gas must be in an enclosed container

Therefore, number of molecules, moles and mass of gas is constant

43
Q

When should you use PV = nRT ?

A

If you are given / if you want the number of moles of gas

44
Q

When should you use PV = NkT ?

A

If you are given / if you want the number of molecules of gas

45
Q

What does the area under a P/V graph give?

A

Energy released expanding from V1 to V2 or energy needed to compress a gas from V2 to V1

46
Q

How do you derive the kinetic theory equation?

A
  1. Gas molecule in a box of dimensions a x b x c
  2. Time to collide with the right wall = 2a/v
  3. Each time molecule collides with left wall, change in momentum = p2 - p1 (so 2p1 = 2mv)
  4. F = p/t where p = 2mv and t = 2a/v, so F = 2mv2/2a or mv2/a
  5. Assuming ⅓ of molecules are travelling left and right, Ftotal = ⅓N * (mv2/a) so Nmv2 / 3a
  6. P = F/A, so Nmv2 /3aA with A meaning area of right wall
  7. a * A = Volume so P = Nmv2 / 3V
  8. Multiply by V so PV = Nmv2 / 3
  9. So PV = ⅓Nm(vavg)2
47
Q

How can the kinetic theory equation be expressed (3 ways)?

A

PV = ⅓Nm(Crms)2

Nm is number of molecules and mass of each.

This is the same as the total mass of gas (M), which can also be expressed as molar mass multiplied by number of moles (nmr)

48
Q

How does the Maxwell-Boltzmann diagram differ for hotter gases?

A

Peak moves rightwards and is lower (avg molecules have higher KE).

The range of KE’s increases (larger upper limit).

Still some molecules with small KE but fewer than in a cooler gas.

49
Q

What can be said of the area under a Maxwell-Boltzmann distribution?

A

The area under the curves remains constant.

The area corresponds to the number of molecules in the gas and so this cannot change as mass is constant.

50
Q

What are the two constants for the gas laws?

A

R (molar gas constant) - for moles
K (Boltzmann constant) - for molecules

51
Q

What are the symbols for molar mass and molecular mass?

A

M - molar mass
m - molecular mass

52
Q

What are the symbols for the number of moles and the number of molecules?

A

n - number of moles
N - number of molecules

53
Q

What does Na mean?

A

Avogadro’s constant

54
Q

What units of mass should you use for Thermal physics?

A

Kg, unless a question gives values of molar mass in grams in which case use grams

55
Q

Should you use the terms ‘hot’ or ‘cold’ when talking about energy transfer in thermal?

A

NO,
Use the terms ‘hotter’ and ‘colder’ to compare objects relative to each other

56
Q

What is the definition for thermal equilibrium?

A

The point at which there is no net transfer of energy between two objects when they are placed in contact

57
Q

What are the stages of reaching thermal equilibrium between two objects?

A
  • The object at the higher temperature will have a greater mean kinetic energy
  • When the objects are placed in contact, molecules from each object will collide
  • During the collisions, energy will be transferred from the molecules with high KE to the lower temperature material with low KE
  • This continues until the mean kinetic energies are equal (same temperature) and there is thermal equilibrium
58
Q

When do we use Q = mcΔθ?

A

When we have a substance changing temperature and we want to know the energy change

59
Q

When do we use Q = ml?

A

When a substance is changing state and we want to know the energy required to do so

60
Q

How do you deal with complex thermal changes (changes in state and temperature)?

A
  • Split the different stages of the process up (do the changing state equations separately to the changing temp)
  • Add the energy of each equation to give total energy input

Henry Physics RSFC (27 decks)

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