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rsfc physics 2023-2025 > 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)

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

20
Q

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

A

Calculate the area under the curve

21
Q

What is the general equation for pressure?

A

P = F / A

(Pressure = Force / Area)

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
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
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
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 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

When would you use this equation?

A

To calculate the mass flowing per kg of a fluid

37
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)

38
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.

39
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.

40
Q

What is wrong with this?

A

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

41
Q

For the Temperature graph of heating up a solid, how is the energy of the molecules changing at each stage?

A

Kinetic Energy changes when temperature changes

Potential Energy changes when it changes states (solid->liquid or liquid->gas )

42
Q

When heat melts a solid why doesn’t the kinetic enrgy of the molecules increase?

A

The heat energy increases the potential energy of the molecules (breaking some of the intermolecular bonds)

43
Q

When heat melts a liquid why doesn’t the kinetic enrgy of the molecules increase?

A

The heat energy increases the potential energy of the molecules (breaking the rest of the intermolecular bonds)

44
Q

What is internal energy?

A

KE + PE for a substance

45
Q

What is the internal energy for a gas?

A

The total KE of all molecules in gas

46
Q

Why is the internal energy for a gas = KE?

A

PE = 0 because all intemrolecular bonds broken for gas

rsfc physics 2023-2025 (25 decks)

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