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AQA A-level Physics > Thermal physics > Flashcards

Thermal physics Flashcards

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1
Q

What energies do particles of all substances have?

A

-potential energy
-kinetic energy

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2
Q

Why is there energy in substances?

A

kinetic energy is due to the speed of the particles
potential energy is due to the separation/bonds between the molecules

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3
Q

Define what temperature is

A

the average Ek of a substance

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4
Q

How can we convert between degrees and kelvin?

A

T= θ + 273

where:
T= kelvin
θ= °C

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5
Q

What do we mean by absolute zero?

A

the temperature at which the molecules in a substance have zero kinetic energy (0K or -273°C)

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6
Q

What do we mean by the internal energy of something?

A

the sum of all the kinetic and potential energies of all its particles

U= ∑Ep + ∑Ek

Where all components are measured in joules

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7
Q

What factors affect the internal energy for a system? (4)

A

-temperature ( ↑temp, ↑Ek)
-phase of matter ( ↑U when in gaseous state, low U when in solid state)
-intermolecular forces between particles
-random motion of particles

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8
Q

Explain how we can increase the internal energy of a system

A

-doing work on the system
this transfers energy to it e.g. increases the Ek or Ep

-adding heat to it
this increases the temperature which as a result increases the Ek leading to an overall increase in internal energy

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9
Q

Explain how we can decrease the internal energy of a system

A

-losing heat to the surroundings
-changing state from a solid to a liquid or a liquid to gas

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10
Q

Why does the internal energy of a substance change when changing state?

A

as the substance changes state the potential energy changes due to the changing bonds whilst the kinetic energy is kept constant

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11
Q

Draw a graph showing how internal energy varies with temperature. Include the different process that occur

A

https://img-aws.ehowcdn.com/877x500/s3-us-west-1.amazonaws.com/contentlab.studiod/3/11/c0e9e8124ce64f48987682be4aaea0b1.png

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12
Q

How can we work out the amount of energy required to change the temperature of a substance?

A

Q = mcΔθ

where:
Q= energy required (J)
m= mass (kg)
c= specific heat capacity (Jkg^-1°C^-1)
Δθ= change in temperature (°C)

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13
Q

What is specific heat capacity?

A

the amount of energy required to increase the
temperature of 1 kg of a substance by 1 °C/1 K, without changing its state

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14
Q

What are the different processes to change state?

A

melting = solid to liquid
vaporisation = liquid to gas
sublimation = solid to gas
freezing = liquid to solid
condensation = gas to liquid

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15
Q

What is specific latent heat?

A

the thermal energy required to change the state of 1 kg of mass of a substance without any change of temperature

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16
Q

What is the equation involving energy required to change the state of a substance and specific latent heat?

A

Q=mL

where:
Q= energy required to change state (J)
m= mass (kg)
L= specific latent heat (Jkg^-1)

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17
Q

What are the two types of latent heat? When are they used?

A

latent heat of fusion (melting)
-melting a solid
freezing a liquid

latent heat of vaporising (boiling)
-vaporising a liquid
-condensing a solid

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18
Q

Define latent heat of fusion

A

the thermal energy required to convert 1 kg of solid to liquid with no change in temperature

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19
Q

Define latent heat of vaporisation

A

the thermal energy required to convert 1 kg of liquid to gas with no change in temperature

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20
Q

What are some things to know when a substance is changing state?

A

-there is no change in temperature
-the potential energies of the particles change, but not their kinetic energies

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21
Q

What does it mean if bonds are being broken/stronger? Explain why

A

-the heat absorbed in melting and boiling causes the particles to move further apart by overcoming the intermolecular forces of attraction
-the heat released in freezing and condensation allows the particles to move closer together and the intermolecular forces of attraction become stronger

Ek is proportional to temperature (as temperature is the avg. Ek)
-if there is no change in temperature, there must be no change in kinetic energy either

22
Q

What are the ideal gas laws? What does it mean if they’re empirical?

A

the experimental relationships between pressure (P), volume (V) and temperature (T) of an ideal gas

meaning they are not based on theory but arose from observation and experimental evidence

23
Q

What is constant for all the ideal gas laws?

A

mass
number of particles

24
Q

State Boyle’s law

A

if the temperature T of an ideal gas is constant, then Boyle’s Law is given by:

p ∝ 1/V

where:
p= pressure (Pa)
v= volume (m^3)

25
Q

How else can Boyle’s law be shown for situations involving a before and after scenario?

A

p1V1= p2V2

26
Q

Draw the graph showing Boyle’s law

A

https://cdn.savemyexams.co.uk/cdn-cgi/image/w=1920,f=auto/uploads/2021/05/6.5.2-Boyles-Law.png

27
Q

State Charles’ law

A

if the pressure P of an ideal gas is constant, then Charles’s law is given by:

V ∝ T

28
Q

How else can Charles’ law be shown for situations involving a before and after scenario?

A

V1/T1 = V2/T2

29
Q

Draw the graph showing Charles’ law

A

https://cdn.savemyexams.co.uk/cdn-cgi/image/w=1920,f=auto/uploads/2021/05/6.5.2-Charles-Law.png

30
Q

State the pressure law

A

if the volume V of an ideal gas is constant, the Pressure law is given by:

p ∝ T

31
Q

How else can the pressure law be shown for situations involving a before and after scenario?

A

p1/T1 = p2/T2

32
Q

Draw the graph showing the pressure law

A

https://cdn.savemyexams.co.uk/cdn-cgi/image/w=1920,f=auto/uploads/2021/05/6.5.2-Pressure-Law.png

33
Q

What is an ideal gas?

A

an ideal gas is one that obeys the relation:
pV ∝ T

Where:
p = pressure of the gas (Pa
V = volume of the gas (m^3)
T = temperature (K)

34
Q

How can we find out the internal energy of an ideal gas?

A

equal to the kinetic energy

35
Q

What are some characteristics of an ideal gas? (5)

A

-has molecules with negligible volume
-collisions which are elastic
-cannot be liquified
-has no interactions between the molecules (except during collisions)
-obeys the (ideal) gas laws (Boyles law, Charles’ law and Pressure law)

36
Q

What are the two equations of state for an ideal gas?

A

pV = nRT

where:
p= pressure (Pa)
V= volume (m^3)
R= molar gas constant (Jkg^-1mol^-1)
n= number of moles (mol)

pV=NkT

where:
p= pressure (Pa)
V= volume (m^3)
k= Boltzmann constant (JK^-1)
N= number of molecules
T= temperature (K)

37
Q

What are the two constants which can be used in the ideal gas equation?

A

k (boltzmann constant)- 1.38 x 10^-23JK^-1
R (molar gas constant)- 8.31Jkg^-1mol^-1

38
Q

How can we find the number of molecules (N) from the number of moles?

A

N = Na x n

39
Q

How can we change the volume of a gas at a constant pressure?

A

do work on it

40
Q

What is meant when gas expands/compresses

A

expands- work is done BY the gas
compresses- work is done ON the gas

41
Q

What is the equation linking work done, pressure and volume?

A

W = pΔV

where:
W= work done (J)
p= pressure (Pa)
V= volume (m^3)

42
Q

What do we mean by the atomic mass?

A

the mass of a proton/neutron (1.67 x 10^-27)

43
Q

State avagadro’s number

A

6.0-2 x 10^23

44
Q

What do we mean by relative atomic mass?

A

the mass number
e.g. H20 relative atomic mass
(2 x 1) + 16= 18g

45
Q

What do we mean by molar mass?

A

the mass in one mole
g mol-1

46
Q

Give the equation that links moles, molar mass and mass

A

https://cdn.savemyexams.co.uk/cdn-cgi/image/w=1920,f=auto/uploads/2021/04/6.5.1-Number-of-Moles-Equation_2.png

47
Q

What is brownian motion?

A

the random motion of larger particles in a fluid caused by collisions with surrounding particles

48
Q

Compare the three laws to the kinetic theory model

A

-the three laws are empirical whereas the kinetic theory model arose only from theory
-the kinetic theory model has mulitple assumptions

49
Q

What are some assumptions of the kinetic theory model? (4)

A

-no intermolecular forces act on the molecules
-the duration of collisions is negligible in comparison to time between collisions
-the motion of molecules in random, and they experience perfectly elastic collisions
-the motion of the molecules follows Newton’s laws
-the molecules move in straight lines between collisions

50
Q

How can we work out the change in internal energy for an ideal gas?

A

ΔU= 3/2kΔT

where:
U= internal energy (J)
k= boltzmann constant
T= temperature (K)

51
Q

How is internal energy and temperature related for an ideal gas?

A

ideal gas molecules are assumed to have no intermolecular forces (this means they have no Ep, only Ek)

as the avg. Ek is the definition of temperature:
ΔU ∝ ΔT

52
Q

Derive the kinetic theory of gases equation (derivation which is needed)

A
  1. determine change in momentum as a single molecule hits the wall perpendicularly
    Δp = final p – initial p = −mv − (+mv) = −mv − mv = −2mv
  2. calculate the number of collisions per second by the molecule on a wall
    time between collisions= distance/speed
    t= 2L/v
  3. calculate the force exerted by the molecule on the wall
    f= rate of change of momentum (mv/t)
    f=2mv / (2L/v)
    f= 2mv^2/L
  4. calculate the total pressure for one molecule
    p= force/area
    p= (2mv^2 / L) / L^2
    p= mv^2 / L^3

AQA A-level Physics (11 decks)

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