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A LEVEL PHYSICS > THERMAL PHYSICS > Flashcards

THERMAL PHYSICS Flashcards

1
Q

1st law of thermodynamics

A

Change in internal energy of object = total energy transfer due to work done and heating

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2
Q

Fun fact about graduations of thermometer

A

Interval distance isn’t the same at the middle of the scale vs ends (measure w travelling microscope) - expansion of liquid isn’t directly proportional to temperature

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3
Q

When does energy transfer occur?

A

When one object exerts a force on another object, causing it to move (does work), or when one object is hotter than another, so energy is transferred via convection, conduction or radiation

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4
Q

Conditions for constant internal energy

A

No work done/energy transfer by heating or energy transfer by heating and work done “balance each other out”

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5
Q

What is internal energy?

A

The sum of the random distribution of the kinetic and potential energies of its molecules

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6
Q

Boyle’s law

A

pV = constant for constant temperature + n

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7
Q

Charles’ Law

A

V/T = constant for constant pressure + n

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8
Q

isobaric meaning

A

At a constant pressure

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9
Q

Pressure and temperature relationship

A

P/T is constant

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10
Q

Work done in terms of p &V

A

E=pΔV

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11
Q

Avocado constant definition

A

Number of atoms in 12g of carbon12

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12
Q

Atomic mass unit defintion

A

1/12 the mass of a carbon12 isotope

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13
Q

1 Mole definiton

Molar mass definition

A

Quantity of a substance (identical particles) that contains Nₐ particles

= (Mass/μ)

Molar mass - Mass in 1 mole = Molxμ

mew is atomic mass

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14
Q

Ideal gas eq

A

pV = nRT

graph of pV against T is straight line through absolute zero

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15
Q

Manipulating ideal gas law (in terms of density and number of molecules)

A

n=pV/RT Mₛ=Mn (mass=molarmassxn)

Mₛ = MpV/RT → ρ=Mp/RT for molar mass M
or nM/V

n=N/Nₐ where N is number of molecules

pV = NkT where k = R/Nₐ (boltzmann constant)

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16
Q

Assumptions made in kinetic theory

A
  • Molecules are point molecules - volume of
    each molecule negligable wrt volume of gas
  • Molecules don’t attract each other (would
    reduce force on impact with container)
  • Move around in continual random motion
  • Collisions with other molecules and
    container are elastic
  • Each collision with the container is of
    much shorter duration than the time
    during impacts
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17
Q

Getting p = NM/3V

A

Pₓ = (NMvₓ^2)/V and etc for Pᵧ and P₂
so 3P = NM/V (Pₓ^2+Pᵧ ^2+ P₂ ^2)

P = NM/3V (c^2) where c = rms

also u^2 = 1/N(u1^2+u2^2…un^2)

as c²=x²+y²+z²
crms = c²+c²+c²…cn²/n
= x²+y²+z² … xn²+yn²+zn²/n
= xrms²+yrms²+zrms²

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18
Q

What happens to arrangement of atoms during state change specifically

A

Energy transferred reduced number of nearest atomic neighbours

(crystalline to amorphous from solid to liquid)
atoms move to centre of vibration

also avoid using velocity instead of speed when talking abt motion

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19
Q

Deductions from experiment where Brownian motion is demonstrated using smoke particles in air

A

The motion is caused by collisions between air molecules
and smoke particles

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20
Q

Ideal gas law + kinetic theory interpretations of absolute zero

A

Ideal gas - where pressure/volume =/(extrapolates to) 0

Kinetic- random motion stops or Ek of particles = 0

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21
Q

Compare mean Ek of two types of particles enclosed in a volume at the same temp

A

System is at equilibrium so same mean Ek

22
Q

Explain using kinetic theory model why a gas exerts a force on a piston

A

Particles collide with piston and change momentum
Force = rate of change of momentum
Pressure = force/area

23
Q

Using the kinetic theory model, two changes that can be made independently to
reduce the pressure exerted by a gas

A

change
the volume could be increased
explanation
which increases the time between collisions OR results in less frequent collisions
(with the piston/wall so reducing the rate of change of momentum)
OR
which increases the area of the piston/wall (and so reduces the pressure)
change
the temperature could be reduced
explanation
which reduces the momentum (change at the wall)

24
Q

Which of the following is not an assumption of the kinetic model of ideal gases?
A. All particles in the gas have the same mass.
B. All particles in the gas have the same speed.
C. The duration of collisions between particles is very short.
D. Collisions with the walls of the container are elastic.

A

C - The particles have a distribution of speeds with a mean (temp)

25
Q

Under what conditions of density and pressure is a real gas best described by the equation of state
for an ideal gas?
A. Low density and low pressure
B. Low density and high pressure
C. High density and low pressure
D. High density and high pressure

A

A - Large distance between particles

26
Q

Derivation of kinetic theory steps

A

Find the change in momentum as a single molecule hits a wall perpendicularly. …
Calculate the number of collisions per second by the molecule on a wall. …
Find the change in momentum per second. …
Calculate the total pressure from N molecules. …
Consider the effect of the molecule moving in 3D space

27
Q

Mean velocity of a gas

A

Zero, as there are equal distributions in all directions (pos and neg)

arguable

28
Q

Mean speed

A

Sum of speeds divided by number of molecules

29
Q

Graph of number of molecules with speed v against v

A

Kinda bell distribution - If temperature of gas increases, curve becomes broader and flatter - greater number of particles moving at greater speeds. Also slightly shifted to right

30
Q

Internal energy of an ideal gas

A

Only due to kinetic energy of molecules

= total Ek/N

= m/2 (c1²+c2²…cn²)/N
mcrms²/2

Higher temperature higher mean Ek

31
Q

Average translational Ek of a gas molecule

A

3/2 KT = 1/2mcrms²
Monatomic atoms have translational Ek, however diatomic have rotational aswel

Monos basically points

32
Q

energy of n moles using 3/2KT

A

3/2nKT aka 3/2nRT from equating pv in ideal gas eq and kinetic theory

33
Q

Limitations of kinetic theory

A

When the temperature is low and the pressure is high, the molecules come closer to each other, and the volume of the gas is compressed. In this scenario, one cannot ignore the forces of attraction between them and the volume of the gas.

Why can’t kinetic theory be applied to liquids and solids?
The kinetic theory modelling assumes that the molecules are very small relative to the distance between molecules. This is certainly not true of liquids and solids

34
Q

Gas laws vs kinetic theory

A

Gas laws are empirical however kinetic theory model arises from theory

35
Q

Brownian motion proving existence of atoms

A

Pollen grains in water move w zigzag motion
- Movement of particles suspended in a fluid - brownian
- Says that random motion is due to collisions with fast, randomly moving particles in fluid.

Seen when large heavy particles (smoke) are moved with Brownian motion by smaller lighter particles (air) travelling at high speeds. Evidence that air is made up of tiny atoms/molecules moving quickly

36
Q

Brownian motion

A

Brownian motion:
Can be observed under a microscope
Provides evidence for the existence of molecules in a gas or liquids
The particles are said to be in random motion, this means that they have:
A range of speeds
No preferred direction of movement
The observable particles in Brownian motion are significantly bigger than the molecules that cause the motion
In most cases, these were observed as smoke particles in air
The air particles cause the observable motion of the smoke particles that we see
This means that the air particles were small and light and the smoke particles were large and heavy
The collisions cause larger particles to change their speed and directions randomly
This effect provides important evidence concerning the behaviour of molecules in a gas, especially the concept of pressure
The small molecules are able to affect the larger particles in this way because:
They are travelling at a speed much higher than the larger particles
They have a lot of momentum, which they transfer to the larger particles when they collide

37
Q

When internal energy may not be due to thermal energy

A

A magnetised iron bar has a greater internal energy than an unmagnetised one due to the magnetic interaction between the atoms in the bar

38
Q

What is a molecule

A

Smallest particle of a pure substance that is characteristic of the substance

39
Q

Solids

A

Atoms/molecules held together by forces due to electric charge of protons/electrons in atoms. Molecules vibrate randomly about fixed positions, higher temperature greater vibration. If enough energy provided molecules vibrate so much they break free from each other (crystalline to amorphous), raising potential energy

40
Q

Liquids

A

Molecules move randomly in contact with each other - higher temperature faster movement

41
Q

Vapours

A

Molecules move randomly at greater distances than those in a liquid

42
Q

Absolute zero

A

No object can have a temperature below absolute 0. An object at absolute 0 has minimum internal energy

43
Q

Improving the thermal contact between thermometer and metal

A

Use a small amount of water/oil
pg41 year 2

44
Q

Before you measure the temperature of a liquid in an experiment

A

Give it a stir

45
Q

Sublimation

A

When an object vaporises immediately when heated

46
Q

Pressure of a gas

A

Force per unit area exerted normally on a surface

Depends on temperature, volume and mass of gas in container

47
Q

Why pv is work done

A

When work is done to change the volume of the gas, energy must be transferred by heating to keep pressure constant.

(for constant pressure)

48
Q

Explaining Boyle’s law experimentally

A

Pressure of a gas at constant T is increased by reducing the volume as the gas molecules travel less distance between impacts with the walls due to reduced volume. More impacts per second - greater pressure

49
Q

Explaining pressure law experimentally

A

Pressure of a gas at constant volume is increased by raising its temperature. Average speed of molecules increases with temperature. Therefore greater change of momentum on impact as well as greater number of impacts per second - greater pressure

50
Q

Molecular speeds

A

In an ideal gas have a continuous spread of speeds. Speed of an individual molecule changes when it collides with another gas molecule, but distribution doesn’t change as long as T is constant

51
Q
A

A LEVEL PHYSICS (17 decks)

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