THERMAL PHYSICS

Question 1

1st law of thermodynamics

Answer 1

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

Question 2

Fun fact about graduations of thermometer

Answer 2

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

Question 3

When does energy transfer occur?

Answer 3

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

Question 4

Conditions for constant internal energy

Answer 4

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

Question 5

What is internal energy?

Answer 5

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

Question 6

Boyle’s law

Answer 6

pV = constant for constant temperature + n

Question 7

Charles’ Law

Answer 7

V/T = constant for constant pressure + n

Question 8

isobaric meaning

Answer 8

At a constant pressure

Question 9

Pressure and temperature relationship

Answer 9

P/T is constant

Question 10

Work done in terms of p &V

Answer 10

E=pΔV

Question 11

Avocado constant definition

Answer 11

Number of atoms in 12g of carbon12

Question 12

Atomic mass unit defintion

Answer 12

1/12 the mass of a carbon12 isotope

Question 13

1 Mole definiton

Molar mass definition

Answer 13

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

= (Mass/μ)

Molar mass - Mass in 1 mole = Molxμ

mew is atomic mass

Question 14

Ideal gas eq

Answer 14

pV = nRT

graph of pV against T is straight line through absolute zero

Question 15

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

Answer 15

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)

Question 16

Assumptions made in kinetic theory

Answer 16

• 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

Question 17

Getting p = NM/3V

Answer 17

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

Question 18

What happens to arrangement of atoms during state change specifically

Answer 18

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

Question 19

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

Answer 19

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

Question 20

Ideal gas law + kinetic theory interpretations of absolute zero

Answer 20

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

Kinetic- random motion stops or Ek of particles = 0

Question 21

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

Answer 21

System is at equilibrium so same mean Ek

Question 22

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

Answer 22

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

Question 23

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

Answer 23

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)

Question 24

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.

Answer 24

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

Question 25

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

Answer 25

A - Large distance between particles

Question 26

Derivation of kinetic theory steps

Answer 26

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

Question 27

Mean velocity of a gas

Answer 27

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

arguable

Question 28

Mean speed

Answer 28

Sum of speeds divided by number of molecules

Question 29

Graph of number of molecules with speed v against v

Answer 29

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

Question 30

Internal energy of an ideal gas

Answer 30

Only due to kinetic energy of molecules

= total Ek/N

= m/2 (c1²+c2²…cn²)/N
mcrms²/2

Higher temperature higher mean Ek

Question 31

Average translational Ek of a gas molecule

Answer 31

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

Monos basically points

Question 32

energy of n moles using 3/2KT

Answer 32

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

Question 33

Limitations of kinetic theory

Answer 33

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

Question 34

Gas laws vs kinetic theory

Answer 34

Gas laws are empirical however kinetic theory model arises from theory

Question 35

Brownian motion proving existence of atoms

Answer 35

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

Question 36

Brownian motion

Answer 36

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

Question 37

When internal energy may not be due to thermal energy

Answer 37

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

Question 38

What is a molecule

Answer 38

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

Question 39

Solids

Answer 39

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

Question 40

Liquids

Answer 40

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

Question 41

Vapours

Answer 41

Molecules move randomly at greater distances than those in a liquid

Question 42

Absolute zero

Answer 42

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

Question 43

Improving the thermal contact between thermometer and metal

Answer 43

Use a small amount of water/oil
pg41 year 2

Question 44

Before you measure the temperature of a liquid in an experiment

Answer 44

Give it a stir

Question 45

Sublimation

Answer 45

When an object vaporises immediately when heated

Question 46

Pressure of a gas

Answer 46

Force per unit area exerted normally on a surface

Depends on temperature, volume and mass of gas in container

Question 47

Why pv is work done

Answer 47

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

(for constant pressure)

Question 48

Explaining Boyle’s law experimentally

Answer 48

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

Question 49

Explaining pressure law experimentally

Answer 49

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

Question 50

Molecular speeds

Answer 50

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