IB HL Physics (2023) - The Particulate Nature of Matter

Density

Mass per unit of volume (Kgm^-3)

Temperature ☆

A measure of the average random kinetic energy of particles in a substance (°C Or K)

Heat

The thermal energy transferred from a body of higher temperature to a body of lower temperature

Absolute Zero ☆

The lowest temperature theoretically possible at which particles have zero kinetic energy (0K or -273°C)

Thermal Equilibrium

When two bodies in thermal contact have the same temperature and no net flow of heat energy

Internal Energy

The sum of the kinetic and potential energies in ln the molecules of a body (J)

Specific Heat Capacity

The amount of thermal energy required to raise 1kg of a substance by 1K (J Kg^-1K^-1)

Thermal Capacity

The amount of energy required to raise the temp of an object by 1K or 1°C (J K^-1)

Specific Latent Heat of Fusion

The energy required to change 1kg of a solid into a liquid (JKg^-1)

Specific Latent Heat Vaporisation

The energy required to change 1kg of a liquid into a gas (JKg^-1)

Methods of Heat Transfer

• Conduction - Through solids

• Convection - Through fluids

• Thermal Radiation - Transfer through the emission of photons

Black Bodies ☆

An object which can absorb and emit all wavelengths of EM radiation (reflects NO radiation)

Intensity

The amount of energy recieved per unit second, per cross sectional area (Wm^-2)

Black bodies and Intensity

For black bodies (like stars) we can plot the intensity of each wavelength emitted on a graph and it always takes this shape

Wien’s Displacement Law ☆

The relationship between the peak wavelength and the absolute temperature of the surface of a black body

T at λ_max = 2.898 x 10^-3 mK (metres Kelvin NOT miliKelvin)

λ_max is the wavelength at which maximum energy is radiated

Luminosity (L)

The Energy radiated by a star per second (W)

Stefan’s Law ☆

The total energy radiated per unit time by a black body is proportional to the fourth power of its absolute temperature and surface area

Apparent Brightness (b)

The amount of energy per second per unit area that arrives at a distance from the star (Wm^-2)

Emissivity (e)

The ratio of power emitted from a body to the power emitted by a black body of the same size

e will always be between 0 and 1

a black body has an emissivity of 1

Albedo

The proportion of incident light that is reflected off a surface

Solar Constant ☆

The intensity of solar radiation across all wavelengths that is incident at the mean distance of earth from the sun

Greenhouse Gases

Gases which make up our atmosphere that trap heat and prevent it from escaping into space

• Carbon Dioxide (CO₂)

• Methane (CH₄)

• Water Vapour (H₂O)

• Nitrous Oxide (N₂O)

The Greenhouse Effect: Excitation ☆

Gas Molecules will absorb certain wavelengths of photons and temporarily gain more energy. This is known as excitation

Eventually they re-emit these photons in all directions

The Greenhouse Effect: Resonance ☆

The natural frequency of greenhouse gases is in the infrared region which means they are prone to absorbing the energy from infrared radiation

Then they later re emit the photons in all directions and sometimes back towards Earth

Energy Balance

the difference between the energy into the earth and the energy leaving the eart

Enhanced Greenhouse effect ☆

the additional radiative forcing resulting from increased concentrations of greenhouse gases induced by human activities

Pressure

The amount of force per unit area that acts perpendicular to the surface of an object (Pa)

Pressure in Solids and liquids

Solids - weight/ area of face

liquids - density*gravity*height

State of a Gas

The specific physical condition of a gas sample at a given time

Ideal Gas Assumptions ☆

• D - duration of collisions are negligible

• R - random motion by particles

• I - Intermolecular forces not present between particles

• V - volume of particles are negligible

• E - elastic collisions between particles

Real Gases and Ideal Gases

Real gases approach ideal gas behaviour at:

Low pressure and High Temperature

Ideal Gas Laws

• Boyle’s Law - Pressure-Volume

• Charles’s Law - Temperature-Volume

• Gay-Lussac’s Law - Pressure-Temperature

Gay-Lussac’s Law ☆

For a fixed mass of gas with constant volume the pressure is directly proportional to the Absolute temperature of the gas

P/T = constant

• higher temp=higher avg speed

• particles collide with the walls more frequently

• rate of change of momentum increases

• greater average force

Boyle’s Law ☆

For a fixed mass of gas at a constant temperature the pressure is inversely proportional to the volume

PV = Constant

with a smaller volume the frequency of collisions increases leading to more force due to an increased rate of change of momentum

Charles’ Law ☆

For a fixed mass of gas kept at a constant pressure the volume is proportional to the temperature

V/T= Constant

When the temperature increases avg speed of molecules increase and so there are more frequent collisions with the side of the container, leading to the volume increasing

System

The body or bodies that we are considering

Surroundings

The area that the system can interact with through the transfer of heat energy

Universe

The system and it’s surroundings

Open System

a system which can exchange energy, work done, and matter with its surroundings

Closed System

a system which can only exchange work done and energy with its surroundings

Isolated System

a system which cannot exchange anything with it’s surroundings

The First Law of Thermodynamics

The Thermal energy (Q) entering a closed system is equal to the sum of the Change in Internal Energy (ΔU) of the system and the Work Done (W) of the system

Q=ΔU+W

Isobaric Change

• a change in state where pressure is constant (ΔP=0)

• temperature and volume change proportionally

• Work is done on or by the gas to change the volume

Isovolumetric change

• a change in state where volume is constant (W=0)

• temperature and pressure change proportionally

• Q=ΔU+0

Isothermal Change

• a change in state where temperature is constant (ΔU=0)

• pressure and volume are inversely proportional

• Q=0+W

• ideally an isothermal change is infinitely slow

Adiabatic Change

• a change in state where no thermal energy is transferred (Q=0)

• volume temperature and pressure can change

• the magnitude of ΔU will be equal but opposite to the W

  • +W = -ΔU

  • -W = +ΔU

• when ΔU is positive temperature increases

Entropy (S)

a measure of the number of possible arrangements of the particles and their energies

The Second Law of Thermodynamics

The entropy of the universe always increases during an irreversible change

The Clausius Clause

Energy cannot be transferred from a body at lower temperature to a body at a higher temperature unless work is done system

Kelvin’s Clause

Energy cannot be extracted from a hot object and transferred entirely into work

Heat Engine

a device that converts thermal energy into mechanical work by transferring heat from a hot reservoir to a cold reservoir

Reversible Process

a process in which there is no overall change in the entropy of the system and its surroundings

Irreversible process

a process which results in an overall increase in entropy of the system and its surroundings

Microstate

a specific molecular configuration

Macrostate

larger scale measurable outcome, resulting from the outcome of each of the smaller microstates

Equations of Entropy

• S= K_B ln(Ω)

• ΔS=ΔQ/T

Stages of a Heat Engine

1. Q in from R_H

2. +W therefore +V

3. (some)Q → R_C

4. -Q therefore -V

5. cycle restarts

Carnot cycle

an idealised theoretical thermodynamic cycle that:

• involves 4 processes

• is reversible

• has the theoretical maximum efficiency of any heat engine

Heat Pumps and Refrigerators

• Heat engines in reverse

• work is done to transfer energy from cold to hot

• refrigerators maximise energy transfer from the CR per joule of W

• Heat pumps maximise energy transfer to the HR per joule of W

Ion

particles with different numbers of electrons and protons

Elementary charge (e)

the magnitude of the charge of a proton or electron

1e = 1.60×10^-19C

Potential Difference (V) ☆

The work done per unit charge moving at a positive charge between two points

Electromotive force (emf) ☆

work done by a cell per unit charge in moving a cell between two terminals

Electron Volt (eV)

the energy gained when an electron is accelerated by a potential difference of 1 volt

W=qV

1eV=1.60×10^-19 J

Conventional current (I)

the amount of positive charge flowing past a point in a second

Conventional current and electron flow

CC= + → -

EF = - → +

Electrical Resistance (R)

the ratio of potential difference across a component to current through it

Electrical Power

rate of energy transfer (amount of work done per second)

Ohm’s Law

potential difference across a metallic conductor is proportional to the current flowing through it, provided the temperature does not change

Non-Ohmic Conductors - Filament Lamp

• As pd across the filament lamp is increased, the electrons collide more frequently with the vibrating fixed ions, transferring energy

• temperature increases which makes resistance increase

Non-Ohmic Conductors - Semi-conducting Diode

• Diodes only allow current to flow in one direction

• for negative V values there is no current

• there is no significant current in the forward direction before the threshold voltage

• high then low resistance in the forward direction

LDRs and Thermistors

• LDR - resistance is inversely proportional to light intensity

• NTC Thermistor (negative temp coeff.) - resistance is inversely proportional to temperature

Resistance of Wires

The physical propertied of a wire change it’s resistance

• Length

• Cross sectional area

• Resistivity

R=ρL/A

Kirchoff’s First Law

The sum of currents into a junction is equal to the sum of currents out of a junction

Kirchoff’s Second Law

in a complete loop the sum of emfs is equal to the sum of potential difference in a loop

Potential Dividers

a circuit made of two or more series resistors that allows us to output any voltage we want that is less than the battery voltage

Potentiometer

a single wire with a sliding contact that can be used to build a potential divider circuit.

Batteries

• a battery consists of two or more cells joined together

• the cells consist of conductive components and a store of chemical energy

Internal Resistance

the resistance of the conductive components of a battery