IB HL Physics (2023) - The particulate nature of matter

B.1-B.3

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 bu 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