Thermal physics

What is internal energy

The sum of the randomly distributed kinetic energy and the potential energies of the particles that move at random in a gas

How is the internal energy of a system increased

When energy is transferred to it by heating or when work is done on it

Is internal energy constant in a system

Only in closed system

Does internal energy change when state changes

Yes but kinetic energy stays the same as potential energy altered not kinetic

What is specific heat capacity

Amount of energy needed to raise the temperature of 1kg of a substance by IK or 1c

Energy change equation

Energy change = specific heat capacity x change in temperature

1st law of thermodynamics

The change of internal energy of the object = the total energy transfer due to work done and heating

What happens at -273 C

• Temperature at which the random motion stops and kinetic energy is zero

• PV=nRT is the temp when pV is zero

• Using as laws it is the temperature at which the volume/pressure o a gas extrapolates to zero

Specific heat capacity c

The energy needed to raise the temperature of a unit of substance by 1K without a change in state. JKG-1K-1

Q=mc^T

Equations for mixing substances

M1c1ΔT1=m2c2ΔT2

For equilibrium: m1c1(Tstart - Tfinish)= m2c2(Tf-Ts) given both reach same finial temp

Inversion tube

• As tube is inverted GPE → kinetic energy then to internal energy (heat

• Ep=mgl

• Mgl=mcΔT

• Tube inverted n times → nmgl =mcΔT

• Assuming all GPE lost transferred to internal energy

• McΔT=mgln → c=gln/ΔT

Metal block experiment

• Need to measure mass of material, start temp, end temp,current, time, pd

• P=VI + P=E/t → E=VIt

• Q=mcΔt → VIt= mcΔt → c=VIt/mΔT

Insulated beaker experiment

• Electrical energy =VIt

• Energy used to heat both liquid an calorimeter

• Eliquid = MlclΔtl + Ecalorimeter = mcccΔtc

• VIt = El + Ec assuming no loss to environment

What happens during a change in state

Energy used to break some of the atomic bonds, kinetic energy and temp stays the same but potential energy increases

The energy transferred reduces the number of nearest atomic neighbours, breaks some atomic bonds and allows atoms to move their centre of vibration (crystalline to amorphous)

Specific latent heat equation

Q=ml

What is specific latent heat of fusion

Energy required to change 1kg of material from the solid to liquid state or melt/fuse without a change of temperature

What is the specific latent heat of vaporisation

Energy required to change 1kg of material from the liquid to gas’s state or boil/condense without a change of temperature

Highest as all intermolecular bonds must be broken

Pressure

Force per unit area, measured in Nm^-2 or Pa, caused by particles randomly moving in all directions, colliding with container walls and each other

Volume

Measured in meters cubed

Amount of substance

Measured in moles or number of particles

Temperature

Average kinetic energy of particles in a substance. Must be in Kelvin

Boyle’s Law

• For a fixed mass a constant temp pV=constant

• P1V1=P2V2

• P∝1/V

• PV=k

What is an isothermic change

A change at a constant temp

What is an ideal gas defined as

• A gas that follows Boyle’s law

• A gas where the volume of the molecules is negligible compared to the volume of the container, no intermolecular forces between particles, molecules move in random motion, all collisions are elastic, molecules move in straight lines between collisions

Charles’ law

• Investigates how the volume of gas varies with temp for a fixed pressure

• Amount of gas irrelevant, volume always 0 at -273.15 C

• Volume directly proportional to temperature V/T=constant

What is an isobaric change

A change at a constant pressure

What happens when work is done to change the volume of a gas

Energy must be transferred by heating it keep pressure constant

Work done=P x change in V

Pressure laws

Investigates how the pressure of gas varies with temperature for a fixed volume

P/T=constant

Molecules in a gas

• Constantly moving indifferent reactions with different speeds

• When they hit a surface they rebound (elastically) causing a Change in momentum (impulse) causing a force

• Sum of forces = pressure

Brownian motion

We cannot see individual gas molecules but we can infer their motion from their impacts with observable objects

Random motion→ Brownian motion

Na

Avagadros constant = 6.023×10²³

One atomic mass unit (u)

1/12th the mass of a 12-C atom

1u = 1.661 × 10^-27

Molar gas constant

R → 8.31 JK-1 mol-1

The Boltzmann constant

K → 1.38×10^-23 JK-1

Molar mass

The mass of 1 mol of the substance. Kgmol-1

N(number of moles) = m(mass)/M(molar mass)

The ideal gas equation

PV/T = constant for a fixed mass of ideal gas

Equal volumes of ideal gases at the same temperature and pressure contain equal number of moles

Can be shown experimentally that one mol of an ideal gas at 273K and pressure 101kpa has a volume of 0.0224m³

PV/T = 8.31 Jmol—1K-1 (molar gas constant R)

Kinetic theory of gases