b4 physics HL

What is thermodynamics?

The study of how energy moves and changes form, the relationships between heat, work, temperature, and internal energy

1st law of thermodynamics? Equation?

Energy cannot be created or destroyed; it can only change form or be transferred between a system and its surroundings (🔺U=Q-W)

2nd law of thermodynamics?

Clausius form: heat cannot spontaneously flow from a colder body to a hotter one (only if you do work, e.g. a fridge)

Open vs closed vs isolated system?

Open → energy/work + matter CAN flow in and out

Closed → energy/work can enter but MATTER cannot

Isolated → energy/work + matter can NOT flow in and out

When is it positive work vs negative work?

Positive work → work done BY gas on piston (gas pushes the piston away / up)

Negative work → work done ON gas by piston (piston pushes piston back / down)

What are the 4 thermodynamic processes?

Isobaric, isovolumetric / isochoric, isothermal, ADIABATIC

Isothermal definition and its effect on Q=🔺U + W?

Temperature is held constant during a change → 🔺U is constant so Q=W

Isovolumetric / isochoric definition and its effect on Q=🔺U + W?

Volume is held constant during a change → no 🔺W (as piston stays same), Q=🔺U

Isobaric definition and its effect on Q=🔺U + W?

Pressure is held constant during a change → (doesn’t affect any values) Q=🔺U+W

ADIABATIC definition and its effect on Q=🔺U + W?

Constant thermal energy transfer during a change → (no Q) so -🔺U=W (change so fast that Q isn’t measurable)

ADIABATIC compression and expression formula? When to use?

P1(V1)^5/3 = P2(V2)^5/3 (INDICE ONLY ON VOLUME)

• when a FIXED QUANTITY (constant no. moles) changes state adiabatically

When is Q positive or negative?

• + when energy put INTO the system and GAINED

• - when energy REMOVED from the system and LOST

On a PV graph, what is the difference between ADIABATIC and isothermal lines?

ADIABATIC is steeper and often results in a change in volume (lower)

Area under a PV graph?

WORK DONE

“Net” work done? What rewritten as?

Total energy transferred by ALL acting forces (in this case just the ADIABATIC part of the gas expanding / compression itself)

Rewritten as: Qh - Qc

What can n= useful / input be rewritten as?

n = Wnet (or Q1 - Q2) / Q2 (or Qh bc u only get energy from hot)

Explain the Stirling Engine in 4 steps! :3

1. Extract heat from the HOT reservoir (Th which is maintained by the fuel and stays hot)

2. Some heat used in useful work to push the piston away as the gas expands

3. Unused heat dumped into the cold reservoir (Tc) as wasted energy, causes a drop in pressure and the piston sinks

4. REPEATS

LIKE A CONVECTION CURRENT!!

What is a Carnot cycle and why is it never 100% efficient?

An ideal theoretical thermodynamic cycle that sets the maximum possible efficiency limit for ANY heat engine

Never 100% efficient → some heat must always be transferred into cold reservoir, always heat loss from Work done (to make the cycle continue, it has to return to starting temperature, pressure, volume etc. so energy always has to be put into the cold >:O )

Does entropy always increase in the universe (isolated system) or in non-isolated systems?

The entropy in a real isolated system almost always increases, while for a non-isolated system, the entropy can decrease locally, but the overall entropy of the system and surroundings still increases.

What is entropy? Units?

The measure of the amount of disorder of the particles in a system (S)

Units are → J/K

What does entropy do over time in ISOLATED systems?

Will INCREASE, goes from low → high entropy (high disorder / spread) as the probability of clumping together is low

Microstate vs Macrostate?

Microstate → specific molecular configurations (e.g. combinations of H,Ts on coins, HHTTHH, HTTHTH)

Macrostates → larger, observable microstates (e.g. 3 head, 2 tail)

Omega → how to work out? What is it?

Number of microstates → (possible outcomes)^(number of dice/??)

When can you use 🔺S=🔺Q/T?

When T is held constant!! When u need to find the CHANGE in entropy

What is Tc / Th equals to?

Qc / Qh

If Wnet → Qh - Qc, what can you describe the energy system as?

Qh = W + Qc, (heat absorbed from the hot reservoir is equal to the sum of useful work and the heat ejected to the cold reservoir)