What happens in isothermal expansion?
• Temperature constant
• Heat absorbed (Qᵢₙ)
• Gas does work
What happens in adiabatic expansion?
• No heat transfer
• Temperature decreases
• Gas does work
What happens in isothermal compression?
• Temperature constant
• Heat released (Qₒᵤₜ)
• Work done on gas
What happens in adiabatic compression?
• No heat transfer
• Temperature increases
• Work done on gas
What is a heat engine?
Uses a temperature difference to produce mechanical work
• Heat flows from hot → cold
• Some heat is always wasted (Second Law)
Otto cycle stages
Adiabatic compression
• Volume ↓, pressure ↑, temperature ↑
• Happens quickly
• Constant volume heating (combustion)
• Spark ignites fuel
• Pressure & temperature increase instantly
• Adiabatic expansion
• Gas pushes piston
• Work done
• Pressure ↓, volume ↑
• Exhaust
• Hot gases expelled
• Fresh mixture enters
Key exam point
• Mechanical work comes from adiabatic expansion
RANKINE CYCLE
Stages
• Pump
• Liquid water compressed
• Pressure ↑ (small work input)
• Boiler
• Heat added (Qᵢₙ)
• Water → steam
• Turbine
• Steam expands
• Work output (Wₜᵤʳᵦᵢₙₑ)
• Condenser
• Steam condenses
• Heat released (Qₒᵤₜ)
REFRIGERATOR (FRIDGE)
• Compression
• Pressure & temperature increase
• Condensation
• Gas → liquid
• Heat released to surroundings
• Expansion valve
• Pressure drops
• Liquid becomes very cold
• Evaporation
• Liquid → gas
• Heat absorbed from fridge interior (cooling)
Exam gold
• Cooling happens during evaporation
• Heating happens during condensation
SECOND LAW OF THERMODYNAMICS
Heat flows hot → cold naturally
• Cannot convert all heat into work
• Some energy always wasted (friction, sound, heat loss)
