FIrst law of thermodynamic Flashcards Preview

a2 physics engineering > FIrst law of thermodynamic > Flashcards

Flashcards in FIrst law of thermodynamic Deck (9)
Loading flashcards...

what does the 1st law decribe

  • how energy is conserved in a system through heating, cooling and doing work



Q = ΔU + W


what is the meaning

what are the conventions

  •  Q
    • energy transferred by heating
    • if energy is transferred away, it will be -ve
  • ΔU
    • change in internal erngy
    • sum of the potential and kinetic energies
  • W
    • work done by system
    • if the work is done on the gas, e.g. compressing it, then the value of W will be negative


isothermal change

constant temperature






how does the 1st law of thermodynamics euqaition change?

  • Q = W
  • U only depends on temperature 
  • any work a system does will cause it to lose an equal amount of heat energy



adiabatic process

  • where no heat is lost or gained by the system



how does the 1st law equation change for an adiabatic change

  • ΔU = -W
  • any change in U is caused by work being done by/on the system
  • if work is done by the system, W will be +ve and U will decrease
  • change in temp ,ust occur





what is W = pΔV used for 

how is it derived

  • constant pressure 
  • W = FΔx
  • P = F / A so
    • F = pA
    • W = pAΔx
  • AΔx = V
    • W = pΔV



what are teh convention for the W = pΔV equation

  • for expansion
    • change in volume and work done a re positive




processes at constant volume


how does first law change


  • no work 
  • Q = ΔU