Engineering physics

• inertia


  The resistance to change in velocity when subjected to a force
• Moment of inertia


  The measure of resistance to rotational acceleration
• Inertia = m x r^2
• moment of inertia =  ∑mr^2
• Adiabatic restriction?


  Q = 0
• Adiabatic 1st law?


  Change in U = - W
• Rapid expansion indicates adiabatic expansion - Work done by system so is +ve work
• Efficiency of engine = net work output / energy supplied from external source
• What are the four strokes in an engine?

  • Suck - air drawn into cylinder (intake)
  • Squeeze - fuel compressed (adiabatic compression) spark ignites (mix burns due to pressure increases)
  • Bang - expansion (extracts work from explosion)
  • Blow - exhaust (gas valve opens and forces out burnt fuel) heat escapes due to pressure decrease
• Otto cycle (petrol) has a spark plug that ignites the mixture. Has a constant volume when heating (combustion / Q in)
• Diesel has no spark plug, the mixture is ignited due to pressure and temperature increasing very quickly (not enough time for heat to exit system)
• Diesel reaches a higher pressure during compression, has no constant volume at combustion, is more efficient (greater area so greater work out)
• Work out = area of graph
• if there is no change in volume?

  there is no work done (W=pxchange in volume)
• Isothermal graph?


  follows isotherms (one line)
• Adiabatic graph?


  very steep jump across isotherms
• Heat engine use heat to do useful work
• Overall efficiency = thermal efficiency x mechanical efficiency
• Thermal efficiency = indicated / input x100
• Mechanical efficiency = output/ indicated x100
• Overall efficiency = output / input x100
• For ideal engines, we assume?

  mechanical efficiency is 100%
• Heat engines extract work by converting heat into work
• heat engines never extract all the work/ heat. There is always an amount of wasted heat (2nd law of thermodynamic)
• Describe heat engine diagram

  high temp reservoir transfers Qh to heat engine (+ve work), then work is extracged from heat engine. Qc also transfers from heat engine to cold temp reservoirs (-ve work).
• In a heat engine, what is Qc?

  Heat lost to surroundings (cold heat)
• Q net = W net
• W net = Qh - Qc
• Thermal efficiency = P indicated/ P input
• Thermal efficiency = W net / Qh
• Thermal efficiency = Qh- Qc / Qh
• For ideal engines, Q is directly proportional to T
• Max theoretical efficiency = Th-Tc/Th
• K = c +273
• max Theoretical efficiency of any engine is less than one. For an engine to be 100% efficient, Tc would have to be absolute 0 (Mte = 1 - Tc/Th) which isnt possible.
• Second law of thermodynamics 


  Heat cannot flow from a cold space to a hotter space without the input of work
• Second law of thermodynamics 


  You can never convert ALL heat into work in a heat engine (some will be transferred to cold space)
• Reverse heat engines 


  Transfers energy from cold space to a hotter space by doing work
• Describe reverse heat engine diagram

  Work is put into the engine. Qc extracted from cold temp reservoir by the reverse heat engine. Qh is transferred to high temp reservoir from reverse heat engine.
• Refrigerators maximise Qc (exctract Qh from Qc) and try to make cold space colder