Reversibility

Reversible Process  Quasi-static processes meant that each step was slo enough to maintain equilibrium.  If the process is reversed the work and heat also just reverse sign.  Heat lost to friction is not reversible. Reversible process is infinitely slow; system stays at equilibrium. Irreversible process has heat loss to friction; system has net loss of useful energy.

Plotting Reversibility  The PV diagram can plot a quasi-static, reversible process.  Irreversible processes cannot be plotted. P V P V

Reversible Cycle  A series of reversible processes can be plotted on a PV diagram.  A reversible cycle is a set of reversible processes that return to the initial state. P V

Carnot Engine  An ideal Carnot engine consists of four processes. 1) expand gas isothermally. 2) expand gas adiabatically. 3) compress gas isothermally. 4) compress gas adiabatically.

Expansion  During the isothermal expansion there is work done with heat in.  There is no heat flow during the adiabatic expansion, but work is done. P V

Compression  During the isothermal compression work is returned with heat out.  There is no heat flow during the adiabatic compression, but work is returned. P V

Carnot Efficiency  The heat and temperatures are related in a Carnot engine. |Q L | / |Q H | = T L / T H|Q L | / |Q H | = T L / T H  This is the ideal engine efficiency.

Underwater  A nuclear plant produces 540 MW of power while the fuel releases 1590 MW.  Steam enters the turbine at 556 K and is discharged at 313 K.  What is the ideal and actual efficiency?  The ideal efficiency assumes a Carnot engine.  The actual efficiency is found from the power usage. next