thermal dynamics Flashcards
equation of first law of thermodynamics
Q=^U+W
condition where w is -ve
gas compresses
condition where q is +ve
heat energy into the system
condition for +ve ^U
temperature increases
condition for +ve w
gas expands
use 1st law compare the signs of each terms in gas ring and microwave
gas ring: +Q,+U,W=0
microwave: -Q,+U,-w
what does isothermal mean
no temp change,^U=0
what does adiabatic mean
no heat transferred to the surroundings Q=0
what does isochoric mean
constant volume, since W=p^V, W=0
what does isobaric mean
constant pressure Q=^U+W
compare graphs of isothermal and adiabatic
isothermal requires greater pressure to reach the same volume as adiabatic. Adiabatic has steeper gradient and adiabatic constant (same initial p for both )
why are there round edges in the engine cycle diagram
Valves take finite time to open
In real engine why heating up an engine can not be taken place at constant pressure
heating can not be controlled
In real engine why cooling can not be taken place at constant volume
the piston would have to stop
How does the max pressure in the real life engine compare to theoretical, why
It is lower than theoretical due to incomplete combustion
Why is there an extra (horizontal) region in the real engine cycle
Real engine needs induction and exhausts strokes(in theoretical cycle same air is used repeatedly )
How does the real engine cycle graph show that the efficiency is much lower than 100%
The area of loop(i.e. the net work done) is less than the area of loop in the theoretical cycle.
Why is the mechanical efficiency of an engine never 100%
-some energy expended to uses like driving oil and water pump
-friction between moving parts wastes energy as heat
Why is the thermal efficiency of an engine never 100%
-compression is not perfectly adiabatic(2nd law of thermodynamics) so max pressure is lower
-Incomplete combustion
-opening and closing valves takes time
Why does diesel engine has a much higher compression ratio than petrol
Diesel engines requires high pressure to reach high enough temp to ignite fuel whereas petrol ignites air mixture at lower pressure/temperature
Explain why the compression of a gas can be considered to be an isothermal
change when the gas is compressed very slowly.
-energy transfer must take place to maintain constant internal energy
-slow change means there is sufficient time for heat to leave the system
