Ignore part b,,, (Ive written my question in the paper) i dont know why they use
ID: 2075031 • Letter: I
Question
Ignore part b,,, (Ive written my question in the paper) i dont know why they use pv=mrt to find work in the air on part a,, could someone also explain why they use that formula?? 2. A piston-cylinder device contains 5.0 x 10 kg of dry air (Ra-287.0 Jkg K). If the initial and final states are: v, = 2.5 x 104 m3, T1-300 K, and V2 = 5.0 x 104 m respectively, determine the work on the air in the cylinder in Joules if the process is (a) a constant-pressure heat-addition process and process. Show and reference all work. (10 points) Note: (b) an isothermal expansion ,.-, ,.sxis.-' /72,200 dv Sin a
Explanation / Answer
In piston-cylinder system, constant pressure heat addition means to add heat we can otherwise say work (since it is a cyclic process, total work done = heat ) at constant pressure. That means we keep the pressure constant and increase the volume. The whole concept here is first law of thermodynamics, i.e. " For a closed system undergoing cycle, the net interaction in the form of HEAT is equal to the net interaction in the form of work".
Since piston-cylinder system undergoes a cyclic process, if we find work done that it self is the heat added. Here at constant pressure, we need to add heat, so we have to increase volume in order to keep pressure as constant i.e. to satisfy Equation of state Pv= mRT.
As you asked why they are using Pv=mRT, every ideal gas follows the equation of state. And air is also a ideal gas so we use that equation to find Pressure from the given value of mass, gas constant, temperature and volume. The reason we use Pv=mRT is to find unknown value of pressure and we use that pressure in flow equation WORKDONE = P[V1-V2]. If we didn't use Pv=mRT , we cannot find pressure and work done.