- #141
Red_CCF
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Chestermiller said:No. The interface/external pressure always balances for a massless frictionless piston. The variation in external pressure does not have to be applied in discrete steps. This is just a specific example that your book presented. The external force variation can also be applied continuously with time, as long as it is done very slowly. Do you want to try modeling some other applied force variations with time to see how that plays out?
I thought quasistatic process is the step-by-step process we solved for but at infintiessimal step-sizes. Does increasing pressure in dP incrment count as a discrete steps? From my recollection of first year calculus, continuous function y(x) is one where infinitessimal dx variation causes infinitessimal dy change.
With regards to the step about "waiting until equilibrium is reached," I was thinking that although interface/external pressure always equal, pressure inside the cylinder does not instantaneously equal the interface pressure. As we approach quasistatic process, I imagine that the time for cylinder pressure to equal interface pressure to approach dt; is this correct?
Out of curiosity, what other kinds of applied force variation can also work for quasistatic processes?
Chestermiller said:You can apply any external force variation with time you desire. In the case of the spring-damper model, you already calculated the motion of the piston necessary to hold the force constant. You are asking an experimental question, rather than a conceptual question. Think about putting a force transducer on the piston and controlling its motion so that the measured force varies in the way that you desire. This can be done automatically (with motors and feedback loops) or, in concept, it can be done manually.
Chet
How would one control the motion of a massless/frictionless piston (making it start/stop), if such an object theoretically exists?
Thank you very much