Question

how to find work in thermodynamics.????

Answer 1

For mechanical work done in thermodynamics there are many simplified formulas for different cases all are derived from basic energy balance.

For closed system,

dQ=dU+dW

If we know the change in heat energy and internal energy of the System, work done can be find from the above formula.

Always be careful with sign convention while using the above formula.

Heat lost from to the System is negative and heat given to the System is positive whereas work done on the System is negative and work done by the System is positive.

Simplified expression for work done for any closed system is

equal to integral pdv

F

or constant volume process it is always zero.

For constant pressure process it is equal to p(v2-v1)

for isothermal process work done Is equal to p1v1ln(v2/v1)

For polytropic process it is equal to p1v1-p2v2/n-1.

For open systems, to calculate the work done always use integral( - vdp).

Most of the devices engineering devices comes under open system.

Examples: pumps, turbines, nozzles, diffusers.

Most of the open system are operating at Steady state.

Hence, use of steady flow energy equation is the best way to calculate work done from the open systems operating under steady state.