Four charge are placed at four corners of a square that as 30cm on each side. Find the potential at the center of the square If (a) the four charges are each +2.0uC and (b) two of the four charges are +2.0uC and two are -2.0uC.

An insulated 40 ft^3 rigid tank contains air at 50 psia and 120°f. a valve connected to the tank is now opened, and air is allowed to escape until the pressure inside drops to 25 psia. the air temperature during this process is kept constant by an electric resistance heater placed in the tank. determine the electrical work done during this process.