An insulated piston-cylinder assembly containing a fluid has a stirring device operated externally. The piston is frictionless, and the force holding it against the fluid is due to standard atmospheric pressure and a coil spring. The spring constant is 500 lbf/ft. The stirring device is turned 10,000 revolutions with an average torque of 0.50 lbf-ft. As a result, the 2.0 ft diameter piston moves outward 2 ft. Find the change in the energy of the fluid, in Btu, if the initial force of the spring, Fs,i = 0.