Problem 1

Consider an electric refrigerator located in a room. Determine the direction of the work and heat interaction(in or out)when the following are taken as the system

(a)the contents of the refrigerator

(b)all parts of the refrigerator including the contents

(c)everything contained within the room during a winter day

Problem 2

Water is being heated in a closed pan on top of a burner while being stirred by a paddle wheel. During the process,30KJ of heat is transferred to the water, and 5KJ of heat is lost to the surrounding air. The paddle-wheel work amount to 500Nm.

Determine the final energy of the system if its initial energy is 10KJ.

Problem 3

Water is pumped from a lower reservoir to a higher reservoir by a pump that provides 20kW of shaft power. The free surface of upper reservoir is 45 m higher than that of lower reservoir. If the flow rate of water is measured to be 0.03 m3/s.

Determine mechanical power that is converted to thermal energy during this process due to frictional effect.

Problem 4

Windmills slow the air and cause it to fill a larger channel as it passes through the blades. Consider a circular windmill with a 7m diameter rotor in a 8m/s wind on a day when the atmospheric pressure is 100kPa and the temperature is 200c.The wind speed behind the windmill is measured at 6.5 m/s.

Determine the diameter of wind channel downstream from the rotor and the power produced by this windmill, presuming that the air is incompressible.

Problem 5

The pump of a water distribution system is powered by a 15-kW electric motor whose efficiency is 90 percent. The water flow rate through the pump is 50 L/s. The diameters of the inlet and outlet pipes are the same, and the elevation difference across the pump is negligible. If the pressures at the inlet and outlet of the pump are measured to be 100kPa and 300 kPa (absolute), respectively, determine the mechanical efficiency of the pump.