Chapter 2 Homework –CHE 340

2-26 A small electrical motor produces 5 W of mechanical power. What is this power in (a) N, m, and s units; and (b) kg, m, and s units?

2-35 A spring whose spring constant is 200 lbf/in has an initial force of 100 lbf acting on it. Determine the work, in Btu, required to compress it another 1 inch.

2-40 For a cycle, is the net work necessarily zero? For what kind of systems will this be the case?

2-45 An adiabatic closed system is raised 100 m at a location where the gravitational acceleration is 9.8 m/s2. Determinethe energy change of this system, in kJ/kg.

2-48 The lighting needs of a storage room are being met by 6 fluorescent light fixtures, each fixture containing fourlamps rated at 60 W each. All the lamps are on during operating hours of the facility, which are 6 a m to 6 p m 365 days ayear. The storage room is actually used for an average of 3 h a day. If the price of electricity is $0.08/kWh, determine theamount of energy and money that will be saved as a result of installing motion sensors. Also, determine the simple paybackperiod if the purchase price of the sensor is $32 and it takes 1 hour to install it at a cost of $40.

2-61 A 75-hp (shaft output) motor that has an efficiency of 91.0 percent is worn out and is replaced by a high-efficiency75-hp motor that has an efficiency of 95.4 percent. Determine the reduction in the heat gain of the room due to higher efficiencyunder full-load conditions.

2-63 A 75-hp (shaft output) motor that has an efficiency of 91.0 percent is worn out and is to be replaced by a high-efficiency

motor that has an efficiency of 95.4 percent. The motor operates 4368 hours a year at a load factor of 0.75.Taking the cost of electricity to be $0.08/kWh, determine the amount of energy and money saved as a result of installingthe high-efficiency motor instead of the standard motor. Also, determine the simple payback period if the purchase prices ofthe standard and high-efficiency motors are $5449 and $5520, respectively.

2-68 The water in a large lake is to be used to generate electricity by the installation of a hydraulic turbine-generatorat a location where the depth of the water is 50 m (Fig. 2-62). Water is to be supplied at a rate of 5000 kg/s. If theelectric power generated is measured to be 1862 kW and the generator efficiency is 95 percent, determine (a) the overallefficiency of the turbine—generator, (b) the mechanical efficiency of the turbine, and (c) the shaft power supplied by heturbine to the generator.

2-71 Water is pumped from a lake to a storage tank 20 m above at a rate of 70 L/s while consuming 20.4 kWof electric power. Disregarding any frictional losses in the pipes and any changes in kinetic energy, determine(a) the overall efficiency of the pump-motor unit and (b) the pressure difference between the inlet and the exitof the pump.