1)Can the combined turbine-generator efficiency be greater than either the turbine efficiency or the generator efficiency? Explain 2) On a hot summer day, a student turns his fan on when he leaves his room in the morning. When he returns in the evening, will the room be warmer or cooler than the neighboring rooms? Why? Assume all the doors and windows are kept closed. 3) Water is being heated in a closed pan on top of a range while being stirred by a paddle wheel. During the process, 30 kJ of heat is transferred to the water, and 5 kJ of heat is lost to the surrounding air. The paddle-wheel work amounts to 500 N · m. Determine the final energy of the system if its initial energy is 10 kJ.
1)Can the combined turbine-generator efficiency be greater than either the turbine efficiency or the
generator efficiency? Explain
2) On a hot summer day, a student turns his fan on when he leaves his room in the morning. When he
returns in the evening, will the room be warmer or cooler than the neighboring rooms? Why? Assume all
the doors and windows are kept closed.
3) Water is being heated in a closed pan on top of a range while being stirred by a paddle wheel. During the
process, 30 kJ of heat is transferred to the water, and 5 kJ of heat is lost to the surrounding air. The
paddle-wheel work amounts to 500 N · m. Determine the final energy of the system if its initial energy is
10 kJ.
4) Consider a hooded electric open burner in an area where the unit costs of electricity and natural gas are
$0.07/kWh and $1.20/therm, respectively (Noting that 1 therm = 29.3 kWh) . The efficiency of open
burners can be taken to be 73 percent for electric burners and 38 percent for gas burners. Assuming both
burners operated for 1 hour. The electric burner consumed 3kWh (i.e a 3-kW hooded electric open burner). If the same utilized energy is to be produced by the gas burner, determine which burner will be
cheaper to operate and by how much
5) A 90-hp (shaft output) electric car is powered by an electric motor mounted in the engine compartment. If
the motor has an average efficiency of 91 percent, determine the rate of heat supply by the motor to the
engine compartment at full load.
6) . A 75-hp (shaft output) motor that has an efficiency of 91.0 percent is worn out and is replaced by a highefficiency 75-hp motor that has an efficiency of 95.4 percent. Determine the reduction in the heat gain of
the room due to higher efficiency under full-load conditions
7) An escalator in a shopping center is designed to move 30 people, 75 kg each, at a constant speed of 0.8
m/s at 45° slope. Determine the minimum power input needed to drive this escalator. What would your
answer be if the escalator velocity were to be doubled?
8) Consider a room that is initially at the outdoor temperature of 20°C. The room contains a 100-W
lightbulb, a 110-W TV set, a 200-W refrigerator, and a 1000-W iron. Assuming no heat transfer through
the walls, determine the rate of increase of the energy content of the room when all of these electric
devices are on.
9) A fan is to accelerate quiescent air to a velocity of 10 m/s at a rate of 4 m3
/s. Determine the minimum
power that must be supplied to the fan. Take the density of air to be 1.18 kg/m3
.
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