Deck 2: Energy, Energy Transfer, and General Energy Analysis

Question 1

A 1.5-kW electric resistance heater in a room is turned on and kept on for 20 min. The amount of energy transferred to the room by the heater is

A)1.5 kJ
B)60 kJ
C)750 kJ
D)1800 kJ
E)3600 kJ

Answer 1

1800 kJ

Question 2

A 200 W vacuum cleaner is powered by an electric motor whose efficiency is 70%. (Note that the electric motor delivers 200 W of net mechanical power to the fan of the cleaner). The rate at which this vacuum cleaner supplies energy to the room when running is

A)140 W
B)200 W
C)286 W
D)360 W
E)86 W

Answer 2

286 W

Question 3

A 40-cm-long, 0.6-cm-diameter electric resistance wire is used to determine the convection heat transfer coefficient in air at 25°C experimentally. The surface temperature of the wire is measured to be 150°C when the electric power consumption is 90 W. If the radiation heat loss from the wire is calculated to be 30 W, the convection heat transfer coefficient is

A)0.48 W/m².°C
B)127 W/m².°C
C)63.7 W/m².°C
D)95 W/m².°C
E)200 W/m².°C "

Answer 3

63.7 W/m².°C

Question 4

A 1.5-m² black surface at 120°C is losing heat to the surrounding air at 30°C by convection with a convection heat transfer coefficient of 18 W/m².°C, and by radiation to the surrounding surfaces at 10°C. The total rate of heat loss from the surface is

A)1483 W
B)2430 W
C)2448 W
D)3913 W
E)2609 W

Question 5

Heat is transferred steadily through a 0.15-m thick 3 m by 5 m wall whose thermal conductivity is 1.2 W/m.°C. The inner and outer surface temperatures of the wall are measured to be 18°C to 4°C. The rate of heat conduction through the wall is

A)112 W
B)3360 W
C)2640 W
D)38 W
E)1680 W