An ideal voltmeter is one that does not change the behavior of a circuit when it is properly used to measure potential difference. The internal resistance of such a meter is
A)
dependent on the circuit.
B)
infinite.
C)
the same resistance as the circuit.
D)
0 ?.
E)
cannot be determined
B
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A. A B. B C. C D. D E. Zero F. AA G. BB H. CC I. DD
If Alex wishes to rotate his skateboard, then he must apply a
A) torque. B) rotational maneuver. C) pause before bearing down on the board.
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As a designer working for a major electric appliance manufacturer you are required to estimate the amount of fiberglass insulation packing (k=0.035 W/m K) that is needed for a kitchen oven shown in figure below. The fiberglass layer is to be sandwiched between a 2 mm thick aluminum cladding plate on the outside and 5 mm thick stainless steel plate on the inside that forms the core of the oven. The insulation thickness is such that the outside cladding temperature does not exceed 40 °C when the temperature at inside surface of the oven is 300 °C. Also the air temperature in the kitchen varies from 15 °C to 33 °C, and the average heat transfer coefficient between the outer surface of the oven and air is estimated to be 12 W/(m2 K). Determine the thickness of fiber glass insulation that
is required for these conditions. What is the outer surface temperature when the inside surface of the oven is at 475 °C? How much fiberglass insulation [k = 0.035 W/(m K)] is needed to guarantee that the outside temperature of a kitchen oven will not exceed 43°C? The maximum oven temperature to be maintained by the convectional type of thermostatic control is 290°C, the kitchen temperature may vary from 15°C to 33°C and the average heat transfer coefficient between the oven surface and the kitchen is 12 W/(m2 K). GIVEN • Kitchen oven wall insulated with fiberglass sandwiched between aluminum cladding and steel • Fiberglass thermal conductivity (k) = 0.035 W/(m K) • Thickness of Al cladding (LAl)=0.002 m, thickness of ss plate (Lss)=0.005 m • Convective transfer coefficient on the outside of wall ( ch ) = 12 W/(m2 K) • Maximum oven temperature (Ti) = 300°C • Kitchen temperature (T?) may vary: 15°C < T? < 33°C FIND • Thickness of fiberglass (L) to keep the temperature of the outer surface of the oven (Two) at 40°C or less ASSUMPTIONS • One dimensional, steady state heat transfer prevails • The temperature of the inside of the wall (Twi) is the same as the oven temperature • The thermal resistance of the metal wall of the oven is negligible PROPERTIES AND CONSTANTS Fiber glass (kfg)=0.035 W/m K Aluminum(kAl)=164 W/m K Stainless steel (kss)=14.4 W/m K