A pn-junction diode at 300 K produces a current density 1.00×104 A/m2 , with a forward bias of 0.10 V. Calculate the current density for a forward bias of 0.300 V.
What will be an ideal response?
There are two equations and two unknowns, J2 and J0. We eliminate J0 by taking the
ratio of J1/J2 and then solve for J2.
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Four resistors are connected in parallel: the resistance values are 4-ohms, 8-ohms,12-ohms, and 16-ohms: calculate the total circuit resistance
A salesperson for insulation material claims that insulating exposed steam pipes in the basement of a large hotel will be cost effective. Suppose saturated steam at 5.7 bars flows through a 30-cm- OD steel pipe with a 3 cm wall thickness. The pipe is surrounded by air at 20°C. The convective heat transfer coefficient on the outer surface of the pipe is estimated to be 25 W/(m2 K). The cost of generating steam is estimated to be $5 per 109 J and the salesman offers to install a 5 cm thick layer of 85% magnesia insulation on the pipes for $200/m or a 10-cm-thick layer for $300/m. Estimate the payback time for these two alternatives assuming that the steam line operates all year long and make a recommendation to the hotel owner. Assume that the surface of the pipe as well as the
insulation have a low emissivity and radiative heat transfer is negligible.
GIVEN
Steam pipe in a hotel basement
Pipe outside diameter (Do) = 30 cm = 0.3 m
Pipe wall thickness (Ls) = 3 cm = 0.03 m
Surrounding air temperature (T?) = 20°C
Convective heat transfer coefficient (hc) = 25 W/(m2 K)
Cost of steam = $5/109J
Insulation is 85% magnesia
FIND
Payback time for
(a) Insulation thickness (LIa) = 5 cm = 0.05 m; Cost = $200/m
(b) Insulation thickness (LIb) = 10 cm = 0.10 m; Cost = $300/m
Make a recommendation to the hotel owner.
ASSUMPTIONS
The pipe and insulation are black (? = 1.0) The convective resistance on the inside of the pipe is negligible, therefore the inside pipe surface temperature
is equal to the steam temperature The pipe is made of 1% carbon steel Constant thermal conductivities
SKETCH
A piece of iron and a cup of water both at room temperature are heated such that the thermal energy of each doubles. Which has the higher temperature?
A) the water, because of its higher specific heat capacity B) the iron, because it doesn't change phase C) both the same D) none of the above
In the halo, we do NOT find:
A) tidal streams. B) globular clusters. C) small satellite galaxies. D) spiral arms. E) old white dwarfs.