Show that the effective conductance. A1 F 1-2 for two black parallel plates of equal area connected by re-radiating walls at a constant temperature is
GIVEN
• Two black parallel planes of equal area connected by re-radiating walls at a constant temperature
FIND
• Show that
SKETCH
Since A1 and A2 cannot see themselves, F1–1 = F2–2 = 0.
The shape factors from a single surface must sum to unity
Substituting this and A1 = A2 into the expression for A1 F12
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Double-Slit Interference: Two radio antennas are 10 km apart on a north-south axis on high mountain tops at the seacoast. The antennas broadcast identical AM radio signals, in phase, at a frequency of 4.70 MHz. A steamship, 200 km offshore, travels toward the north at 15 km/h and passes east of the antennas. A radio on board the ship is tuned to the broadcast frequency. The reception of the radio signal on the ship is a maximum at a given instant. How much later until the next occurrence of maximum reception? (c = 3.00 × 108 m/s)
A. 5.1 min B. 3.8 min C. 6.4 min D. 7.7 min E. 8.9 min
How do small stars die? About how large are their remnants?
A. They become white dwarfs, about the size of the Earth. B. They go supernova, then form neutron stars about the size of a country. C. They become black holes, about the size of a marble. D. They go nova, then form black holes about the size of a person. E. They become neutron stars, about the size of a city.
A radio wave transmits 1.59 W/m2 average power per unit area. What is the peak value of the associated magnetic field? (?0 = 4? × 10^?7 T?m/A and c = 3.00 × 10^8 m/s)
a. 1.33E–14 T b. 3.46E+1 T c. 1.15E–7 T d. 7.28E–5 T e. 8.16E–8 T
Why do hot water heaters lose their efficiency quicker in households with hard water?
A) The hard water calcium and magnesium ions corrode the heating element. B) The hard water calcium carbonate deposits oxidize the inner tank wall. C) The calcium carbonate on the inner surface of the hot water tank builds up to the point that it insulates the water from the heating element. D) The hard water calcium ions exchange with iron ions from the inner metal wall which react with the water heater base to form rust and deteriorate the bottom of the tank.