The photoelectric effect was discovered by:
a. Maxwell.
b. Einstein.
c. Hertz.
d. Planck.
c
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In the experimental determination of the heat transfer coefficient between a heated steel ball and crushed mineral solids, a series of 1.5% carbon steel balls were heated to a temperature of 700°C and the center temperature-time history of each was measured with a thermocouple as it cooled in a bed of crushed iron ore that was placed in a steel drum rotating horizontally at about 30 rpm. For a 5-cm-diameter ball, the time required for the temperature difference between the ball center and the surrounding ore to decrease from 500°C initially to 250°C was found to be 64, 67, and 72 s, respectively, in three different test runs. Determine the average heat transfer coefficient between the ball and the ore. Compare the results obtained by assuming the thermal conductivity to be infinite
with those obtained by taking the internal thermal resistance of the ball into account.
GIVEN
Heat steel balls are put in crushed iron ore
Balls are 1.5% carbon steel balls
Initial temperature of balls (To) = 700°C
Ball diameter = 5 cm = 0.05 m
Temperature difference between the ball center and the ore
Center temperature of the balls decreases from 500°C to 250°C
Time taken was found to be 64, 67, and 72 s, respectively, in three different test runs
FIND
The average heat transfer coefficient between the ball and the ore.
Compare the results obtained
(a) by assuming the thermal conductivity to be infinite with
(b) those obtained by taking the internal thermal resistance of the ball into account
ASSUMPTIONS
Temperature of the iron ore is uniform and constant
SKETCH
Semiconductors: The energy gap between the valence and conduction bands in a certain semiconductor is 1.25 eV. What is the threshold wavelength for optical absorption in this substance? (c = 3.00 × 108 m/s, h = 6.626 × 10-34 J ? s)
A. 599 nm B. 959 nm C. 873 nm D. 994 nm
If the outer air temperature in Problem 1.11 is –2°C, calculate the convection heat transfer coefficient between the outer surface of the window and the air assuming radiation is negligible.
GIVEN
• Window: 1 m by 3 m
• Thickness (L) = 7 mm = 0.007 m
• Surface temperatures
? Inner (Ti) = 20°C
? outer (To) = 17°C
• The rate of heat loss = 1040 W (from the solution to Problem 1.11)
• The outside air temperature = –2°C
FIND
• The convective heat transfer coefficient at the outer surface of the window ( h c)
ASSUMPTIONS
• The system is in steady state and radiative loss through the window is negligible
SKETCH
As a 6 solar-mass star leaves the main sequence on its way to becoming a red supergiant, its luminosity
A) remains roughly constant. B) decreases. C) first increases, then decreases. D) increases. E) first decreases, then increases.