A coal-fired plant generates 600 MW of electric power. The plant uses 4.8 × 106 kg of coal each day. The heat produced by the combustion of coal is 3.3 × 107 J/kg
The steam that drives the turbines is at a temperature of 300°C, and the exhaust water is at 37°C.
(a) What is the overall efficiency of the plant for generating electric power?
(b) What is the maximum efficiency that this plant could possibly have using the same temperature extremes that it presently uses?
(c) How much thermal energy is exhausted each day by this plant?
What will be an ideal response?
Answer: (a) 33% (b) 46% (c) 1.1 × 1014 J
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In the construction of a space platform, two structural members of equal size with surfaces that are considered black are placed relative to each other as shown schematically below. Assuming that the left member attached to the platform is at 500 K, the other is at 400 K and that the surroundings are treated as though black at 0 K, calculate (a) the rate at which the warmer surface must be heated to maintain its temperature, (b) the rate of heat loss from the cooler surface to the surroundings, (c) the net rate of heat loss to the surroundings for both members.
GIVEN
• Two black surfaces as shown above on a space platform
FIND
(a) The rate of heating of the warmer surface (q1)
(b) Net rate of heat loss to the surroundings (qs)
(c) The rate of heat loss from the cooler surface to the surroundings (q2s)
ASSUMPTIONS
• Steady state
• The surroundings behave as a blackbody enclosure at Ts = 0 K
• The plate also lose heat from their back surface
PROPERTIES AND CONSTANTS
the Stephan-Boltzmann constant (?) = 5.67 × 10–8 W/(m2 K4)
Greta and her family float higher in the dense water of the Dead Sea because
A) less volume of water is displaced. B) more volume of water is displaced. C) of a greater buoyant force.
The unit of electric resistance, the ohm, is equivalent to which of the following?
a. V/A b. V?m c. A/s d. A/m
As ice floats in water, about 10% of the ice floats above the surface of the water. If we float some ice in a glass of water, what will happen to the water level as the ice melts?
a. The water level will rise 10% of the volume of the ice that melts. b. The water level will rise, but not as much as the 10% indicated in answer a. c. The water level will remain unchanged. d. The water level will become lower.