What spectacular development in organisms marks the beginning of the Paleozoic era?
What will be an ideal response?
Answer: Organisms developed ability to secrete calcium-carbonate and calcium-phosphate for the formation of hard shells and skeletons. Hard shells provided protection from predators and UV radiation, and made organisms better able to be preserved as fossils.
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A metal-clad electrical heating element of cylindrical shape, as shown in the sketch below, is immersed in water at atmospheric pressure. The element is a 5-cm-OD, and heat generation produces a surface temperature of 300°C. Estimate the heat flux under steady state conditions and the rate of heat generation per unit length.
GIVEN
? Cylindrical, electrical heating element
? Heat generation produces a 300°C surface temperature
FIND
(a) Heat flux
(b) Rate of heat generation
ASSUMPTIONS
? The system operates at atmospheric pressure
? The heater surface is black
SKETCH
PROPERTIES AND CONSTANTS
Inspection of the boiling curve in Figure (9.1) indicates that at 300°C surface temperature, the heating system must operate in the film boiling regime. Hence, a vapor layer covers the surface of the heater and it is appropriate to evaluate the physical properties at the mean film temperature of (100 + 300)/2 = 200°C.
From Appendix 2, Table 35
From Appendix 2, Table 13
A mass attached to the end of a spring is pulled out and released on a surface with friction. The work
? done on the mass by the force exerted by the spring
A. ?has the same sign as the change in energy owing to friction during one half of each cycle. B. ?never has the same sign as the change in energy owing to friction. C. ?always has the same sign as the change in energy owing to friction. D. ?never has the same sign as the change in energy owing to friction if the force of friction is greater than the spring force. E. ?always has the same sign as the change in energy owing to friction if the force of friction is greater than the spring force.
You pull on a crate with a rope. If the crate moves, the rope's pull on the crate must have been larger than the crate's pull on the rope, but if the crate does not move, both of these pulls must have been equal
A) True B) False
A light bulb used in a slide projector draws a current of 6 amperes when operating on 120 volts.The resistance of the light bulb is 20 ohms
a. True b. False Indicate whether the statement is true or false