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Chapter 7: Q18 CQ (page 259)
Most electrical appliances are rated in watts. Does this rating depend on how long the appliance is on? (When off, it is a zero-watt device.) Explain in terms the definition of power.
No, the rating of the electrical appliances does not depend on how long the appliance is on.
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Using values from Table 7.1, how many DNA molecules could be broken by the energy carried by a single electron in the beam of an old-fashioned TV tube? (These electrons were not dangerous in themselves, but they did create dangerous x rays. Later model tube TVs had shielding that absorbed x rays before they escaped and exposed viewers.)
Suppose a star 1000 times brighter than our Sun (that is, emitting 1000 times the power) suddenly goes supernova. Using data from Table 7.3:
(a) By what factor does its power output increase?
(b) How many times brighter than our entire Milky Way galaxy is the supernova?
(c) Based on your answers, discuss whether it should be possible to observe supernovas in distant galaxies. Note that there are on the order of 1011 observable galaxies, the average brightness of which is somewhat less than our own galaxy.
a) Calculate the work done on a 1500-kg elevator car by its cable to lift it 40.0 m at constant speed, assuming friction averages 100 N. b) What is the work done on the lift by the gravitational force in this process? c) What is the total work done on the lift?
(a) Use of hydrogen fusion to supply energy is a dream that may be realized in the next century. Fusion would be a relatively clean and almost limitless supply of energy, as can be seen from Table 7.1. To illustrate this, calculate how many years the present energy needs of the world could be supplied by one millionth of the oceans鈥� hydrogen fusion energy.
(b) How does this time compare with historically significant events, such as the duration of stable economic systems?
A pogo stick has a spring with a force constant of , which can be compressed. To what maximum height can a child jump on the stick using only the energy in the spring, if the child and stick have a total mass of? Explicitly show how you follow the steps in the Problem-Solving Strategies for Energy.
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