I am confused about Ch 20 #80. I know Power is the rate of work and I can use that to calculate current . I would also used the magnetic field equation B = N (uI)/2r to calculate the number of turns required to run power at max power (i.e use the calculated Imax). But I don't know B and I am not sure how to calculate that from V.
Ahh... ok, it took me a minute to figure this one out, but the trick is that it is using things you know from previous chapters (tricky folks these text book writers are). Do not be distracted by the equation they give you immediately! Think back to when you looked at electrical power:
P=IV
So, you know V, and Pmax... you can directly determine Imax from this. Then you can use the equation they give to determine the maximum magnetic field that can be generated.
Oftentimes half the trouble in figuring out a problem is determing what information you need to use and when. Most of the problems in textbook are divided up into the appropriate sections where you learn the material so it is easy to determine how to approach a given "type" of problem. The trouble is in the real world there are no "types" of problems. You have to take each situation and consider what factors are present and relavent. Every now and again textbooks (and exam writers!) throw in questions that test your ability to discern what is needed to help train you to solve problems more generally. Learning how to do this takes some experience (which unfortuantely just requires time and practice), and requires that you think about the concepts and how they apply, not just classify the problem as a particular type.
Hope that helps you out.
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