GR0177 #86



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Comments 
Copperknickers 20160708 06:46:05  Is there a simpler way to figure out the cos(wt), sin(wt) part? Without having to remember any formula by rote, that is? asdfman\'s solutions seems nice. Any alternative ones?
asdfasdfasdf 20160907 14:53:11 
I\'m not sure there\'s a simpler way to figure out the trig part. The only \"memorization\" needed is: (a) what are sin(0) and cos(0)? (b) what is the derivative of sin(wt) or cos(wt)? The question provides the initial condition of the n vector being parallel to the yaxis. When that is the case, there is no magnetic flux through the coil, right? So, when t=0, we should have B=0. sin(0)=0, so we\'ll use sine, not cosine. And you know you need to use some sort of periodic function since it\'s rotating...once it rotates such that the n vector lies parallel to the xaxis, we have the maximum flux possible, right? sin(90)=1, so that confirms that we should use sine.Then, since we get our emf from d(phi)/dt, we need the derivative of sin(wt), which is w*cos(wt).Does that help? [also, sorry if this text gets garbled...I\'m not sure why slashes are being randomly inserted.]

  John Smith 20131017 02:02:09  I am a foreigner, the time when I solve this question, I can hardly notice that the FUNK MILLIamperes. That will cost me lots of time if it happened in the real test.   asdfman 20091102 23:47:03  it says the coil starts at which means and therefore a function.
must therefore be a cosine.
Do some algebra and you realize that it'll be something like 1.5^2, so it'll have something ~2.25 or 2.5. The only answer that fits the bill is E.   theevilmachines 20090712 17:58:53  I don't know about you guys, but my 0177 test doesn't have as an answer choice. Choice E is . Am I missing something?
theevilmachines 20090824 15:31:09 
Oh it says in milliamperes, not amperes.

  duckduck_85 20081103 22:20:12  how do you know that the resistance given in the problem isn't for the entire coil? (so that you needn't take the # of turns into account)
elzoido238 20081107 19:17:30 
The problem states that "...the coil resistance is 9 , which I interpret as the resistance of the entire coil. Since Yosun found the flux for 1 turn of the coil, you would need to account for the number of turns in the coil by dividing the total resistance by N (if, however one found the flux for the entire coil  i.e. =NBsin(t)  one would not need to find the resistance per turn.)
Thanks Yosun for this kickass site! :)

  sblmstyl 20081015 21:39:49  thanks yosun!   naama99 20061129 21:16:46  What do you mean by \"one uses Ohm's Law in Faraday's Law"? Can you be more specific about it? It seems to me that you are taking the R given in the question and merely using it as L
Richard 20071029 09:53:54 
I'm pretty sure she just means,
take Ohm's law and substitute the expression for the induced voltage given by Faraday's law:
Of course you need to take into account the number of turns...

  radicaltyro 20061028 21:53:31  You are missing the in your final answer.  

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