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Phy 122
Your 'question form' report has been received. Scroll down through the document to see any comments I might have inserted, and my final comment at the end.
** Question Form_labelMessages **
Z axis
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I hope this is the last of the bombardment of questions I've thrown at you.
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No problem. Most of the questions are the result of errors on my end, so I couldn't complain even if I wanted to.
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I'm looking at problem set 4 #18. It states:
Problem
A segment of wire is 8 cm long and carries a current of 5 amps. If the current is directed along the positive x axis and a magnetic of .01 Tesla is directed at 56 degrees with the z axis, with the field vector parallel to the y-z plane, then what will be the magnitude and the direction of the force exerted on the wire by the field?
Solution
The magnitude of the force on a charge q moving with velocity v in a direction which makes angle `theta with a magnetic field B is F = (q * v) * B * sin(`theta). From this it can be concluded that the force on a current I in a wire of length L is F = ( I * L ) * B * sin(`theta). (the sum of all the q * v contributions in the wire is easily shown to be I * L).
It the current was moving perpendicular to the field, sin(`theta) would be 1 and the force would be
force = (I * L) * B = ( 5 amps * 8 cm) * .01 Tesla = .4 amp m * .01 N / (amp m) = .004 N.
Since the current and field are at an angle of 56 degrees, the force will be
force = I * L * B * sin(`theta) = .004 N * sin( 56 deg) = .003316 N.
The direction of the force is found from the right-hand rule by 'turning' the velocity vector toward the field vector and checking the direction of the thumb. The fingers will point along the velocity vector, is in the x direction, with the palm turned so it faces the direction of the velocity vector (imagine holding the palm so it faces vertically upward, then turning it 56 degrees toward the y axis). The thumb will point 56 degrees below the y axis, in the y-z plane, and this will be the direction of the force vector.
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I get everything up until the last sentence. As I see it the the thumb would not by 54 degrees below the y axis, but more like 34 deg.(90 deg-54deg) below the y axis.
Or maybe I could see it as being 56 degrees above the z axis.
I have a tendency to get muddled in when z comes in, but that's how I'd visualize it.
???What might I be messing up???
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You are correct. The angle would be 90 deg - 56 deg = 34 deg.
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Once more it's my error.
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