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Unformatted text preview: tude, and under these circumstances, the motion of
the bar is given by the kinematic equations. The initial velocity of the bar is zero so distance
is given by 1/2 *a*t^2.
Some of you who had physics in high school realized that there is a confusing issue
associated
with the fact that the magnetic ﬂux is decreasing (and of course, all of you will now know this
issue since you know about Faraday's Law). In these problems where there is a current that
exists in the wire, we assume that the induced current from Faraday's Law is small compared
to the original current and can be ignored. If you are
confused by this sort of thing by a question on the ﬁnal, make sure to ask. We will tell you if
we want you to worry about the induced currents as well as the original one. Physics 7D
Name: Quiz7B
ID: 1. A metal stick (with mass m=0.5kg) can slide along the wire loop in the Bfield
(B=10T)pointing into the paper. The length of the stick is Lo =8cm. It has a constant
current Io=1A. Initially, it stays at rest at the distance Lo away from the left end of the
loop. Then how long will this stick take to reach the left end of the loop? 2. A current I=1A flows in a plane rectangular current loop with width w=20cm and length
b=40cm. The loop is placed into a uniform magnetic field B=10T in the way that the
sides of width w are perpendicular to B. And there is an angle =45 degree between the
sides of length b and the field B. Calculate , the magnitude of the torque about the
vertical axis of the current loop due to the interaction of the current through the loop
with the magnetic field. Solutions: 1.
t= (2*0.5/10/1)^1/2=0.316s 2. =w b cos( )B I=0.2*0.4*cos(45)*10*1=0.566N.m The key to problem 1 is to recognize that the force on the bar from the magnetic ﬁeld is
both constant in direction and magnitude, and under these circumstances, the motion of
the bar is given by the kinematic equations. The initial velocity of the bar is zero so distance
is given by 1/2 *a*t^2.
Some of you who had physics in high school realized that there is a confusing issue associated
with the fact that the magnetic ﬂux is decreasing (and of course, all of you will now know this
issue since you know about Faraday's Law). In these problems where there is a current that
exists in the wire, we assume that the induced current from Faraday's Law is small compared
to the original current and can be ignored. If you are
confused by this sort of thing by a question on the ﬁnal, make sure to ask. We will tell you if
we want you to worry about the induced currents as well as the original one....
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This note was uploaded on 09/23/2013 for the course PHYSICS 7D 7D taught by Professor Barwick during the Spring '11 term at UC Irvine.
 Spring '11
 Barwick

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