7DW13-Week8-DiscQuiz

The paper the length of the stick is lo 5cm it has a

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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 B-field (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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