Download Electromagnetism: Induced Currents and Magnetic Forces and more Exams Physics in PDF only on Docsity! 1. (25 pts) a) (5 pts) What would happen in a circuit loop if the induced current did not oppose the changing magnetic flux? As the induced current produces more magnetic field, the flux changes more, creating more induced current, etc. You’d just get more and more current! This would violate conservation of energy. b) (5 pts) A conducting loop that is lying in the plane of the page is moving to the right as it enters a magnetic field that is directed into the page. Will an external agent be required to maintain the motion of the loop at a constant velocity? Explain. (i.e. would you need to pull the loop or would it just keep moving at the same velocity by itself?) An external force would be required. The magnetic force on the current in the wire will be directed such that the wire slows down. In order to maintain a constant velocity, some source of external force is needed to counter this. In fact, it is the work done by this force that is converted into the electrical energy of the induced current since the magnetic force cannot perform any work on the charges in the wire. c) (5 pts) Under the conditions specified above, in which direction will the current induced in the wire loop flow? (A quick sketch may help.) With the magnetic field in and the wire entering the field from the left, the current would be counter- clockwise: the magnetic flux is increasing so we need the magnetic field from the induced current to be out of the page. Thus the current must flow counter-clockwise. d) (5 pts) Which would be more hazardous for the wearer: wearing a metal bracelet near a wire carrying several thousand amps of current that is constant or near a wire carrying a few tens of amps that changes direction 120 times each second? Explain. A constant current would not induce any current since the magnetic field is not changing. However, the rapidly changing (though smaller) current would produce a lot of changing magnetic flux which would induce current in the bracelet. This could be either a shock or burn hazard to the wearer. Note that the second wire is not changing direction. It’s the current in the wire that is changing direction. Also note that the magnetic flux is much greater for the large, steady current. But since it is not changing, it will not induce any current in the bracelet. e) (5 pts) A current is induced in a wire loop when a magnet is brought closer to it. Yet there is no magnetic force acting on the charges in the wire since they are at rest! What is it that exerts a force on the charges in the wire to start them moving? An electric field is produced by the changing magnetic field that then accelerates the (originally mo- tionless) charges. Just saying that “the changing flux induces the current” is not enough. We’re looking for the physical mechanism that actually exerts force on the charges.
