Exam 2 practice problems solutions

# A sketch a graph of what the current in the coil will

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a. Sketch a graph of what the current in the coil will be as a function of time. If the current is flowing upwards on the side of the coil facing you indicate that with a positive current on the graph. Indicate a current flowing the other way as negative. The coil is connected to the power grid at the top, so it is part of a complete circuit. b. Predict the magnitude of the EMF (voltage-like effect) this design will produce during the time it takes the magnets to rotate 180 degrees. F

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The magnetic field lines within the coil point to the right, but as the poles of the two magnets rotate away the number of field lines pointing to the right will decrease. The induced magnetic field will try to restore these field lines as they disappear. Right hand rule #2 (grab a wire on the front face of coil with fingers inside the coil pointing to the right) predicts that the induced current will be flowing downward on the front face of the coil. This is a negative current according to the convention described in the problem. As the magnets complete their 180 degree rotation the magnetic field lines will be pointing to the left since the north and south poles have changed places. Still the induced current will be down in order to produce an induced field that cancels the newly appearing leftward field lines. Once the poles begin rotating away again these leftward field lines start to disappear. To try to restore these lines as they are disappearing a positive current is needed (use RHR #2 with the field within the coil pointing to the left.). The induced current continues to alternate directions every 0.1 s . To predict the magnitude of the EMF I’ll use Faraday’s law. At 0 t s the magnetic field lines within the coil point to the right. I’ll define that to mean a positive magnetic flux through the coil.    2 2 2 5 cos cos 1 1 2 2 3.5 2000 0.5 1.1 10 0.1 0 f f f i i i B f i f i f i B A B A B B BN r N N N r t t t t t t t T V s s  13. You’ve learned that it is possible to use the magnetic field to cause an electric current to flow in a circuit without needing a power supply. You have the following equipment available to design an experiment to demonstrate this phenomena: A circular coil of wire. The coil has 50 windings and a radius of 5 cm. It has a resistance of 0.50 . An electromagnet. Basically it acts like a bar magnet except that it can be turned on and off. It creates a magnetic field with a maximum average strength of 3.00 T . It takes 0.50 s for the electromagnet to come up to maximum strength from zero, and 0.50 s to shut down from maximum strength. An ammeter to measure current. a. Describe your experiment. Include a labeled diagram. Make sure your diagram includes any relevant magnetic fields and the direction of the induced current.
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