Lab 6 - Faraday's Law

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Brigham Young University, Idaho *

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106

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Electrical Engineering

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Apr 3, 2024

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Lab 6: Faraday’s Law Full Name(s) of group member(s): Mckenna Fears Chad Wilson Alyanna Rodelas Titan Harker Joseph Dougherty Jase Robinson Learning goals: Students will be able to:  Understand Faraday’s Law and its application to an induced current  Apply Lenz’s law to predict the behavior of a changing magnetic flux  Explain how a step-up and step-down transformer work  Apply Lenz’s law and associated right hand rules to predict behavior of an eddy current Background Material: A voltage is induced in a loop of wire whenever there is a change in the magnetic flux, Φ B , passing through the loop. If B ⊥ is the perpendicular component of a uniform magnetic field through the plane of the loop and A is the area bounded by the loop, then Φ B = B ⊥ A and the induced potential difference, ΔV , is given by Faraday's Law: ΔV =− N ΔΦ B Δt The polarity of the induced potential difference is explained by Lenz's Law, which states that the polarity of ΔV will tend to oppose the change in flux, ΔΦ B , which caused it. If, for example, the magnetic field, B , in a loop were increased, the polarity of ΔV would produce an induced current and corresponding induced magnetic field, B induced , that would tend to cancel the increase in B and keep Φ B constant (recall that the direction of a magnetic field created by a current is given by the right hand rule). In all of these activities, please be careful not to drop the magnets. Part 1: Induced currents Using a coil and a bar magnet, experiment with induced voltage by moving a magnet toward and away from the coil. Verify the basic ideas of Faraday’s Law and Lenz’s Law. 1. Under what conditions does an induced emf appear in the coil? What can increase the magnitude of the induced voltage? a. The induced emf is proportional to the rate of change of the magnetic flux. When there is a high magnetic flux, it is due a fast speed in the coil turns.

2. Push one end of the magnet toward the coil. Consider the speed of the moving magnet, the direction of the motion, and the direction of the magnetic field. a. Measure the induced voltage. i. Is the voltage positive or negative? A. Positive. b. Repeat using the other end of the magnet and explain the similarities/differences. i. What conditions will create a positive voltage measurement? A. The speed the magnet goes in and out of the coil will cause a positive voltage. The speed is directly proportional to Φ B , this is why we increase in voltage when we increase speed. ii. What conditions will create a negative voltage measurement? A. When it is in a direct current you can get a negative voltage because the current will only go in one way. So, if you go against the current you will get a negative voltage. Alternating current means you can go either way. 3. Determine the polarity of the unknown test magnet. a. From the measured induced voltage, the direction of the coil windings, and how the voltage probe is connected, determine which end of the magnet is north. b. Confirm which end of the magnet is north by using a compass or a magna-probe. Part 2: Transformers 1. Explain the basic functioning of the transformer, including why the input and output voltages can be different. a. It helps transfer electric energy to other circuits that are either increasing or decreasing in voltage. The input and output voltages can be different because of the ratio of loops in the coils. 2. Verify the theoretical relationship between the input and output voltages, ΔV s ΔV p = N s N p a. Measure the output voltage using an equal number of turns on the primary and secondary coils. i. 4V b. Replace the secondary coil so that a step-up transformer is created. Measure the output voltage.

i. 7.95V c. Replace the secondary coil so that a step-down transformer is created. Measure the output voltage. i. 2.75V d. Create a table to compare the measured voltages with the ratio of the number of turns on the primary and secondary coil. Discuss your observations. Primary Coil Secondary Coil Voltage 400 Turns 400 Turns 4V 400 Turns 800 Turns 7.95V 800 Turns 400 Turns 2.75V Part 3: Eddy Currents 1. Use Faraday and Lenz’s laws to understand the behavior of a coil of wire moving through a magnetic field a. Predict the direction of the current within a loop of wire as it is passed through a magnetic field. i. Clockwise. b. Allow the coil of wire to swing freely through the magnetic field, note the direction of the current will determine the color of the LED. Discuss your observations and compare them with your prediction. i. Clockwise, then counterclockwise because the light goes green then red. 2. Use Faraday and Lenz’s laws to understand the behavior of a metal sheet moving through a magnetic field a. Predict the behavior of a metal sheet as it is passed through a magnetic field that is directed through the sheet. Consider the effects that the field will have on charges within the metal when they are inside and outside of the magnetic field. i. It will swing freely. b. Allow the plate to swing freely through the magnetic field. Discuss your observations and compare them with your prediction. i. It did not swing as freely as predicted. Surprising.

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