Lab 8: Faraday’s Law
Faraday’s Law of Induction – PHY 202 Introduction The purpose of this laboratory is to study Faraday’s Law of Induction and Lenz’s Law, qualitatively, by analyzing the voltage induced in a conducting coil when the flux Φ of a magnetic field through the coil is changing. Theory According to Faraday’s Law of Induction, a changing magnetic flux through a coil induces an EMF given by: Ɛ = -N (d/dt) Eq. (1) Then, the induced EMF Ɛ is proportional (with opposite sign) to the rate of change of the magnetic flux and proportional (with opposite sign) to the number of turns in the coil. The minus sign in Faraday’s Law of Induction is a manifestation of Lenz’s Law which states that “the direction of any magnetic induction effect is such as to oppose its cause”. For example, in the figure below there is a uniform magnetic field passing through a coil. The magnitude of the field is increasing, so there is an induced EMF driving a current that creates a new, induced, magnetic field opposing the first one (increasing in the opposite direction). Procedure 1) Moving a bar magnet manually (within a simulation) Open the PhET simulation Faraday’s Law: https://phet.colorado.edu/sims/html/faradays-law/latest/faradays-law_en.html a) Click on the field lines to see them. Move the magnet so that it enters and then leaves the loop, travelling from right to left with its north pole in front. b) Describe the changes in the voltage meter and light bulb as the magnet: • Enters the loop • Moves through the loop • Leaves the loop c) What are the directions of the current and the induced magnetic field in these three stages (make a diagram to show)? d) How do you need to move the magnet to increase the voltage deflection in the meter (i.e. to increase the induced EMF)? e) Describe what happens if you repeat this virtual experiment with the magnet entering the loop with the south pole in front. 2) Induced EMF as a Function of the Number of Loops Link to Video: https://youtu.be/WYQhZnK25p8 a) Click on the 2- and 4-loop figure, describe the changes in the voltage meter as the magnet enters, moves through, and leaves the 2-loop and the 4-loop coil. b) If you were to compare the ratio of the produced EMF in the 2- and the 4- loop coil what function would you use. c) Using the appropriate function, what would be the ratio of the produce EMF in the 2- and the 4-loop coil? Why? d) Is your result consistent with Faraday’s Law? Explain. 3) Summarizing Question A 2 cm long bar magnet is dropped from 4 cm above a coil of wire 10 cm long with many loops. Assuming the bar magnet passes straight through the coil, north pole first (as in the diagram below): a) What would a graph of EMF as a function of time look like? Sketch your answer in a graph like the one shown below where the middle horizontal line represents 0 V (pay attention to polarity). b) What is the direction of the current in the coil wire as the magnet enters the coil? c) What is the direction of the induced magnetic field as the magnet enters the coil? d) What is the direction of the current in the coil wire as the magnet leaves the coil? e) What is the direction of the induced magnetic field as the magnet leaves the coil? Hint: it may be helpful to draw the magnetic field lines around the magnet before attempting to solve. In your report include the answers to all the questions (with diagrams when needed).
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