Barry C. Walker Copyright University of Delaware, 2001. Materials may be distributed for Department of Physics and Astronomy educational purposes from . html and the links [email protected] 302- 831- 2673 contained therein. Not to be sold or otherwise used commercially in whole or part. 1 Lab 7: Electric Guitar and Faraday’s Law Introduction: •Similar to E fields from charged material, B fields come from magnetic matter. One of the most basic characteristics of a magnetic field is its flux, or quantity of field lines per unit area: . The link between moving charges and magnetic fields is expanded in this lab with investigations of changing magnetic flux. Changes in the flux through an electrical circuit will induce an electromagnetic force ( emf ) on the charges in the circuit. This relationship is given succinctly by Faraday’s law: Faraday’s law has a variety of applications in the real world. For example, the electric guitar is a popular instrument whose function relies completely on Faraday’s Law. Throughout this lab we will be examining the behaviors of induced current from changing magnetic fields. By the end of the lab you should: •Understand the concept of Faraday’s Law •Understand the emf that influences the induced current in a coil. •Measure time dependent voltages with an oscilloscope Materials: • multimeter • magnets (a) • Field coil (b) • Magnetic field sensor • Electric guitar string/pickup apparatus (c) • Oscilloscope ∫ ⋅ = Φ A d B r r dt d emf B Φ - = (a) (a) (c) (b)
Barry C. Walker Copyright University of Delaware, 2001. Materials may be distributed for Department of Physics and Astronomy educational purposes from . html and the links [email protected] 302- 831- 2673 contained therein. Not to be sold or otherwise used commercially in whole or part. 2 Experiment 1: Induced Current If you move charge, you create magnetic fields. Conversely, when a magnetic field moves their can be a force acting on charges. This is the physics behind Faraday’s law, , where emf is the electromotive force causing the charges in the conductor to move and d Φ /dt is the changing magnetic flux. Since the current is induced there is a negative sign, i.e. field is being put into the loop rather than created by the loop. This phenomenon can be examined in the following experiment. You have a coil of N turns and a variety of magnets. Connect the coil to a multimeter and examine the emf voltage across the coil when the magnet is in motion near the coil. Answer the following questions to explore Faraday’s Law and its characteristics: •When does the sign of the emf change? Does it matter if you use the S or N
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- Magnetic Field, Department of Physics and Astronomy, Copyright University of Delaware, Barry C. Walker