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Unformatted text preview: Lab 8: Faraday’s Law, generators, and motors 1 Introduction Most major power plants rely on Faraday’s law for the conversion of mechanical energy into electrical en- ergy. These power plants are commonly designed to derive mechanical energy from water, wind, and nu- clear fission reactions, as well as from thermal energy obtained by burning coal, natural gas, or oil. In all these cases, the mechanical energy is used to rotate specially designed coils placed in a magnetic field. This arrangement is known as a generator . The changing magnetic flux through the generator coils produces an induced emf ( e lectro m otive f orce), which results in a flow of induced current to electrical devices connected to the generator. The torque acting on a current carrying loop of wire placed in a magnetic field can be engineered to rotate the wire loop. This idea finds widespread application in electric motors that are used universally - in com- puter hard drives and washing machines. The torque is the result of the force acting on a current carrying wire placed in a magnetic field. This lab will allow you to learn the basic principles associated with Faraday’s law, and learn how genera- tors and motors work. The first section will allow you to perform several qualitative tests using simple appa- ratus consisting of a coil of wire, a bar magnet, and a galvanometer (a current detector). In the second sec- tion, you will analyze the predictions of Faraday’s law in a quantitative manner. You will be able to control the rate of change of a magnetic field and measure the induced emf. In the final section, you will experiment with generators and motors and get a better idea of how they function. EXERCISES 1, 4, 5 AND 6 PERTAIN TO THE BACKGROUND CONCEPTS AND EX- ERCISES 2-3 AND 7-15 PERTAIN TO THE EXPERIMENTAL SECTIONS. MT73 MT66 MT70 MT70 MT109 Figure 1: Top view of rectangular coil placed in uniform mag- netic field. The arrows indicate the direction of current flow and forces acting on the coil, which produce a torque that rotates the coil. 2 Background Consider a coil with N turns, and area of cross section A . If the coil carries a current I , its magnetic dipole moment-→ m is defined as,-→ m = NIA b n (1) Here b n is a unit vector drawn perpendicular to the area of the coil. You can figure out the direction of the magnetic dipole moment by using the right hand rule. If the fingers of your right hand are aligned along the direction of current in the coil, the thumb gives the direction of-→ m . When the coil is placed in a uniform magnetic field-→ B , the net force on the coil is zero. However, there is a torque acting on the coil. This torque-→ τ is perpendic- ular to both-→ m and-→ B . It tends to rotate-→ m parallel to the direction of-→ B (figure 1) and is expressed as,-→ τ =-→ m ×-→ B (2) The behavior of the coil is analogous to the behavior of a bar magnet. This is a consequence of the fact that the magnetic dipole moment of the magnet is...
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