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19-3 Magnetic Force ► Charged particles at rest are not affected by magnetic field (stationary) ► When moving, charged particles can be deflected by magnetic fields ► Used in TV tubes to create picture on screen ► Earth’s magnetic field deflects charged particles from outer space (cosmic rays) Magnetic Force on a charged particle 𝐹𝑚𝑎𝑔 = 𝑞𝑣𝐵 q = charge v = velocity B = magnetic field FB is a centripetal force ● A charge moving through a magnetic field follows a circular path Magnetic Forces and Current-Carrying Wires ► Current-carrying wire placed in magnetic field is deflected ► Reversing current cause deflection in opposite direction ► Greatest deflection occurs when current is perpendicular to magnetic field Force on a wire 𝐹𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑐 = 𝐼𝐿𝐵 ► Two parallel conducting wires exert a force on one another ► When two conductors are parallel, the force on a wre is based on magnetic field created by the other wire ► When current is in the same direction, force is attractive Meters ► Meters use magnetic needle Current moving through wires in meter creates magnetic field that interacts with magnetic field of needle Cause needle to be deflected (like repel, etc.) Amount of deflection indicates amount of current Loudspeakers Galvanometer use of magnetic force on a wire to produce sound waves device that operates on a torque acting on a current loop in a magnetic field that causes a coil to twist 20-1 Electricity from Magnetism ► The process of creating a current in a circuit loop by changing the magnetic flux in the loop is called electromagnetic induction. Electromagnetic Induction ► Imagine a wire moving to the right as shown. According to the RHR the magnetic force is directed upward This force will separate the charges. As negative charges move upward, the wire will develop a potential difference. Electromagnetic Induction ► Three ways to induce a current in the coil ► Vary the size of the coil (ex: as coil enters the field the area increases) ► Change the orientation of the coil ► Change the strength of the magnetic field Chapter 20 Characteristics of Induced Current ► Lenz’s Law The magnetic field of the induced current is in a direction to produce a field that opposes the change causing it. ► This rule can be used to find the direction of the current in the loop. ► Note: the induced current does not oppose the applied field, but rather the change in the applied field. ► The magnitude of the induced emf can be predicted by Faraday’s law of magnetic induction. Faraday’s Law ► The magnitude of the induced emf depends on the number of loops (N), the magnetic flux (M), and the rate of change. ► M = AB cos Classroom Practice Problem ►A coil with 25 turns of wire is moving in a uniform magnetic field of 1.5 T. The magnetic field is perpendicular to the plane of the coil. The cross-sectional area of the coil is 0.80 m2. The coil exits the field in 1.0 s. Find the induced emf. Determine the induced current in the coil if the coil’s resistance is 1.5 . ► Answers: 3.0 101 V, 2.0 101 A