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chapter32

# chapter32 - Chapter 32 Inductance L and the stored magnetic...

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Chapter 32 Inductance L and the stored magnetic energy RL and LC circuits RLC circuit

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Resistance, Capacitance and Inductance Ohm’s Law defines resistance: V R I Capacitance, the ability to hold charge: Q C V Capacitors store electric energy once charged: Resistors do not store energy, instead they transform electrical energy into thermo energy at a rate of: 2 2 V P V I I R R = = = ( 29 2 2 1 1 2 2 E Q U C V C = = Inductance, the ability to “hold” current (moving charge). Inductors store magnetic energy once “charged” with current, i.e., current flows through it.
Inductance, the definition I When a current flows through a coil, there is magnetic field established. If we take the solenoid assumption for the coil: E E L + 0 B nI μ = When this magnetic field flux changes, it induces an emf, E L , called self-induction: ( 29 ( 29 0 2 0 B L d NAB d NA nI d dI dI n V L dt dt dt dt dt μ Φ μ = - = - = - = - - E or: L dI L dt R - E This defines the inductance L , which is constant related only to the coil. The self- induced emf is generated by current flowing though a coil. According to Lenz Law, the emf generated inside this coil is always opposing the change of the current which is delivered by the original emf. For a solenoid: 2 0 L n V μ = Where n : # of turns per unit length. N : # of turns in length l . A : cross section area V : Volume for length l .

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Inductor We used a coil and the solenoid assumption to introduce the inductance. But the definition holds for all types of inductance, including a straight wire. Any conductor has capacitance and inductance. But as in the capacitor case, an inductor is a device made to have a sizable inductance. L L dI dt R - E An inductor is made of a coil. The symbol is Once the coil is made, its inductance L is defined. The self-induced emf over this inductor under a changing current I is given by: L dI L dt = - E
Unit for Inductance The SI unit for inductance is the henry (H) Named for Joseph Henry: 1797 – 1878 American physicist First director of the Smithsonian Improved design of electromagnet Constructed one of the first motors Discovered self-inductance A s V 1 H 1 =

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Discussion about Some Terminology Use emf and current when they are caused by batteries or other sources Use induced emf and induced current when they are caused by changing magnetic fields When dealing with problems in electromagnetism, it is important to distinguish between the two situations
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chapter32 - Chapter 32 Inductance L and the stored magnetic...

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