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Chapter 32

# Chapter 32 - 32 Inductance CHAPTER OUTLINE 32.1 32.2 32.3...

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32 CHAPTER OUTLINE 32.1 Self-Inductance 32.2 RL 32.3 Energy in a Magnetic Field 32.4 Mutual Inductance 32.5 Oscillations in an LC Circuit 32.6 The Circuits Circuit RLC Inductance ANSWERS TO QUESTIONS Q32.1 The emf induced in an inductor is opposite to the direction of the changing current. For example, in a simple RL circuit with current flowing clockwise, if the current in the circuit increases, the inductor will generate an emf to oppose the increasing current. Q32.2 The coil has an inductance regardless of the nature of the current in the circuit. Inductance depends only on the coil geometry and its construction. Since the current is constant, the self-induced emf in the coil is zero, and the coil does not affect the steady-state current. (We assume the resistance of the coil is negligible.) Q32.3 The inductance of a coil is determined by (a) the geometry of the coil and (b) the “contents” of the coil. This is similar to the parameters that determine the capacitance of a capacitor and the resistance of a resistor. With an inductor, the most important factor in the geometry is the number of turns of wire, or turns per unit length. By the “contents” we refer to the material in which the inductor establishes a magnetic field, notably the magnetic properties of the core around which the wire is wrapped. Q32.4 If the first set of turns is wrapped clockwise around a spool, wrap the second set counter-clockwise, so that the coil produces negligible magnetic field. Then the inductance of each set of turns effectively negates the inductive effects of the other set. Q32.5 After the switch is closed, the back emf will not exceed that of the battery. If this were the case, then the current in the circuit would change direction to counterclockwise. Just after the switch is opened, the back emf can be much larger than the battery emf, to temporarily maintain the clockwise current in a spark. Q32.6 The current decreases not instantaneously but over some span of time. The faster the decrease in the current, the larger will be the emf generated in the inductor. A spark can appear at the switch as it is opened because the self-induced voltage is a maximum at this instant. The voltage can therefore briefly cause dielectric breakdown of the air between the contacts. Q32.7 When it is being opened. When the switch is initially standing open, there is no current in the circuit. Just after the switch is then closed, the inductor tends to maintain the zero-current condition, and there is very little chance of sparking. When the switch is standing closed, there is current in the circuit. When the switch is then opened, the current rapidly decreases. The induced emf is created in the inductor, and this emf tends to maintain the original current. Sparking occurs as the current bridges the air gap between the contacts of the switch.

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Chapter 32 - 32 Inductance CHAPTER OUTLINE 32.1 32.2 32.3...

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