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Inductors1 ELECTROMAGNETIC INDUCTION All conductors that carry a current produce a magnetic field As the magnet is moved in and out of a coil of wire in a closed circuit an induced current will be produced All dynamos and generators produce electricity using this effect Electromagnetic induction takes place when the magnetic field around a conductor changes If the magnetic field is made to change quickly, the size of the current induced is larger

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Inductors2 INDUCTANCE Inductance is a phenomenon in which a changing current in a circuit builds up a magnetic field which induces an electromotive force either in the same circuit and opposing the current (self-inductance) or in another circuit (mutual inductance) A component designed to introduce inductance into a circuit is called an inductor It is usually in the form of a coil of wire The energy stored in the magnetic field of the coil is proportional to its inductance and the current flowing through it The magnitude of the voltage induced in a coil depends directly on the rate of change of the current through it, where L is the inductance Symbol Unit of inductance is the henry (henries) (H)

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Inductors3 FUNCTION OF AN INDUCTOR (1) Consider the following circuit Without inductor, the bulb lights up when the switch is closed Light bulb is a resistor – resistance creates heat to make bulb filament glow The coil of wire around a piece of iron in the inductor has a much lower resistance When the is switch closed, the bulb burns brightly, and gets dimmer When the switch is opened, the bulb burns very brightly then quickly goes out

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Inductors4 FUNCTION OF AN INDUCTOR (2) When current starts flowing in the coil (inductor), the coil wants to build up a magnetic field As the field builds up, the coil inhibits current flow Once the field is built, current flows normally through the wound wire When the switch is opened, the magnetic field around the coil maintains current flow in the coil until the field collapses The current keeps the bulb lit for a period of time, even though the switch is open In other words, an inductor can store energy in its magnetic field An inductor also tends to resist any change in the amount of current flowing through it

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Inductors5 VOLTAGE ACROSS AND CURRENT THROUGH INDUCTOR EXAMPLE Sketch the voltage across a 9.87mH inductor, when the current through it changes with time as shown in the graph below

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Inductors6 PRACTICAL EXAMPLE OF AN INDUCTOR An inductor is used in a car ignition system Objective is to produce a high voltage that’s sent to the air gap in the spark plugs Its operation depends on the fact that the induced voltage is directly proportional to the rate of change of current (Δi / Δt) through it For large Δi / Δt, DC voltage from the battery is switched into and out of the circuit at a high rate Switching is accomplished by the contacts in the distributor

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Inductors7 INDUCTORS IN CIRCUITS The total equivalent inductance for series connected inductors is L T = L 1 + L 2 +…+ L n For parallel connected inductors 1/L T = 1/L 1 + 1/L 2 +…+ 1/ L n When an inductor having no current flowing through it is first switched into a circuit, it behaves like and open circuit because the current cannot change instantaneously from its zero initial value After the circuit has been switched on for a long time, the current has reached a state where its value is not changing (steady state value) anymore, hence the inductor acts as a short circuit The energy stored (W) in an inductor with inductance (L) is given by W = (1/2)LI 2 Joules

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Inductors8 INDUCTORS IN CIRCUITS EXAMPLES Find the initial and steady-state (final) voltage across and current through every component after the switch is closed at t = 0. The inductor in the circuit below has an inductance of 0.2H, and a winding resistance of 400Ω. Find the energy stored in the inductor and the rate at which energy is dissipated by the winding under steady-state conditions.

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