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Chapter 28: Problem 3

There's an insulating gap between capacitor plates, so how can current flow in an AC circuit containing a capacitor?

Short Answer

Expert verified
Even though there's an insulating gap between the plates of a capacitor, an alternating current can 'flow' in an AC circuit with a capacitor. This is not because electrons physically move across the insulator, but due to the concept of displacement current which is caused by the changing electric field between the plates. When the a.c. source reverses, the capacitor discharges and recharges in the opposite sense: this gives the impression of a current flow.

Step by step solution

01

Understanding of Capacitor

A capacitor is a two-terminal electrical component that can store electrical energy in an electric field. Although it is composed of two metal plates separated by an insulator, this configuration does not prevent the flow of alternating current (AC).
02

Concept of Displacement Current

When there's an alternating current (AC) in the circuit, the voltage across the capacitor plates changes continuously. This causes the charge stored on the plates to change as well as the electric field between the plates. Although no physical movement of electrons occurs across the insulator, this changing electrical field creates something known as displacement current, which gives the effect of a current flowing.
03

Alternating Current (AC) and Capacitor

In an AC circuit, the direction of the current changes periodically. When the current direction changes, the capacitor alternately charges and discharges, reversing the polarity of the plates. This charging and discharging of the capacitor gives the illusion of current flow, as it results in a current in the external circuit.

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Most popular questions from this chapter

A series \(R L C\) circuit has \(R=18 \mathrm{k} \Omega, C=14 \mu \mathrm{F},\) and \(L=0.20 \mathrm{H}\) (a) At what frequency is its impedance lowest? (b) What's the impedance at this frequency?

Show that the unit of both capacitive and inductive reactance is the ohm.

Your university's FM station broadcasts at \(89.5 \mathrm{MHz}\). The \(L C\) circuit that establishes this frequency has a 47 -pF capacitor. What's the corresponding inductance?

You're concerned about a circuit that will be used in a remote communications installation. The series RLC circuit with \(R=5.5 \Omega, L=180 \mathrm{mH},\) and \(C=0.12 \mu \mathrm{F}\) is connected across a sine-wave generator. The inductor can safely handle 1.5 A of current. The peak generator output when it's tuned to resonance will be \(8.0 \mathrm{V} .\) Will the inductor current stay within a safe limit?

One-eighth of a cycle after the capacitor in an \(L C\) circuit is fully charged, what are the following as fractions of their peak values: (a) capacitor charge, (b) energy in the capacitor, (c) inductor current, (d) energy in the inductor?
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