EEE105: Circuit Theory Chapter 5

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Questions and Answers

What is the main difference between resistors and capacitors?

Resistors store energy, while capacitors dissipate energy.

Resistors are active elements, while capacitors are passive elements.

Capacitors are used in AC circuits, while resistors are used in DC circuits.

Capacitors store energy, while resistors dissipate energy. (correct)

What is the purpose of the dielectric material in a capacitor?

To reduce the voltage across the capacitor.

To separate the two conducting plates. (correct)

To decrease the capacitance of the capacitor.

To increase the capacitance of the capacitor.

What is the unit of capacitance?

Hertz (Hz)

Siemens (S)

Farad (F) (correct)

Ohm (Ω)

What is the symbol for a capacitor?

--| |-- Signup and view all the answers

What happens to the current flow when the capacitor is discharging?

It flows out of the positive terminal Signup and view all the answers

How does the capacitance of a parallel-plate capacitor change when the distance between the plates is increased?

It decreases. Signup and view all the answers

What is the relation between voltage and current in a linear capacitor?

i = Cdv/dt Signup and view all the answers

What happens to the charge on a capacitor when the voltage across it is increased?

The charge increases. Signup and view all the answers

What is the voltage across a capacitor dependent on?

The past history of the capacitor current Signup and view all the answers

When is a capacitor being charged?

When current flows into the positive terminal. Signup and view all the answers

What is the expression for the energy stored in a capacitor?

W = Cv^2/2 Signup and view all the answers

What is the definition of capacitance?

The ratio of the charge on one plate to the voltage difference between the plates. Signup and view all the answers

What happens to the current through a capacitor when the voltage is not changing?

It becomes zero Signup and view all the answers

Why does a capacitor resist an abrupt change in voltage?

Because it has memory Signup and view all the answers

What is the power in a capacitor given by?

p = vi Signup and view all the answers

What is the condition for a capacitor to be linear?

i = Cdv/dt Signup and view all the answers

What is required for a discontinuous change in voltage?

An infinite current Signup and view all the answers

What is a characteristic of an ideal capacitor?

It does not dissipate energy Signup and view all the answers

What is typically included in a nonideal capacitor model?

A parallel-model resistance Signup and view all the answers

What is the expression for the current through a capacitor?

i(t) = C dv/dt Signup and view all the answers

What is the voltage across the 5µF capacitor in Example 1?

v(t) = 10 cos 6000t V Signup and view all the answers

How much is the current through the capacitor in Example 1?

-0.3 sin 6000t A Signup and view all the answers

Study Notes

Capacitors and Inductors

Capacitors

Capacitors store energy in their electric field.
A capacitor consists of two conducting plates separated by an insulator (dielectric).
The charge stored is proportional to the applied voltage, q = Cv.
Capacitance (C) is the ratio of the charge on one plate to the voltage difference between the plates.
Unit of capacitance: farad (F).
Capacitance depends on physical dimensions of the capacitor.

Capacitor Characteristics

The current flows into the positive terminal when the capacitor is being charged.
The current flows out of the positive terminal when the capacitor is discharging.
Linear capacitors satisfy Equation 5.3: i = C dv/dt.
Voltage-current relation: v = (1/C) ∫ i(t) dt + v(t0).
Capacitor voltage depends on the past history of the capacitor current – has memory.

Power and Energy

Instantaneous power: p = vi = Cv dv/dt.
Energy stored: w = (1/2) Cv^2 or w = q^2/2C.

Important Points

A capacitor is an open circuit to dc; the voltage across a capacitor must be continuous.
The ideal capacitor does not dissipate energy.
A real, nonideal capacitor has a parallel-model linkage resistance.

Examples

Example 1: Calculate the current through a 5µF capacitor with a voltage v(t) = 10 cos 6000t V.
Example 2: Calculate the voltage across an initially charged 1-mF capacitor with a given current waveform.

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Description

This quiz covers the fundamentals of capacitors and inductors, including their construction and energy storage. It explores the differences between resistors and storage elements, and provides an introduction to electric fields.