ENERGY STORAGE ELEMENTS part 1, Lecture notes of Electrical Circuit Analysis

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ELECTRIC CIRCUIT

BEJ 10403

CHAPTER 1

CHAPTER 1:

ENERGY STORAGE ELEMENTS

BEJ 10403

4 1.1.1 Introduction

 Capacitor and inductor do not dissipate

energy.

 Reason: Capacitors and inductors are

called “storage elements”.

 Contrast: Resistors dissipate energy.

 Types of passive elements (absorb and

store energy).

 Circuit analysis techniques: Equally

applicable

5 1.1.2 Concepts

 Capacitors: passive elements designed to store

energy in its electric field.

 Used extensively in electronics, communications,

computers, etc:

 Tuning circuits for radio receivers

 Dynamic memory elements in computer system.

 In general a capacitor is constructed by two

plates separated by an insulating (dielectric)

material

7 Cont…

 Definition: A capacitor consist of two conducting

plates separated by an insulator or dielectric.

 For practical applications:

 Plates  aluminum, foil

 Dielectric  air, ceramic, paper, mica.

 DIELECTRIC:

 Having the property of transmitting electric force

without conduction; insulating

8 1.1.3 Types of capacitors Small capacitors used in electronic equipment

10

1.1.2 Working Principles of a Capacitor

 Consider the following figure.

V

+q

• q

11 Cont…

 When a voltage source, V is connected to

the capacitor

 the source deposits a positive charge (q) on

one plate and a negative charge (-q) on the

other

 the capacitor is said to store the energy

 The amount of charge stored = q (directly

proportional to the applied voltage)

13 Cont…

 Capacitance:

 Ratio of the charge, q per plate to the applied

voltage, V

 It does not depend on q or V.

 But depends on the physical dimensions of the

capacitor.

14 Cont…

 For a parallel-plate capacitor, the capacitance is

given by:

 Where:

 A = surface area of each plate (m

2

 d = distance between the plates (m)

 ε = permittivity of the dielectric material (F/m)

d εA C  Metal plates, each with area A Lead (^) Dielectric with permittivity є Lead d

16

17 Cont…

 Three factors determine the values of the

capacitance:

 The surface area of the plates

 Area ↑, capacitance ↑

 The spacing between the plates

 Spacing ↓, capacitance ↑

 The permittivity of the material

 Permittivity ↑, capacitance ↑

1.1. Relationship of the Capacitor

20 Current – voltage relationship

 Basically, q = CV

 To obtain the C-V relationship, take the

derivative of both sides

dt dv i C dt dq ,since i dt dV C dt dq dt dCV dt dq    