Basic Concepts

1
Chapter 1

• Basic Concepts

2
Introduction

• An electric circuit is an interconnection of
  electrical elements.
• Functions
• To transfer energy from one point to another.
• Basic concepts
• Charge.
• Current.
• Voltage.
• Power.
• Circuit elements.
• Energy.

3
Charge q(t)

• Basic quantity in an electric circuit.
• Defined as an electrical property of materials.
• Exist as negative (electron) and positive
  (proton) charges.
• Measured in Coulombs (C).
• 1 electron -1.60210-19C.
• Positive and negative charges move in different
  direction.
• Creates electric current.
• Consider electric current as movement of positive
  charge.
• Charge may be constant/varying.

4
Current i(t)

• Defined as the charge flow rate.
• Measured in Ampere (A).
• Current may be constant/varying.

5
Voltage v(t)

• Defined to be the charge rate of doing work.
• Energy required to move a unit charge through an
  element.
• Measured in volts (V).
• Voltage may be constant/varying.

6
Power p(t)

• Defined to be the time rate of doing work.
• Measured in watts (W).
• Power can be absorbed or supplied by circuit
  elements.
• Positive power ? element absorbs power.
• Negative power ? element supplies power.
• Sign determined by voltage and current.
• An ideal circuit
• ?Psupplied ? Pabsorbed 0.

7
Energy

• Defined as the capacity to do work.
• Measured in joules (J).

8
Circuit Elements

• An element is the basic building block of a
  circuit.
• Electric circuit is interconnecting of the
  elements.
• Types of elements
• Active elements ?Capable of generating energy
  (i.e. batteries, generators).
• Passive elements ? Absorbs energy (i.e.
  resistors, capacitors and inductors).
• Voltage and current sources ? the most important
  active elements.

9
Source

• Divided into
• Independent source ? Does not depend to other
  elements to supply voltage or current.
• Dependent source ? Reverse of independent.
• Constant voltage source
• Voltage same for all elements.
• Constant current source
• Current same throughout the circuits.

10
Examples

• Determine the current flowing through an element
  if the charge flow is given by q(t)(9t22t-2)C.
• Find the charge q(t) flowing through a device if
  the current (i(t))(2t5) mA and q(0)0.
• The charge entering a certain element is shown
  below. Determine current at
• t 1 ms
• t 6 ms
• t 10ms

11
Examples

• Given the graph, plot the corresponding current.
• Calculate total charge that pass through an
  element at
• t 1s
• t 3s
• t 5s

12
Examples

• Find the power absorbed by each of the elements.
  
• Assignment 1 download _at_ http//staff.edu.my/hadib
  ahmr