Resistance DC Circuits, Slides of Electrical and Electronics Engineering

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ELECTRICAL AND

ELECTRONIC PRINCIPLES

Resistance & D.C Circuits

Lecture 1

AENG002-4-

Introduction to electrical quantities

Scientific and Engineering notation

Scientific notation

prefixes

5

2. 47  10^ is scientific notation
3. 7  104^ is not a scientific notation

Table 1.

Introduction to electrical quantities

Example 1

Circuit Diagrams

Pictorial Diagram

• describes a system in detail

Figure1.2: A pictorial diagram- The battery is referred to as

a source and the lamp is referred to as a load

Electrical charge

1. 6  10 ^19 C

• electron is negatively charged
• proton is positively charged
• The charge of an electron is

where C is in Coulombs

Figure 1.4: A Simplified atomic model

Coulomb’s law

Figure 1.5 Coulomb’s law of forces

Coulomb’s law states

where

the constant k  9  109

r is the distance between the charges in meters F is in Newtons

F/m

A practical voltage source

Figure 1.7: A carbon zinc cell. Voltage is created due to the separation of charge due to chemical reaction. Normal cell voltage is 1.5 V

Potential energy and voltage

• The concept of voltage is tied into the concept of potential energy.
• The voltage between two points is one volt if it requires one joule of energy to move one coulomb of charge from one point to other , given as:

1.

where W is energy in joules, Q is charge in Coulombs and V is the resulting voltage in volts.

• Note carefully that voltage is defined between points. For this reason, voltage is also called potential difference

Symbol for Voltage sources

Figure 1.8: Battery symbol. The long bar denotes the Positive terminal and the short bar the negative terminal. Thus, it is not necessary to put + and - signs on the diagram. For simplicity, some books use (a) for battery source.

Current

Figure 1.9: Electron flow in a conductor. Electrons(-) are attracted to the positive (+) pole of the battery. The flow of electrons or charges is called an electric current.

The Ampere

Example

If 840 coulombs of charge pass through the imaginary plane of Figure 1.9 during a time interval of 2 minutes, what is the current?

Solution

Convert t to seconds. Thus,

I = Q/t = 840C/(2 x 60) s = 7A

Circuit Elements

• Circuit element is a mathematical model of a

real device

• Active element : is capable of generating energy Examples: generators, batteries
• Passive element : absorbs (dissipates) energy Examples: resistors, capacitors, inductors

Resistance

Resistance of a material depends on several factors

• Type of material
• Length of the conductor
• Cross-sectional area
• Temperature

Resistance of a material is given as

1.

where

What happens if ρ increases? R is directly proportional to ρ What happens if A increases? R is inversely proportional to A

Resistivity

Table 1.3: Resistivity of materials 

Most conductors are circular and its cross-sectional area is worked out as: (^) 1.

Figure 1.10: Conductor with circular cross-section