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Electrical Fundamentals - Introduction to Direct Current (DC) Theory

A. Bhatia, B.E.

Course Outline

A direct current (DC) is one in which the current does not change with time. This is in contrast to alternating current (AC), where the current changes continuously. In this 4-hour course, the readers will be introduced to the fundamentals of direct current (DC) theory, including in-depth coverage of Ohm's law and its relation to voltage, current, resistance and power. The course is followed by the practical applications of electrical circuits connected in series, parallel and combination.

The course material is based entirely on Naval Education and Training Materials (NAVEDTRA 14173), Electricity and Electronic Training Series; Module-1 "Introduction to matter, energy and direct current" and covers Chapter 3.

The text is arranged to let you progress at your own pace, and concepts and terms are introduced as you need them, with many detailed examples and illustrations. This course contains much of the material that forms the foundation of electrical knowledge.

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of course materials.

Learning Objective

At the conclusion of this course, the student will be able to:

• State Ohm's Law and the properties of power in an electrical circuit;
• Understand how current reacts and voltage functions in a DC series and parallel circuit;
• Understand how to calculate resistance in a DC series and parallel circuit;
• Understand open and short circuits and describe their effects on a circuit;
• Understand the meaning of the term source resistance/equivalent resistance and describe its effect on a circuit;
• Understand how the applied voltage divides in a series circuit and how to apply the voltage divider rule effectively;
• Understand how current dividers work in a DC parallel circuit;
• Understand how to calculate power in a DC series and parallel circuit;
• Understand the meaning of the term equivalent resistance and how to compute its value in Series, Parallel and Combination circuits;
• Understand Kirchhoff's Laws and its impact on the analysis of DC circuits;
• Understand in terms of circuit values the circuit condition needed for maximum power transfer; and
• Recognize the safety precautions associated with the hazard of electrical shock.

Intended Audience

This course is aimed at students, professional engineers, service technicians, energy auditors, operational & maintenance personnel, facility engineers and general audience.

Course Introduction

On a DC circuit, current flows in one direction only. Voltage can remain at a level or change, but it always has the same polarity. The main source of DC is from batteries, photocells, fuel cells, rectifiers and DC generators.

The most fundamental equation describing DC circuits is Ohm's Law. While doing experiments on how well metals conduct electricity, German physicist G.com Ohm discovered the equation V = I * R, which is fundamental to both the DC and AC circuits as well. Knowing two items in this equation allows you to calculate the third.

DC circuits can be arranged in different ways. If component current has only one path to follow, it is connected in series. If there are two or more paths, the components are connected in parallel. Components may also be interconnected to provide both situations. These are called series-parallel combinations.

This course explains the DC circuits in simplified text and will be extremely helpful to individuals who require a basic knowledge of electrical principals and equipment to better their primary responsibilities.

Course Content

In this course, you are required to study Naval Education and Training Materials (NAVEDTRA 14173), Electricity and Electronic Training Series; Module-1 "Introduction to Matter, Energy and Direct Current" Chapter 3:

Introduction to Matter, Energy and Direct Current (Chapter 3, NAVEDTRA 14173)

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Course Summary

The DC circuits can be arranged in series, parallel and series-parallel combination circuits. The following rules apply to the respective circuits:

Series Circuit

• A series circuit connects components end to end to provide a single path for current flow.
• Current throughout any series circuit maintains the same value.
• Total resistance in a series circuit is equal to the sum of all resistive values.
• Total power dissipated by a series circuit is equal to the sum of the calculated individual resistors' power consumption.
• Kirchhoff's voltage law states that the sum of the voltage drops around a series circuit is equal to the source voltage applied.
• A voltage divider is a series circuit used to divide up the applied voltage among series-connected resistors.

Parallel Circuits

• The source voltage across all components in parallel is always the same and is normally used as a reference. Due to this fact, branch current is inversely proportional to the resistance in that branch.
• A parallel circuit has two or more paths for current to flow. As Kirchhoff's current law states, the sum of all the currents entering any junction is equal to the sum of all the currents leaving the same junction.
• Because total parallel circuit resistance is less than a series connection containing the same number and value of resistors, current flow is larger.
• The total resistance in a parallel circuit is the same as the reciprocal of the circuit's total conductance. Its value is always less than the value of the smallest branch resistance.
• If only two resistors are connected in parallel, the product over the sum formula may be used to find total resistance. When all resistors are of the same value, one value divided by the number of resistors may be used. Other situations require the reciprocal formula.
• Total power in a parallel resistive circuit is equal to the sum of all power losses, which is the same as with series circuit components.
• A resistive short in a parallel circuit causes maximum branch current flow and resistance to decrease, whereas a circuit open stops branch current, increases resistance, and causes total current to decrease.

Series-Parallel Circuits

• Resistance totals for series-parallel circuits can be calculated by an equivalent resistance of series-connected components, parallel-connected components, and the remaining series-connected resistances.
• Current totals for series-parallel circuits can be calculated by total resistance calculations for series-connected components, and the addition of individual parallel-connected components.
• Voltage division in series-parallel circuits can be calculated by equivalent resistance, total current, and then determining series and parallel component voltage drops.
• Power dissipation and division in series-parallel circuits can be calculated by determining component current, and then adding the individual power consumptions.

Quiz

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom. 