Basic Electric Circuits

(Engineers In Training Series)

by

James and Ronald Broadnax

Current Date: August 14, 2007

Contents

About the authors

Useful Links:

Batteries and Basic Electric Circuits

Introduction

Working as a professional engineer in today's ever-changing society requires a vast amount of training, education, and inter-personal relations. Today's engineer or scientist must be willing to relocate, negotiate, and participate in a number of schemes. Evaluation and decision-making are routine for today's professionals. This ever challenging profession requires lots of science and math skills, but it all starts at the beginning, with basics. For those pursuing electrical engineering, a knowledge of simple circuits and components is essential. Graduating from the simple to the more advanced is the road in which most engineers travel to success.

Activity Overview

This activity is intended to provide students with an introductory knowledge of basic electric circuits . Simple mathematical notation will be introduced, as well as Ohm's Law for circuits. The lesson provides for both theory and hands-on application, and requires computer manipulation of the circuit to enhance the computational science element. Students will be exposed to the importance of formula manipulation from an engineering perspective.

Preliminary Information

Participating students should have basic mathematical skills. Algebra is very helpful, but not essential. Knowledge of units of measure are needed. All students must have access to the internet to run computer applets. Java must be installed on computers in order to run the online applets. Students will be required to write reflections of their experiences at the end of class each day.

Activity Time Requirement

Students will learn to work in teams.

This course will require almost two hours to complete. It can be sub-divided into multiple sessions totaling two hours.

Teacher Preparation

Required Tools and Equipment

Following is a list of items needed to support the class activities.

Procedure

This section discusses the basic electric series circuit using resistors, a switch, and a battery. The student should learn basic Ohms' Law theory and the relationship between voltage, current, and resistance, and their respective units of measure.

Basic Electric Circuit-Water Hose Analogy

The flow of electric current through a wire can be made analogous to that of water flowing through a garden hose. For example, assume that the hose is the wire, the water flowing through the hose is electric current, and the amount of pressure applied at the faucet is the voltage. If we turn the faucet down so that the pressure goes down, what happens to the amount of water flowing through the hose? (Answer: decreases) As we increase the pressure, the amount of water flow is increased through the hose. Now, if we decide to put a crease in the hose, such that there is some restriction, or resistance, to water flow, the amount of water through the hose decreases. If we remove the hose and replace it with a larger hose, then more water can flow for the same amount of pressure. Likewise, a smaller hose will resist (offer more resistance to) the flow of water

Give each student a small drinking straw and a larger drinking straw. Ask each student to blow through the smaller straw and notice how hard it is to blow air through it. Ask the students to blow through the larger straw. Question: Which straw is easier to blow through? (Ans: larger straw) The smaller straw offers more resistance to air flow than does the larger straw. The resistance of the smaller straw is greater, or larger. Likewise, depending on the particular circuit component, its resistance may be large or small.

In similar fashion, Ohm's Law can be used to describe the action taking place in a simple electric circuit. Wire replaces the hose in our example. The flow of electricity, or current (I), through the wire is analogous to the flow of water in the hose. The voltage (V) of the battery is analogous to the water pressure at the faucet. The restriction offered by the hose, including any crimps, is considered to be analogous to resistance (R) in the wire. Then, Ohm's Law can be briefly summed up in the the relationship,: The amount of current (I) flow through a wire with resistance (R) is directly proportional to the amount of voltage (V) applied across the resistor (R). Current is equal to the voltage divided by the resistance, or, I=V/R, where I is in amperes, V is in volts, and Resistance is in ohms. The designation for ohms is the Greek letter omega. Similarly, the voltage V across the resistor R is equal to : V=I*R, and R=V/I.

Ohm's Law basic formulas.

  1. Show the student some resistors and other circuit components or boards that you may have on hand.
  2. If you are familiar with the resistor color code, show the students how to calculate the Ohmic value of the resistor using its color bands.
  3. Explain the function of each component that is used in the circuit and give its symbolic representation - V = battery voltage, A = current in amperes, or mA = current in milliamps, R = the resistor, and the Greek symbol omega is the resistance value in Ohms..

Units of Measure

On many occasions, Engineers will use scientific notation to indicate the circuit component values contained in a circuit. For example, the value of resistorR1 may be listed in Kohms, or kilohms, or ohms x 1000. The the current may be written in milliamps, or mA, or even in microamps, depending on the particular circuit. It is important to be familiar with the decimal multipliers when working with circuits.

A simple Series Circuit

Let's examine a simple series circuit. The circuit is called series because current must flow through all components in the circuit before it can be a complete circuit. our circuit will have a battery, a switch, and a resistor.

  1. Draw a series circuit on the board.
  2. Ask the students to name the components in the circuit.
  3. Assign values to the battery and resistor. Use simple numbers.
  4. Use the drawing to describe a series circuit.
  5. Adopt a convention for current flow. Some engineers use flow from (+) to (-); others, (-) to (+). The convention used will not affect the overall outcome of Ohm's Law.
  6. Use the circuit diagram to discuss Ohm's law and the relationship between voltage and current, and how resistance plays a part in it.
  7. Show that the voltage V, the battery voltage, also appears across the resistor R, since the two componentst are electrically connected when the switch is set to On, or the closed position.
  8. When the switch is Off, or in the open position, no voltage is present across the resistor R. There is no current flow.
  9. Using Ohm's Law, calculate the current through the circuit when the switch is closed, or On. I=V/R.
  10. Ask the students "What effect will increasing the resistance of R have on the current?". (Current will decrease).
  11. Add another resistor R2 in series with R. What will be the effect on the current? (current decreases) How much current will flow through R and R2. Will the current be the same through both resistors? (yes).
  12. How will this affect the battery voltage V? (no effect)
  13. Show the students that the total resistance in the circuit, RT, is the ohmic values RT = R + R2.

Ohm's Law Calculations

Ask the students to perform the following calculations.

Power

As in mechanical motion, the flow of electrical current encounters friction, or resistance, which causes energy to be dissipated. This energy, relatively speaking, can be described as power, or the amount of power dissipated. Since resistors, by design, offer electrical friction, or resistance, power is dissipated by the resistor in the form of heat. This power is expressed in watts, W. The basic ohmic formula for calculating power consumption or power dissipation is: Power P=I*V in watts, W. For example, the amount of power dissipated by a 100 ohm resistor with 10A of current I through it is:

P = IV = I*(IR) = 10*(10*100) = 10 kW

Related formulas include: P=I**R and P=V**/R, which can be derived from Ohm's Law.

Most common carbon composition resistors that are used in radio and TV circuits are usually rated at one watt or less. Generally, high wattage resistors are reserved for industrial or commercial use, and can dissipate tremendous amounts of power in the form of heat.

Ask the students to perform the following calculations

Computer-Aided Circuit Analysis

A computer can be used to help perform simple or complex analysis of an electric circuit. special programs called applets can often allow the computer to act as a very capable tool in circuit analysis. This computational tool can reduce countless hours of mathematical manipulation and mistakes to a minimum. This course allows the use of such applet.

  1. Arrange the students in teams according to space and computer availability.
  2. Each student (or team) must have a logon ID and password.
  3. Ask each team to power On the computer and bring up a browser.
  4. Go to the URL:Circuit applet
  5. The circuit applet should load with aRLC circuit with switch shown on the screen. Don't worry, the C (capacitor) and L (inductor) will be removed.
  6. When the applet loads, the positive voltage is denoted in green, the negative voltage in red, and ground by grey. Yellow dots show the movement of current.
  7. Moving the mouse over a component will highlight a description of that component.
  8. Right clicking or (Control click on a MAC) will bring up a window to allow modification of that component.
  9. Clicking on the switch will open and close it.
  10. For instructions on how to use these applets with a Mac or PC, click below.

Build a Circuit (Optional)

In this section, the student will build a circuit using the switch, resistor, and battery as circuit components. The activity is optional, but, if performed, can give the student an overview of what is involved in a real physical electric circuit. Use the following components.

  1. Explain to the students how connectivity is made on the breadboard.
  2. Use the circuit on the applet as a template.
  3. Connect the red lead of the battery clip to the spst swittch.
  4. Connect the other end of the switch to the 1K ohm resistor.
  5. connect the free end of the resistor to the Black lead of the battery clip.
  6. Insure that the switch is set to Off.
  7. connect the battery to its clips.
  8. Measure the voltage across the battery and record the measurement.
  9. Set the switch to the On position.
  10. Measure the voltage across the resistor and record the result.
  11. Using the formulas introduced by Ohm's Law, calculate the current through the resistor and the power dissipated by the resistor.
  12. What can be said about the voltage across the resistor and the battery? (They are the same)
  13. Set the switch to Off.
  14. Remove the 1K ohm resistor and replace it with the 470 ohm resistor
  15. Set the switch to On and measure the resistor's voltage.
  16. Calculate the current and power dissipation for the resistor.
  17. Set the switch to Off.
  18. Disconnect the black battery lead from the 470 ohm resistor.
  19. Connect the long end of the LEd to the 470 ohm resistor.
  20. Connect the short end of the LED to the black battery lead.
  21. Set the switch to On and measure the voltage across the resistor and across the LED.
  22. Does the sum of the voltages across the resistor and LED equal to the battery voltage?
  23. Calculate the current through the resistor and the power dissipated. Did the values increase or decrease? Note the brightness of the LED.
  24. Set the switch to Off and replace the 470 ohm resistor with the 1K ohm resistor.
  25. Set the switch to On and note the brightness of the LED.
  26. Measure the voltage across the resistor and LED and calculate the current through the resistor and the power dissipated by the resistor.
  27. Set the switch to Off and dismantle the circuit.
  28. Return all components to the instructor or assistant.
  29. Discuss the results with the class.

Assessment

Document History

Current Date

August 10, 2007

Original Published

August 10, 2007

Update History

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