Lesson 3:

Ohm's Law and Power

Objectives:

Students finishing this chapter should be able to define Ohm's Law and Power and be able to do simple calculations using Ohm's Law and the Power Laws.

This is a long chapter, perhaps too long to be taught in one class unless the students have some prior knowledge of electronics and are good at math. If time permits, you may want to teach it in two sessions. If you elect to teach this chapter over two classes then be prepared for a lot of questions at the beginning of the second class.

Questions will inevitably arise about reconfiguring the Ohm's Law equations to extract the parameter that is needed at the moment. Be sure that the students all understand this rearrangement since it arises throughout the course. Since some of the students will catch on quickly and others will not, avoid boring the faster ones by assigning them to assist the slower ones. The review questions provide a source of practice material.

Apparatus:

  1. VOM

  2. Resistors of various values and power ratings

  3. Clip leads

Lesson Plan:

  1. Answer any questions from homework.

  2. Put the Ohm's Law Triangle on the board (available as Slide 1) and review the methods by which equations are re-arranged.

  3. Have the students attempt Chapter 3 Quiz questions 7, 8, and 9.

  4. Stress proper setup:
  5. Have volunteers put them on the board. Tell them that their homework will contain lots of practice.

  6. Have the students attempt Chapter 3 Quiz questions 15 and 22. Again, stress proper setup. Have volunteers put them on the board. Tell them that their homework will contain lots of practice. Calculating the total resistance of resistors in parallel can be tricky for some so emphasize the techniques used on Page 3-6.

  7. Use the VOM and a DC circuit to show how voltage drops across a resistor.

  8. Show resistors of different power ratings to emphasize the differences in the sizes of the leads and the bodies of the resistors.

  9. Put the Power Laws on the board but tell the class that you want them to see what they can do on their own with this.

Demonstration:

There are a few "gotchas" in the following demonstrations. If you are using a battery and not a regulated power supply, and if you have a relatively large current, this load may be sufficient to reduce the available voltage to the test jig. Either use relatively large resistors (10 K or better), or be prepared to explain the real world!

Resistance
Resistors in Series and Parallel

Slide 3 shows the circuits that are needed for this demonstration.

Resistors in Both Series and Parallel

Homework:

  1. Review any questions they had about the homework from the previous class.

  2. Read Chapter 4, or if the chapter is split into two sessions, the last part of chapter 3.

  3. Do the Chapter 3 Quiz. If you elect to do this chapter over two classes then you will have to break this quiz up into two parts.

Chapter 3 Quiz:

Printable copy of the Review Quiz

Answers to Chapter 3 Quiz
1 A 13 A 25 C 37 C 49 C
2 D 14 A 26 B 38 A 50 A
3 B 15 A 27 D 39 D 51 B
4 A 16 D 28 C 40 A 52 B
5 B 17 A 29 C 41 A 53 C
6 B 18 A 30 D 42 C 54 C
7 C 19 A 31 D 43 C 55 C
8 B 20 A 32 C 44 C 56 B
9 A 21 B 33 A 45 C * *
10 A 22 D 34 B 46 D * *
11 D 23 C 35 B 47 B * *
12 A 24 B 36 B 48 B * *

Overhead Slide Versions of the Diagrams

  1. Slide 1
    3.1 Ohm's Law Triangle

  2. Slide 2
    3.2 Circuit with Open Switch
    3.3 Circuit with Closed Switch

  3. Slide 3
    3.6 Circuit with Two Series Resistors
    3.7 Circuit with Two Parallel Resistors
    3.8 Circuit with Three Parallel Resistors

  4. Slide 4
    3.9 Two Resistors, Voltage Divider
    3.11 Series Parallel Resistors
    3.12 Parallel Resistors in Series