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:
- VOM
- Resistors of various values and power ratings
- Clip leads
Lesson Plan:
- Answer any questions from homework.
- Put the Ohm's Law Triangle on the board (available as Slide 1) and
review the methods by which equations are re-arranged.
- Have the students attempt Chapter 3 Quiz questions 7, 8, and
9.
- Stress proper setup:
- What do I have?
- Are all units expressed in base form? ( e.g. millivolts as
volts)
- What do I want?
- How do I re-arrange Ohm's Law equations?
- Have volunteers put them on the board. Tell them that their
homework will contain lots of practice.
- 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.
- Use the VOM and a DC circuit to show how voltage drops across a
resistor.
- Show resistors of different power ratings to emphasize the
differences in the sizes of the leads and the bodies of the
resistors.
- 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
- Use the VOM to measure the resistance of a number of resistors.
Have the class predict in advance from the colour code on the resistor
what the result will be. Show that the variation in the measured
resistance falls within the tolerance as found from the tolerance band
on the resistor. Have the students predict what the tolerance will be.
- Using the VOM or a 100 mA ammeter and a (9 V) battery, use the clip
leads to connect the VOM in series with the battery and a resistor.
Calculate the expected current, then close the circuit and measure the
current. Be careful that the current is within the range of the meter!
- Now use the VOM to show the consequences of connecting resistors in
series and parallel. To the circuit you just set up, add another
resistor in series. Measure the current and show by calculation that it
is the current that would flow if the total resistance were the sum of
the two resistors in the circuit. You have just derived the equation
for resistors in series.
Resistors in Series and Parallel
Slide 3 shows the circuits that are needed for this
demonstration.
- You have already seen resistors in series so review the equation for
that. Measure the voltage at the junction of the two resistors and show
how it arises from the voltage drop across each resistor, with the same
current through each. Show that the sum of the voltage drops is always
equal to the supply voltage.
- Connect two equal resistors in parallel and measure the voltage
across each. Explain that the same voltage appears across each resistor
in a parallel connection.
- Calculate the current that should flow in each resistor from Ohm's
Law and demonstrate by measurement that this is the current flowing in
the resistors.
- Measure the current through the entire circuit and demonstrate that
it is the sum of both the resistor currents.
- Finally, do the same with resistors of different values and with
more than two resistors
- Reinforce the point that the total current in a parallel circuit is
the sum of the current in all the individual branches.
Resistors in Both Series and Parallel
- Connect up a circuit containing two resistors in parallel and a
single resistor in series with the parallel combination
- Apply power and measure the voltages and currents across and through
each resistor.
- Explain that almost all practical electronic circuits are some
combination of series and parallel resistances, although not always
physical resistors.
Homework:
- Review any questions they had about the homework from the previous
class.
- Read Chapter 4, or if the chapter is split into two sessions, the
last part of chapter 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
- Slide
1
3.1 Ohm's Law Triangle
- Slide
2
3.2 Circuit with Open Switch
3.3 Circuit with Closed
Switch
- Slide 3
3.6
Circuit with Two Series Resistors
3.7 Circuit with Two Parallel
Resistors
3.8 Circuit with Three Parallel Resistors
- Slide 4
3.9
Two Resistors, Voltage Divider
3.11 Series Parallel Resistors
3.12
Parallel Resistors in Series