Lesson 4:
Inductors and Capacitors
Objectives:
When the students have finished this chapter they should be able to:
- define the terms inductance, capacitance, inductive and capacitive
reactance, and explain the factors affecting each
- do simple calculations involving capacitance and inductance
- explain the role of the inductor and capacitor in an electronic
circuit.
Apparatus:
- Several different types of inductors and capacitors (fixed
values and variable)
- Oscilloscope
- Low voltage AC and DC sources
- Small gauge insulated wire
- 10, 15 cm steel spikes
- Masking tape
- VOM (preferably one that will measure capacitance).
Handouts:
All students should have a copy of the Formulas Handout from
the last lesson.
Lesson Plans:
4.1 Demonstration of the effect of inductance in an AC
circuit
4.2 Demonstration of the transformer effect
and turns
ratio
This chapter may take two classes to cover. Where you split it is
hard to pin down. The homework from the previous chapter is the first
major test of your students' problem-solving skills and you may find
yourself tied up with this. It is important that they can do the
questions from Chapter 3 before going on to Chapter 4.
- Go over the homework from Chapter 3.
- Pass around sample of inductors. Show them examples of a roller
inductor, a tapped coil, inductors of fixed value, and an inductor with
a core.
- Using low voltage DC and AC sources show how an inductor blocks AC
but passes DC. See figure 4.1 above for the setup of this
demonstration. It is difficult to show voltage drop across an inductor
in a meaningful way. The suggested circuit measures the voltage drop
across the resistor instead. By shorting out the inductor with the
jumper you can show that the initial voltage in the circuit was
6.3 VAC, while with the inductor in it is less. Do not discuss
the phase components at this point! 10H power supply chokes
used to be common but are rarely used now; if you can't find
one, use the primary of an old (tube circuit) audio transformer.
- Point out that calculating total inductance of circuits containing
inductors in series and parallel are approached the same way as for
calculating resistors in series and parallel. This is a good time for
reviewing shifting from one unit of inductance to another. Have the
class do Chapter 4 Quiz Question 44.
- Have the students build a small step up transformer. Wind 20 turns
of #18 or #20 wire around a steel spike, ensuring that the turns are as
close as possible. See figure 4.2. Leave leads of about 10 cm length.
Use masking tape to hold the turns in place. Bare the end of each lead
and label these as your primary. Close beside or on top of the 20 turns
of the primary, wind 40 turns of new wire in two rows to form a
secondary. Tape in place and again leave leads of about 10 cm in length.
Bare the end of the leads and label these as your secondary. Place a
load of about 100 to 1000 Ohms across the secondary. Without the
load the actual secondary voltage may be unpredicatble. Attach a low
voltage AC source to the primary and measure the voltage across the
secondary with a VOM. This is also good place to cover turns
ratios.
- Use the transformer you have built to show the
concept of a step down transformer by reversing the leads– the previous
primary leads will now be the secondary leads and the secondary leads
will now be the primary leads. Attach a low voltage AC source to the
primary and measure the voltage across the secondary with a
VOM.
- Show the class various types of transformers, big and small. Show
them a laminated transformer core taking it apart if possible to show
the plates. Show them a toroid core.
- Pass around capacitors of various types and sizes: big and small
electrolytics, ceramic by-pass, surface mount; your options here are
endless. If you have access to a VOM that also has a capacitance
measurement feature included you can show them how to measure
capacitance and the units with which it is measured. The big
electrolytics all have the working voltage stamped on the outside of the
can. You cover this topic, S4.9 in the RACSG, very easily at
this point.
- A big variable helps to show the various factors:– plate area,
distance between the plates and dielectric. If you can connect a device
that measures capacitance you can show the result of turning the rotor.
If you can obtain two identical units and are prepared to sacrifice one
you can remove alternate plates on the stator and rotor so that distance
is double. By inserting pieces of polyethylene between the plates you
can show the concept of the dielectric.
- Remind the class that formulas calculating the total
capacitance for capacitors in series and parallel are OPPOSITE to the
formulas used for resistors and inductors in series and parallel. Have
them try Question 27 in the Chapter 4 Quiz. This is also a good time for
reviewing shifting from one unit of capacitance to another. Have the
class to Chapter 4 Quiz Question 43.
- Put the formula for calculating Inductive Reactance on the board.
Have the class do Chapter 4 Quiz Question 13, stressing again good form
and the necessity of shifting to proper base units.
- Put the formula for calculating Capacitive Reactance on the board.
Have the class do Chapter 4 Quiz Question 14, stressing again good form
and the necessity of shifting to proper base units.
- The easiest way to introduce resonance is with a long narrow tray
of water. (2 feet of eaves trough is perfect) If the end of the tray is
raised and lowered at a random frequency, then the water splashes about
in the tray but does not develop waves. On the other hand, there is a
frequency, the resonant frequency, at which a small amount of movement
can raise a large wave. The energy in the wave is transferred
alternately between the height of the wave above the floor of the tray
and the physical motion of the water. This is analogous to an electrical
resonant circuit where the energy is transferred between the capacitor
and inductor once per cycle. In both cases there is energy loss which
causes the amplitude of the variations to die down in the absence of
any new input: in the water through friction; in the electrical circuit
through resistance. This reflects the 'Q' of the circuit.
- Touch on impedance, tuned circuits, and Q. What is
covered in S4.14, S4.15, and S4.16 is more than sufficient for their
needs.
Homework:
- Read Chapter 4.
- Do the Chapter 4 Quiz If you elect to do this chapter over two
classes then you will have to break this quiz up into two
parts.
- Skim Chapter 5.
Chapter 4 Quiz:
Printable copy of
the Review Quiz
Answers to Chapter 4 Quiz
1 |
D |
10 |
C |
19 |
C |
28 |
B |
37 |
A |
2 |
D |
11 |
C |
20 |
C |
29 |
C |
38 |
D |
3 |
C |
12 |
D |
21 |
A |
30 |
A |
39 |
D |
4 |
C |
13 |
B |
22 |
A |
31 |
D |
40 |
D |
5 |
A |
14 |
A |
23 |
A |
32 |
D |
41 |
A |
6 |
A |
15 |
A |
24 |
B |
33 |
D |
42 |
C |
7 |
C |
16 |
A |
25 |
D |
34 |
A |
43 |
C |
8 |
D |
17 |
D |
26 |
B |
35 |
B |
44 |
D |
9 |
B |
18 |
B |
27 |
C |
36 |
A |
45 |
B |
Overhead Slide Versions of the Diagrams
- Slide
1
4.1 Left Hand Rule
4.2 Magnetic Field in a Core
- Slide
2
4.3 Inductors
4.4 Tapped Inductor
4.5 Variable
Inductor
4.10 Toroid Inductor
- Slide
3
4.6 Inductors in Series
4.7 Inductors in Parallel
- Slide
4
4.8 Transformer - Physical
4.9 Transformer -
Schematic
- Slide 5
4.11
Capacitor - Schematic
4.12 Capacitor - Electrical
Field
4.13 Capacitor - Charging Circuit
- Slide 6
4.14
Variable Capacitor - Physical
4.15 Compression Variable Capacitor -
Physical
- Slide 7
4.17
Capacitors in Parallel
4.18 Capacitors in Series
- Slide 8
4.19
Inductor in AC Circuit
4.20 Capacitor in AC Circuit
- Slide 9
4.21
Series LC Circuit
4.22 Parallel LC Circuit
4.23 Graph of
'Q'