Lesson 4:

Inductors and Capacitors

Objectives:

When the students have finished this chapter they should be able to:

Apparatus:

  1. Several different types of inductors and capacitors (fixed values and variable)

  2. Oscilloscope

  3. Low voltage AC and DC sources

  4. Small gauge insulated wire

  5. 10, 15 cm steel spikes

  6. Masking tape

  7. 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.

  1. Go over the homework from Chapter 3.

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. 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.

  14. 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:

  1. Read Chapter 4.

  2. 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.

  3. 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

  1. Slide 1
    4.1 Left Hand Rule
    4.2 Magnetic Field in a Core

  2. Slide 2
    4.3 Inductors
    4.4 Tapped Inductor
    4.5 Variable Inductor
    4.10 Toroid Inductor

  3. Slide 3
    4.6 Inductors in Series
    4.7 Inductors in Parallel

  4. Slide 4
    4.8 Transformer - Physical
    4.9 Transformer - Schematic

  5. Slide 5
    4.11 Capacitor - Schematic
    4.12 Capacitor - Electrical Field
    4.13 Capacitor - Charging Circuit

  6. Slide 6
    4.14 Variable Capacitor - Physical
    4.15 Compression Variable Capacitor - Physical

  7. Slide 7
    4.17 Capacitors in Parallel
    4.18 Capacitors in Series

  8. Slide 8
    4.19 Inductor in AC Circuit
    4.20 Capacitor in AC Circuit

  9. Slide 9
    4.21 Series LC Circuit
    4.22 Parallel LC Circuit
    4.23 Graph of 'Q'