Resistors

Question 1:

Shown here is the schematic symbol for a resistor: 
 

What is the purpose of a resistor? What function does it perform? Also, draw an illustration of what a real resistor looks like. 

The purpose of a resistor is to provide a precise amount of electrical resistance in a circuit. Here is an illustration of a small (1/8 or 1/4 watt) resistor: 
 

It is also good to know that the zig-zag symbol shown in the question is not the only symbol used to represent resistors. Another common resistor symbol is shown here: 
 

Notes:
Students may (properly) ask, "Why is there such a thing as a component whose sole purpose is to impede the flow of electrons?" While resistors may seem rather pointless at first, they end up being extremely valuable electrical/electronic components. If asked, you may cite several uses of resistors in circuits: 

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To limit maximum circuit current to a safe value.

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To ßplit" a voltage into proportions.

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To ßcale" meter movements, for precise measurement of current and voltage.

•

To provide a non-shorting path to discharge static electricity.

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Question 2:

Resistors are sometimes represented in electrical and electronic schematic diagrams by a symbol other than this: 
 

Draw this other symbol next to the one shown above. 

 

Notes:
It might be a good idea to occasionally draw schematic diagrams for your students using the öther" resistor symbol, just so they are not taken by surprise when they see this symbol in real schematics. Just be sure to remain consistent in your symbolism within each diagram: never mix the two different symbols within the same schematic! 

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Question 3:

A primitive resistor may be formed by sketching a thick line on a piece of paper, using a pencil (not an ink pen!): 
 

How may the end-to-end electrical resistance of this pencil mark be increased? How may it be decreased? Explain your answers. 

The electrical resistance of a pencil mark may be increased by increasing its length. It may be decreased by increasing its width. 

Notes:
Creating a resistor on paper using a pencil is a very easy experiment to perform, the resistance of which may be measured with an ohmmeter. I strongly recommend this as a classroom exercise! 

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Question 4:

When a resistor conducts electric current, its temperature increases. Explain how this phenomenon is significant to the application of resistors in electric circuits. In other words, why would we care about a resistor's temperature increasing? 
Also, what does this indicate about the technical ratings of resistors? Aside from having a specific resistance rating (i.e. a certain number ofohms), what other rating is important for proper selection of resistors in electric circuits? 

The heating effect of electricity through a resistance is significant because that resistance may be damaged by excessive temperature. To avoid damage, resistors must be selected to be able to withstand a certain amount of heating. 

Notes:
Students need to understand that resistance alone does not fully dictate the selection of a resistor for electrical service. Failure to heed the dissipation ratings of a resistor can result in catastrophic failure! 
A good follow-up question to this is to ask what the unit of measurement is for this kind of thermal rating. 

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Question 5:

Many resistors have their electrical resistance shown by a set of color codes, or "bands," imprinted around their circumference. A standard color code associates each color with a specific decimal digit (0 through 9). Associate each of the following digits with its respective color: 

0

=

1

=

2

=

3

=

4

=

5

=

6

=

7

=

8

=

9

=

0

= Black

1

= Brown

2

= Red

3

= Orange

4

= Yellow

5

= Green

6

= Blue

7

= Violet

8

= Grey

9

= White

Notes:
Several limericks have been invented to remember this color code, most of them "politically incorrect." I often challenge students to invent their own limericks for remembering this color code, and screen the inappropriate creations from general class discussion. 

Hide Answer

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Question 6:

Observe the following "4-band" resistors, their color codes, and corresponding resistance values (note that the last color band is omitted, since it deals with precision and not nominal value): 
 

What patterns do you notice between the color codes (given as three-letter abbreviations, so as to avoid interpretational errors resulting from variations in print quality), the resistance values, and the physical sizes of the resistors? 

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Question 7:

There is more than one answer to this question! On some resistors, the last band represents the tolerance (also known as precision) for that resistor, expressed as a percentage. On other resistors, the last band represents a reliability rating for that resistor. 

Notes:
This question is worded simply and directly enough that students might think there is only one right answer. However, upon doing some research they should find that there is more involved than one simple answer can encompass! Discuss with your students the different color code types, and what applications one might find resistors with "reliability" color codes in. 
Regarding precision, nothing in life is perfectly accurate. However, the absence of perfect accuracy does not necessarily imply total uncertainty. In science, especially, it is important that all data be qualified by a statement of precision (or tolerance). Your students may be familiar with "margins of error" stated for public opinion polls. With resistors, this "margin of error" (expression of uncertainty) is explicitly given in the form of a separate color band. 

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Question 8:

Determine the nominal resistance values of these resistors, given their band colors, and also express the allowable tolerance in ohms. 
For example, a 25 kΩ resistor with a 10% tolerance rating would have an allowable tolerance of +/- 2.5 kΩ. 

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Red, Org, Blu, Gld =

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Brn, Blk, Grn, Sil =

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Blu, Blk, Brn, Gld =

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Yel, Vio, Red, Sil =

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Grn, Brn, Yel =

•

Wht, Blu, Blk, Sil =

•

Gry, Grn, Org, Gld =

•

Org, Org, Gld =

•

Vio, Red, Sil, Gld =

•

Brn, Red, Blk, Sil =

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Question 9:

Observe the following "5-band" precision resistors, their color codes, and corresponding resistance values (note that the last color band is omitted, since it deals with precision and not nominal value): 
 

What patterns do you notice between the color codes (given as three-letter abbreviations, so as to avoid interpretational errors resulting from variations in print quality) and the resistance values of each resistor? Why do precision resistors use a "5-band" color code instead of a "4-band" color code? 

The first three color "bands" for precision five-band resistors denote three digits and a "multiplier" value, respectively. A five-band color code is necessary to express resistance with a greater number of significant digits than a four-band code. 

Notes:
The normal way to teach students the resistor color code is to show them the code first, then show them some resistors. Here, the sequence is reversed: show the students some resistors, and have them figure out the code. An important cognitive skill is the ability to detect and apply patterns in sets of data. Exercises such as this help build that skill. 
It should be noted that there is a 5-band color code for non-precision resistors as well, with the first four bands serving the same purpose as in a 4-band code, the extra band indicating resistor reliability. This scheme was developed for military purposes and is seldom seen in civilian circuitry. 

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Question 10:

Determine whether or not the following resistors measure within the resistance range specified by their color codes: 

•

(Org, Org, Red, Blk, Blu) Measured resistance = 332.5 Ω

•

(Brn, Blk, Blk, Gld, Red) Measured resistance = 9.7 Ω

•

(Blu, Vio, Brn, Red, Grn) Measured resistance = 67.43 kΩ

•

(Red, Wht, Grn, Yel, Vio) Measured resistance = 2.949 MΩ

•

(Yel, Vio, Org, Gld) Measured resistance = 44.68 kΩ

•

(Gry, Red, Brn, Sil) Measured resistance = 905 Ω

•

(Grn, Blu, Gld) Measured resistance = 6.73 Ω

•

(Vio, Brn, Red, Gld, Brn) Measured resistance = 70.82 Ω

•

(Wht, Org, Blu, Brn, Grn) Measured resistance = 9.38 kΩ

•

(Red, Blk, Wht, Grn, Vio) Measured resistance = 20.86 MΩ

Assume that all five-band resistors listed here use the precision color code as opposed to the military 5-band code where the fifth band indicates resistor reliability. 

•

(Org, Org, Red, Blk, Blu) Measured resistance = 332.5 Ω Within tolerance

•

(Brn, Blk, Blk, Gld, Red) Measured resistance = 9.7 Ω Out of tolerance!

•

(Blu, Vio, Brn, Red, Grn) Measured resistance = 67.43 kΩ Within tolerance

•

(Red, Wht, Grn, Yel, Vio) Measured resistance = 2.949 MΩ Within tolerance

•

(Yel, Vio, Org, Gld) Measured resistance = 44.68 kΩ Within tolerance

•

(Gry, Red, Brn, Sil) Measured resistance = 905 Ω Out of tolerance!

•

(Grn, Blu, Gld) Measured resistance = 6.73 Ω Out of tolerance!

•

(Vio, Brn, Red, Gld, Brn) Measured resistance = 70.82 Ω Within tolerance

•

(Wht, Org, Blu, Brn, Grn) Measured resistance = 9.38 kΩ Within tolerance

•

(Red, Blk, Wht, Grn, Vio) Measured resistance = 20.86 MΩ Out of tolerance!

Notes:
This question serves as a great review for the mathematical concepts of scientific notation and percentages. They will have to calculate the allowable range of resistance values for each resistor in order to determine whether or not the measured value falls within that range. 

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Question 11:

Find one or two real resistors and bring them with you to class for discussion. Identify as much information as you can about your resistors prior to discussion: 

•

Resistance (ideal)

•

Resistance (actual)

•

Power rating

•

Type (carbon composition, metal film, wire-wound, etc.)

If possible, find a manufacturer's datasheet for your components (or at least a datasheet for a similar component) to discuss with your classmates. 
Be prepared to prove the actual resistance of your resistors in class, by using a multimeter! 

Notes:
The purpose of this question is to get students to kinesthetically interact with the subject matter. It may seem silly to have students engage in a ßhow and tell" exercise, but I have found that activities such as this greatly help some students. For those learners who are kinesthetic in nature, it is a great help to actually touch real components while they're learning about their function. Of course, this question also provides an excellent opportunity for them to practice interpreting color codes and/or component markings, use a multimeter, access datasheets, etc. 

Sources of electricity

Question 1:

What is the difference between DC and AC electricity? Identify some common sources of each type of electricity. 

DC is an acronym meaning Direct Current: that is, electrical current that moves in one direction only. AC is an acronym meaningAlternating Current: that is, electrical current that periodically reverses direction (älternates"). 
Electrochemical batteries generate DC, as do solar cells. Microphones generate AC when sensing sound waves (vibrations of air molecules). There are many, many other sources of DC and AC electricity than what I have mentioned here! 

Notes:
Discuss a bit of the history of AC versus DC in early power systems. In the early days of electric power in the United States of America, there was a heated debate between the use of DC versus AC. Thomas Edison championed DC, while George Westinghouse and Nikola Tesla advocated AC. 
It might be worthwhile to mention that almost all the electric power in the world is generated and distributed as AC (Alternating Current), and not as DC (in other words, Thomas Edison lost the AC/DC battle!). Depending on the level of the class you are teaching, this may or may not be a good time to explain why most power systems use AC. Either way, your students will probably ask why, so you should be prepared to address this question in some way (or have them report any findings of their own!). 

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Question 2:

Suppose you are building a cabin far away from electric power service, but you desire to have electricity available to energize light bulbs, a radio, a computer, and other useful devices. Determine at least three different ways you could generate electrical power to supply the electric power needs at this cabin. 

There are several different devices capable of producing electrical power for this cabin of yours: 

•

Engine-driven generator

•

Solar cell

•

Thermopile

•

Windmill

For each of these devices, what is its operating principle, and where does it obtain its energy from? 

Notes:
For each of these electric power ßources," there is a more fundamental source of energy. People often mistakenly think of generator devices as magic sources of energy, where they are really nothing more than energy converters: transforming energy from one form to another. 

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Question 3:

Where does the electricity come from that powers your home, or your school, or the streetlights along roads, or the many business establishments in your city? You will find that there are many different sources and types of sources of electrical power. In each case, try to determine where the ultimate source of that energy is. 
For example, in a hydroelectric dam, the electricity is generated when falling water spins a turbine, which turns an electromechanical generator. But what continually drives the water to its üphill" location so that the process is continuous? What is the ultimate source of energy that is being harnessed by the dam? 

Some sources of electrical power: 

•

Hydroelectric dams

•

Nuclear power plants

•

Coal and oil fired power plants

•

Natural gas fired power plants

•

Wood fired power plants

•

Geothermal power plants

•

Solar power plants

•

Tidal/wave power plants

•

Windmills

Notes:
A great point of conversation here is that almost all ßources" of energy have a common origin. The different ßources" are merely variant expressions of the same true source (with exceptions, of course!). 

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Question 4:

There is a fundamental Law in physics known as the Law of Energy Conservation. This law states that energy can neither be created nor destroyed, merely transformed from one form to another. 
In regard to this Law, is it possible to make an electrical battery that lasts forever, and never becomes exhausted? Explain why or why not. 

According to the Law of Energy Conservation, it is not possible to make a battery that lasts forever. 

Notes:
This question may lead into a discussion on perpetual motion machines, a subject that seems to be perpetual itself in the popular interest. 

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Question 5:

The mathematical equivalence between watts and horsepower is approximately 746:1. Given this equivalency, how many watts of electrical energy may theoretically be produced by a generator, if turned by an engine rated at 50 horsepower? 

37.3 kW of electrical power. 

Notes:
Unit conversion problems are handled most easily by using ünity fractions" comprised of a quotient of two different numerical quantities having the same physical value. For instance: 
 

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Question 6:

Suppose a person decides to attach an electrical generator to their exercise bicycle, so as to do something useful with their "pedal power" while they exercise. The first time this person uses their bicycle generator, the electricity is used to power a single 60-watt light bulb. However, the next time this person uses their bicycle generator, a second 60-watt light bulb is connected to the generator, for a total load of 120 watts. 
When pedaling with the additional load, the person notices the bicycle is much more difficult to pedal than before. It takes greater force on the pedals to maintain the same speed as before, when there was only a single 60-watt light bulb to power. What would you say to this person if they asked you, the electricity expert, to explain why the bicycle is more difficult to pedal with the additional light bulb connected? 

I won't give you the answer directly, but here is a hint: the Law of Energy Conservation. 

Notes:
This phenomenon is more easily understood when experienced directly. If you happen to have a hand-powered generator available for a classroom demonstration, help your students set it up to demonstrate this principle. 
An excellent topic of discussion related to this question is the effect that using more electrical power has on the generators at power plants (hydroelectric, nuclear, coal-fired, etc.). What would happen at the power plants supplying electricity to the nation's electrical "grid" if everyone simultaneously turned on all their electrical loads at home? 

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Question 7:

Where does the energy come from that causes a battery to be a source of electricity for powering electrical devices? Ultimately, what is the energy source of a battery? 

Electrical batteries derive their energy from chemical compounds stored within. 

Notes:
Battery electrochemistry is quite complex, and not easily addressed in a class environment unless all students have a good background knowledge of chemistry. It is not the point of this question to address issues of electrochemistry, but merely to point out the nature of a battery's energy source. 

Hide Answer

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Question 8:

Describe what a fuel cell is, and what the practical importance of such a device might be. 

"Fuel cells" are essentially batteries externally supplied with chemicals for their source of energy. 

Notes:
Much attention has been directed toward fuel cells as energy conversion devices, for their high efficiency and environmentally "clean" operation. Your students should have no trouble finding current information on fuel cell technology. 

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Question 9:

What will happen, electrically speaking, when the shaft of the machine is turned? 
 

A voltage will be ïnduced" in the wire coil as the magnet passes by. 
Follow-up question: do you think the voltage generated by this machine would be DC or AC? Explain your answer. 

Notes:
Although this machine would not be very efficient in the form shown in the illustration, it would work to generate electricity when turned. As such, it demonstrates the principle of electromagnetic induction. 

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Question 10:

Suppose there was something wrong in this electrical system. When the shaft of the generator is turned, the light bulb does not light up: 
 

What are some of the possible causes of this failure? Please be specific. Also, what could you do to either confirm or deny these specific possibilities? 

There may be an öpen" fault in the circuit somewhere, and/or the light bulb could be improperly sized for the generator's rated voltage output. I'll let you determine how certain diagnostic checks could be made in this system to determine the exact nature of the fault! 

Notes:
This question poses an excellent opportunity to group discussion on troubleshooting theory and technique. There are several different ways in which the nature of the fault may be determined. Encourage your students to think creatively!