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: 

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Engine-driven generator

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Solar cell

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Thermopile

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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: 

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Hydroelectric dams

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Nuclear power plants

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Coal and oil fired power plants

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Natural gas fired power plants

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Wood fired power plants

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Geothermal power plants

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Solar power plants

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Tidal/wave power plants

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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. 

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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! 

Soldering

Question 1:

Both soldering irons and soldering guns are tools used in the process of electrical soldering. Describe what each of these tools looks like, and how they function. 

A modern soldering ïron" is a pencil-shaped device with an electrical heating element in its end. Soldering "guns" have a pistol-shaped body, with a loop of thick copper wire at the end acting directly as a heating element. Soldering irons have no on/off switch, while soldering guns do, which should tell you something about the speed at which they heat. 

Notes:
Antique soldering irons were really nothing more than iron wedges with wooden handles, which were set directly in a fire for heat, much like clothes irons used to be nothing more than an anvil-shaped mass of iron that was set on top of a hot stove for heat. I've used one of these old soldering irons to solder sheet metal pieces together, and the technique of use was exactly the same as it is for a modern (electric) soldering iron. 
If you have soldering irons and guns available in your classroom, show them to your students during discussion. 

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Question 2:

What does it mean to tin a piece of wire prior to making a soldered connection with it? 

"Tinning" is the act of pre-soldering the bare wire end, so that its appearance is like solder instead of copper. 

Notes:
Some wire may be purchase "pre-tinned" to save you this step, but most electrical wire is not. Here is a helpful hint: tinning the ends of a piece of stranded copper wire really helps to prevent individual strands from bending and breaking away when the wire end is clamped in a terminal strip! 

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Question 3:

All electrical soldering, and most other forms of soldering as well, requires the addition of a substance known as flux. What is the purpose of adding "flux" to the soldering process, and what type of flux is typically used for electrical soldering? 

"Flux" is a substance that chemically cleans the metal surfaces prior to soldering. For electrical work, the preferred flux is "rosin". 

Notes:
This question, and the corresponding answer, point to one of the fundamental requirements for a good solder joint: clean metal surfaces. Flux is designed to chemically clean the joint prior to and during the soldering process, but it is no substitute for proper preparatory cleaning beforehand. 
Discuss this point with your students. If possible, set up some wire connections to be soldered, and show the effects of a small amount of oil or grease on a solder joint (even when using flux). 

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Question 4:

As in all technician tasks, safety must be your first priority. Soldering harbors specific dangers of its own that you need to be aware of. Explain what precautions you need to take with regard to the following aspects of soldering: 

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Safe handling of soldering iron/gun

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Safe handling of solder and flux

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Personal protective equipment

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What to do (and not do) after soldering

Safe handling of soldering iron/gun 

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Never hold it by the metal end - always by the handle!

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Place iron on a stand when idle, and be careful that no flammable materials contact it.

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Never leave the iron on, unattended.

Safe handling of solder and flux 

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Never hold solder in your mouth, as it contains heavy metals that are not healthy for your body.

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Avoid breathing fumes - use a ventilation hood if available.

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Do not eat food while soldering.

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Wash your hands after soldering.

Personal protective equipment 

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Safety glasses or goggles - an absolute must!

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If soldering large wires, use heat-resistant gloves.

What to do (and not do) after soldering 

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Clean up worksite.

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Give the soldering iron enough time to cool down before putting it away.

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Put soldering equipment away where it belongs, and where others can find it.

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Never leave the iron on, unattended.

Notes:
Much of this is common sense, but some of it is not. Students must realize that even though soldering is a relatively low-temperature process, it is still hot enough to start fires and burn skin. Also, molten droplets of solder can cause major damage to the eyes, so safety glasses/goggles are imperative. 

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Question 5:

Semiconductor components such as diodes and transistors are easily damaged by the high temperatures of soldering, so care must be taken to protect these components during the soldering process. One way to do this is to use a heat sink to draw heat away from the component without cooling the connection point too much. Heat sinks made out of sheet metal may be temporarily clipped to the component leads, one at a time, to prevent the solder's high temperature from thermally conducting all the way to the component body: 
 

In the absence of a formal heat sink, can you think of any ways to fashion your own ïmpromptu" heat sinks out of commonly available tools and/or objects? 

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Question 6:

When soldering a wire into a metal lug, which of these positions would be considered best for soldering iron and solder? 
 

Explain why you think one of these positions is better than the others for producing a sound solder connection between the lug and the wire. 

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Question 7:

What is a cold solder joint? Identify its causes, its appearance, and the means of correction. 

A "cold solder joint" is a poor solder connection resulting from motion of the pieces during the cooling process or insufficient temperature. 

Notes:
Ask your students where they obtained their information regarding the causes, appearance, and remedy for cold solder joints. There is a lot of information available on soldering, and it is always interesting and informative to discover new sources of information for future reference. 

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Question 8:

After a period of use, the tip of a soldering iron (or a soldering gun) becomes dark and ëncrusted" with oxidation and flux residue. Describe why this accumulation impedes the soldering process, and how it may be removed. 

Any oxidation or foreign material on the soldering tool tip will impede heat flow to the item being soldered. Routine cleaning is easy: just touch the hot tip to a water-soaked sponge or rag. 

Notes:
Discuss with your students why a wet sponge or rag works to clean the end of a heated soldering tool. Also, discuss what to do to the tip of a soldering tool that has been abused by lack of periodic cleaning. 

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Question 9:

What may be done if you solder a component into a circuit board and then realize you soldered it in the wrong place? What, exactly, must you do to move this component to a different location on the board? 

You must desolder the component from the board, using any number of desoldering tools: 

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De-soldering "wick"

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De-soldering bulb

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De-soldering "pump"

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Vacuum station

Notes:
Ask your students to describe how some common desoldering tools (wick, bulb, plunger, vacuum system) work, and also ask them where they obtained their information. 

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Question 10:

Explain what surface-mount devices are, and how the soldering and desoldering processes for them differ from the processes used to solder and de-solder "through-hole" components on printed circuit boards. The following illustration contrasts "through-hole" component construction versus surface-mount construction for a voltage divider circuit (three resistors connected in series): 
 

"Surface-mount" components solder to the surface of a printed circuit board, and have no mechanical means of attachment, unlike "through-hole" components. Because of their small size and lack of attachment prior to soldering, special soldering equipment must be used. 

Notes:
Ask your students to describe some of the special soldering equipment that must be used for surface-mount devices? How does it differ in terms of power from regular soldering irons and guns? Why are surface-mount soldering tools rated at a different power level? 
Ask your students to describe how surface-mount components may be held to the board during the soldering process. Are all surface-mount components identical, or are there different types? What special soldering tool attachments are available for working with surface-mount devices? 

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Question 11:

The equation for converting degrees Celsius (^oC) to degrees Fahrenheit (^oF) is as follows: 

TF = 9 5 TC + 32

Use this equation to convert the following temperatures from Celsius to Fahrenheit: 

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0^oC =

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100^oC =

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-40^oC =

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250^oC =

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310^oC =

Now, algebraically manipulate the equation so that you may solve for degrees Celsius from a given figure in degrees Fahrenheit. Show all your work, then check your results by working backward through the five calculations above to arrive at the original figures in degrees Celsius. 

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0^oC = 32^oF

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100^oC = 212^oF

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-40^oC = -40^oF

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250^oC = 482^oF

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310^oC = 590^oF

TC = 5 9 (TF − 32)

Notes:
The last instruction given in the question - working backwards through the five calculations to see if you get the original (given) figures in degrees Celsius - is actually a very useful way for students to check their algebraic work. Be sure to make note of this in class! 

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Question 12:

What is soldering? What is solder? 

"Soldering" is a low-temperature form of welding commonly used to join electrical conductors together for permanent connections. 
"Solder" is a special alloy of metals designed to melt at a low temperature, to make permanent electrical connections. 

Notes:
It should be noted that not all forms of soldering are "low temperature." Silver soldering, for example, takes place at much greater temperatures than regular electrical soldering. However, all soldering is a form of welding. 

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Question 13:

Find a printed circuit board with soldered components on it, and bring it with you to class for discussion. If you have access to a scrap bin from an electronics or computer repair shop, this would be an excellent place to find a printed circuit board. On your PCB, try to identify: 

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Number of layers on board

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"Traces"

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"Lands"

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Cold solder joints (if any)

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Solder bridges (if any)