EE101 FAQ

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FALL 2007 Q & A
Responses to common, interesting, and useful questions asked in the HW questions:

Course Material and Logistics

1. Can I get my homework or questions returned to me before the next class?

   As soon as I have your homework and questions graded, I put them in the appropriate slot on my office door. You can stop by any time to see if it is ready. I usually grab this on my way to class and bring them to you. Questions will usually be there by the end of the week, homework may be there as early as monday, but often not until tuesday morning.

2. Should we write the questions ans/or draw the diagrams in the homework answer sheet or simply provide the answers with our work?

   The homework you hand in should be more than an answer sheet. I would like it to be self-contained, which means you should restate the problem and draw the diagram, in addition to showing your work and the answer/s. Please also circle or otherwise highlight your answer so it is easy to find. Tidy homework is much easier to grade and fosters an organized approach to problem solving, so be organized and present your work in an orderly fashion.

3. On Number X part Y, I had trouble, got the wrong answer, didn't understand something, etc...

   Stop by for help with these issues. If you are extremely specific I can sometimes answer written questions like this, but by the time you hand it in and get it back, it's too late to use the information for the homework. So, if you have questions, no matter how large or small, please find me and we'll get you through it. Remember you can always email me too.

4. Will I be able to fix basic circuitry after taking this course?

   Yes and No. We will learn some basic tools for working with circuits, but it is more of an introduction to circuit theory and lab skills, rather than a comprehensive circuit design/repair course. By the end you should be able to design troubleshoot and fix at least the kinds of circuits we study in this class.

5. Will we use Nodal Analysis in the real world?

   Sure. The things we use it for are common uses even in the real world as well. Another example is when using an oscilloscope to make voltage measurements. An oscilloscope has two probe connections like a volt meter, but one of them is connected to ground inside the scope. This means that it can only measure voltages with respect to ground. Therefore, to measure across a circuit component that is not connected to ground, you'll have to measure with respect to ground on both sides of it and subtract one from the other to get the voltage drop across the component.

General Electrical Concepts

1. What keeps animals that perch on power lines from getting elecrocuted?

   There are two answers to this question. They depend on how you envision the problem.

   • If birds perch on a power line without touching anything else, then they are not completing a circuit between the power line and another voltage potential, therefore current does not flow through the bird.

   • If you're wondering why it doesn't get electrocuted by current flowing from one foot to the other, then think about this. With both feet on the wire, the bird is putting itself in parallel with the small length of wire between its feet. While we typically model ideal wires as having no resistance, it actually does have a small resistance. In this case the resistance of the bird's skin and body from foot to foot is so much greater than the tiny resistance of that short length of wire, that only a negligable (if any) current will flow through the bird. Don't try this at home. :-)

2. What side of the circuit should I start on when I group resistors for circuit reduction?

   Wherever you can. There's really no rule for this. In the problems we've seen so far, there's been only one place you can start. In any case, you only want to do as much work as you need to, so start as close as you can to the value you're trying to calculate.

3. What's all this conductance mumbo-jumbo in the book?

   We won't use conductance in this class, other than to understand the basic concept. Conductance is the inverse of resistance, and is mainly a concept of convenience for us. At times when we have more complex values for a resistance (or later an impedance) it's easier to manage the math if we invert the value, especially when dealing with parallel components. In our case, resistances are generally integer values so the fractions are not cumbersome. However, in later courses you'll use the same techniques on impedances which are complex, such as 1/jωc where ω w is a frequency, c is a capacitance, and j is the imaginary number. With these sorts of values in the denominator, it's sometimes easier to do the math if we move them to the numerator by converting them into a conductance.

4. Suggestion for the resistor color code mnemonic:
   Big Blunders Really Organize Your Great Big Views Giving Wisdom

   I like it. Get the word out. Hey everybody, keep them coming, lets get lots of them.

5. Can you always turn a circuit with one power source and any number of resistors into just a power source and one resistor?

   Yes. At least I can't think of any exceptions. We will discuss another technique for circuit reduction toward the end of the semester called Thevenin & Norton Equivalent circuits. With these techniques we can model more complex circuits with a single power source and a single resistor.

6. How many voltage sources can you have in one circuit?

   As many as you want or need. The electric boost system in my hybrid car uses 120 1.2v batteries in series to make an equivalent 144v supply!

   I also once built a UPS for a large phone system (15,000 phone lines) which ran on 48v DC. I used twelve banks (in parallel) of four car batteries in series. That's 48v per bank, 12 banks at about 100 amps each makes 1200 amps. That's a lot of energy. It's no wonder it made sparks and arc-welded my wrench to the battery terminal when I accidentally shorted across the positive and negative posts while connecting the cables. Oops. I covered the wrench handle with an entire roll of electrical tape after that, but I had to let it cool down for a while first. :-) Don't try this at home!

7. What is the purpose of a capacitor?

   A capacitor has many purposes:

   • To store energy: It can charge and discharge faster than a battery so it is well suited for applications that require bursts of energy. Camera flashes and emergency vehicle lights are examples of this. Also to start a motor it takes a burst of energy to overcome its inertia (or think of it as a spike in voltage to get the inductor current flowing, a motor is an inductor after all).

   • Filters: A capacitor voltage can not change voltage instantaneously so it is useful as a way to filter out unwanted changes in voltage such as noise or certain signal frequencies.

   • To block DC: Since current flows through a capacitor only when its voltage is changing, when a constant voltage is applied it is blocked by the cap. It looks like an open circuit once it is fully charged.

   • For power factor correction: Inductive loads sometimes require a capacitor to offset the inductance in the circuit so that it doesn't overload the power supply (like the motor example above), and vice versa.

   • Time travel: As in the flux capacitor. See below. :-)

Oddball Musings and Smart Remarks

1. What kind of question should I ask next time?

   A better one than this!

2. Is is really possible to store flux (as in the "Flux Capacitor")?

   Flux is generally used when talking about some sort of field penetrating a surface, volume, or path. EE folks commonly associate flux with electric and magnetic fields, though there are many others such as heat, fluid, or gas flow, and such. Mathemeticians simply like the process and often don't bother much with the reality of the field in question. Dr. Starrett once proposed calculating flux of the "Love" vector field through a person. Is it merely a coincidence that he looks a lot like Doc Brown? :-) Here's another explanation from people with too much time on their hands

3. How much voltage does it take to kill a person?

   Don't try this at home! Or anywhere else for that matter. It's really current that kills. High voltage is something we come across safely on a daily basis. Static electricity is a good example, and because it is very low current it is relatively harmless. Though it's always wise to be careful with electricity. It only takes about 100 mA to 200 mA for a fatal shock. Yes, that's milliamps, which common batteries are capable of generating. Therefore, always be safe around electricity no matter how small or harmless you may percieve a circuit to be. Here's a good link for more info. And here's a brief answer.

4. I'm considering making a [coil gun, projectile launcher, tazer, other prankster/dangerous item], can you help me find parts or solve some problem with it?

   No. :-) Good luck. Wear safety glasses. And of course, don't try this at home.

5. I'm considering making a [safe circuit with a useful purpose to expand my knowledge and express my creativity with EE material], can you help me find parts or solve some problem with it?

   Absolutely! :-) I have lots of parts, catalogs, books, resources (and a little knowledge) to help you with these kinds of things. Please stop by and share your project. DO try this at home!

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