Foundations Continued

  • Read about resistors. This continues our understanding of the components we are working with this term.

Below are some questions that come from previous and the current reading; please complete the questions before class in Tuesday.

The questions below can be considered practice/prep for your exam. Once you have completed them, I will share answers with you.

Comprehension Questions

You may have to do some additional research to answer some of these; however, the majority should be answerable through a combination of material in the Manga Guide as well as other reading we’ve done.

After you complete the concepts practice, I will share representative solutions with you. However, the important thing is not that your answers match mine exactly, but instead that you are engaged in re-reading material and thinking about the concepts in preparation for the exam. the process of revisiting the material, thinking about it in new ways, and trying to figure things out is the critical learning process that will help you both on the exam and throughout the rest of the course.

Representative Answers

These are not necessarily “the” answers. However, they are representative of the kind of answers you might be thinking about. In many cases, I will simply refer to the source where you are most likely to find representative answers—my answer, in these cases, is not necessarily going to be “more right” than your own work that was based on your reading.

  • What is voltage, and what analogy is typically used to explain it.

  • What is resistance, and what analogy is typically used to explain it.

  • What is current, and what analogy is typically used to explain it.

All three of these questions come from your reading in the MGtE.

  • What is the name of the mathematical relationship between voltage, resistance, and current?

Ohm’s Law.

  • If we build a circuit with a 220Ω resistor (meaning resistance is constant), and we turn up the voltage, what will happen to the amount of current flowing through the resistor?

V = IR, so we can infer that as the voltage goes up, the current will go up proportionately. (Specifically, the resistance is constant, so the voltage and current will go up by equivalent amounts.) Eventually, we will burn out our resistor.

  • What equation relates wattage, voltage, and current?

P=IV, or (as sometimes known) Jules’s Law.

  • The resistor in the circuit is a 220Ω resistor rated at 1/4W. At 5V, are we in danger of exceeding the wattage that the resistor can handle? (This may require combining both equations.)

We worked this in class; it involves using both V=IR and P=IV. Because we know resistance and voltage, we can solve for I using V=IR. Then, we can solve for P using I and V. The result roughly .11W, which is less than 1/4W, or .25W. In other words, the resistor can handle the power we are putting through it.

  • What is the resistance of the resistor depicted in the circuit above?

I can’t remember; I think it was around 2.7K? Either way, this question was a direct V=IR question, but you had to be careful. The amperage was given in milliamps, meaning that you first had to divide 4.4556 mA by 1000 to get amps.

  • What is the difference between an insulator and a conductor?

We discussed this in class; again, any reasonable answer.

  • How could a battery, an LED, and some lengths of wire be combined to test materials for conductivity?

This question was unclear; I was essentially trying to suggest that you could wire up the battery and LED such that you could touch the wire ends to objects and see if you had current flow. It would be a crude form of continuity tester.

The next two questions were about multimeters.

The above video (~5m) is a useful review. I should have given it to you sooner. I will not be assuming you’ve watched it before the exam, but I recommend watching it at some point.

  • What does it mean when we say a switch is a SPST switch? Find an example of one online, and include a URL to the image. (Google Image search is a good tool for this; you can right-click and say “Copy Image URL” to include the image link here.)

  • What is a DPDT switch? Again, find an image example. Compare your answers against what you read in the Encyclopedia of Electronic Components.

These questions were to drive you to Google and to do some research on your own. I know some of you were still wrestling with switches and concepts surrounding them. I was hoping this would help.

The above video is a bit… dry, but it has some nice visualizations. Again, I wish I had found it sooner, but I didn’t. (You can always do your own Googling for good resources, and share them with me. I like that.)

  • You are repairing a lamp. It has a knob that you can turn, and it sets the light bulb to three different intensities of light. Based on your current understanding of switches and resistors, describe what you think might be going inside the lamp.

This was a thinking question. You had to, essentially, design a circuit that might offer a possible explanation of what is going on inside your lamp. The clue is that you have:

  1. A voltage source.
  2. A multi-way switch.
  3. Resistors.
  4. A light bulb.

I imagined a circuit something like this:

The switch selects between different levels of resistance, which control the amount of current that flows to the bulb; this, in turn, effects how bright the bulb is.

Could you have come up with that idea? Maybe, maybe not. It stretches you a bit, though, which I sometimes like to do on exams.