Current Events: Resistance is Futile! (updated)

You will be assimilated – or perhaps it is waht you will have to assimilate. Whatever – the introduction to simple direct current (DC) circuits is a very simple topic. What is more difficult is understanding the underlying principles and concepts – What is a current? What causes it start “flowing”? What exactly is “voltage”? What does it do? Where does it go?

These questions are a lot more difficult (and a lot more interesting!) than the level of  DC circuit analysis that we do in unit 1 Physics. However, it is necessary to concentrate on the Circuit analysis before we get back into the fun stuff.

Let us meditate:

Ommmmm

Oommmm

Ohmmmm

Ohmmm’s LAW!

Ohm’s law is one of two critical underpinnings of DC circuit analysis; the other is Kirchoff’s Laws (Yes, Kir, you do have laws with your name. Does that mean the class can hold you responsible for having to learn them?). Let’s deal with Ohm’s Law first.

The simplest statement of Ohm’s Law states that in a resistive circuit (i.e. one containing only direct resistance elements such as resistors (e.g. heating elements, incandescent light globes)) that current and “voltage” are directly related to resistance (given that temperature remains constant) (note that this is a problem, because resistance creates a change in temperature!)

This is often seen in the formulation of V = IR, but this rearrangement doesn’t reflect the central concept of the relationship between current and voltage as simply. We also frequently arrange Ohm’s Law in a triangle, as this make rearrangements unnecessary; the correct formula is revealed by covering the unknown quantity.

We call circuit components that follow this principle Ohmic Devices, and when you measure the “voltage” across them and the current through them you find the data points produced will form a linear relationship. In the graph below, only device B is an ohmic device:

Other types of devices exist (as shown by lines A, C & D) but it is critical to remember that the V-I graph of an ohmic device is not only linear but must pass through the origin and have a defined gradient (i.e. not zero or infinite).

Next, some videos about Ohm’s Law, and going on to Kirchoff’s Laws. There are 10 in total, so you’ll need to set some time aside to watch them all. I strongly advise that you watch them with a pen in hand. We will be investigating the same concepts in class, but it will make much more sense if you have some idea what is going on first!

Furry Elephant has a very good animation on what actually happens when a current “flows”.

To be continued (again!)


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16 Comments on “Current Events: Resistance is Futile! (updated)”

  1. Patrick Says:

    isn’t that same triangle used for a number of formulas in physics as well? such as the speed,distance, and time? my only question is how do we know what goes on top, and what fills up the other 2 places?

    • CyberChalky Says:

      Yes it is – the same structure can be used with any formula with a similar relationship. It is very useful to speed up the rearrangement part of solving for an unknown quantity.

    • Kane L Says:

      The way we know is just to remember. For Ohm’s law i just think VIR, which helps put them in the right order, either way you have to remember, no formula that i know of.

  2. Loughlin Says:

    As far as my understanding goes, superconductive cooling works by having something so cool that it’s molecules are not vibrating nearly as quickly, meaning that there is almost no friction between the flowing electrons and the walls of the conductor?
    And does that mean that as current passes through a circuit, is gets more heated, therefore decreasing its effectiveness at conducting electricity?
    It appears that this means thermal conductivity should be considered when determining internal resistance.

    • CyberChalky Says:

      Absolutely correct; however the factor of heat for determining resistance is relatively small short of extreme temperatures. If you would like to do an investigation of this, we have a set up you can use to demonstrate the relationship.

  3. Loughlin Says:

    I’m having a little bit of trouble explaining the final voltage CUP (3c), how P and Q have a voltage difference of 2/3 between them. I’ve been able to apply my understanding of the theory and the formulas involved to confidently explain the other questions.

    Is it because, along the path, the first globe takes next to no voltage because it has next to no resistance compared to the gap between P and Q? Which means that P and Q must take two thirds of the voltage, with the last third going to the final globe, where the path connects up again?

    • CyberChalky Says:

      You are thinking about this the right way. If you treat the path with the broken circuit as not present, then two thirds of the current drops across the other branch. Thus, two thirds also would drop across the broken path. Potential drop is in proportion to the resistance, and a open circuit has a very high resistance, thus the majority (~99.9%) of the potential will drop across PQ.

  4. Loughlin Says:

    Hey Sir,
    When’s a good projected time to have those 70-90 physics questions done by, in time for the test?

    • CyberChalky Says:

      We will be having the “test” (actually a summary report) last lesson next week. I will be requesting the questions be submitted in two weeks from today (i.e early may).

  5. cormac Says:

    I hate that triangle! I = V/R is much better, well said.

  6. sdtacoma Says:

    Thank you so much for the triangle. I needed a device like that to help me remember the formula.

  7. James Says:

    I have final completed all the tasks,in regards to the prac, which I was away for, I have completed a detailed report, which I hope will demonstrate my understanding to you.

  8. James Says:

    I found myself remembering many of your examples, re the hill and hall light, when completing my work. Wanted to say thanks.


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