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:
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!)