AE237 WEEK-2: Ohm's Law and the Power Formula
    (updated October 2014)

    A more in-depth look at Ohm's Law and its variations via the Power Formula.  The Formula Circle (below) shows how all of the circuit variables interact.


    Resistor Color Code

    Voltage Division: Introduction to Series and Parallel Circuits

    Resistors, Light Bulbs (Incandescent Lamps) and Bungee Cords have "resistance" in common.  In conventional track lighting, all lamps are in parallel and independent of one another in the sense that one lamp can burn out without affecting the others.  Adding another lamp merely increases the illumination, again, without affecting the others.

    In a string of "holiday" lights, the lamps are all in series - if one lamp is removed, ALL lamps go out.  Oddly, one or more lamps can burn out and the rest stay lit because an internal resistor is in parallel with each lamp filament.  This design allows the user to find the bad lamps before the whole string goes out.  This happens if too many lamps burn out, stressing the hiddent resistors into failure.

    The Bungee Analogy helps to explain what it feels like to be a filament.  Stretch a bungee for as long as possible and you will warm up - that's what happenes when electrical current flows.  A filament is literally white hot when voltage is applied to it.  Imagine the ceiling and the floor as the difference in potential voltage, let's say one-volt per foot, for a distance of ten feet.  Now, imagine an eye hook in both locations to which a single bungee is stretched.  Pretty taut, eh?

    Now, connect two bungees of equal elasticity in series between floor and ceiling.    Where do they meet?  In the middle, of course, dividing 10-volts in half. The bungees are now half as taut as one signle bungee was.   If each was a 100-watt light bulb, they'd now be operating at 50-watts.

    CLICK for Voltage Division Exercise

    Ohm's Law is one example of how math can describe what happens to electrons as they make the journey from power source to components and back  It is ALWAYS a round trip, what is referred to in wiring as Continuity. 

    Ohm's Law defines the relationships between (P) power, (E) voltage, (I) current, and (R) resistance. One ohm is the resistance value through which one volt will maintain a current of one ampere.  Students should know two primary formulae - Ohm's Law and the Power Formula - from which they can derive all the others. 

    This image courtesy of

    This image courtesy of