When you hear the acronym AC/DC, what is the first thing that pops into your head? Is it the Australian rock band formed in 1973 by brothers Malcolm and Angus Young? Possibly. For me, however, it has to be electricity, for AC/DC also stands for Alternating Current and Direct Current.
For all intensive purposes, electricity (electric current) can be defined as the flow of charges (typically electrons) from one place to another with a capability of doing work. For an electric circuit to be present, there must be a closed path in which an electric current can travel from an energy source (positive terminal), to an element in the circuit, and back to the energy source (negative terminal). Conventionally, the direction of the current is considered to be the movement of positive charges from + to - when in reality, it is the movement of negatively charged electrons from - to +. The common elements of a circuit are a battery (energy source), wires, and loads (lights, resistors, etc.) that are arranged in a closed loop with no breaks or gaps.
A DC (Direct Current) circuit is one in which the current travels in one direction continuously (a constant flow of electrons). An AC (Alternating Current) circuit is one in which the current is continuously and rapidly changing directions (the electrons are moving back and forth).
A Series Circuit:
A series circuit is one in which the elements in the circuit (lights, in this example) are arranged in line with each other, from end to end. Take, for example, the circuit below:
A Parallel Circuit:
A parallel circuit is one in which the elements are connected in parallel, meaning that each have a direct connection to the energy source (the lead from the energy source is split into many paths that lead to the elements).
Kirchhoff's current law (Conservation of Charge), the total current of the circuit equals the sum of the currents through each branch. The equivalent resistance of the circuit can be found by taking the inverse of the sum of the inverses of each individual resistance (each new resistance added decreases the equivalent resistance).
A Complex Circuit:
A complex circuit is a combination of elements in series and elements in parallel into one circuit.
When working with circuits, do not memorize a certain method that can be used in every situation. Circuits are like puzzles; they take thought to work through every situation.
AC/DC Photo: Photo by Yannick Croissant - http://www.flickr.com/photos/yannick-croissant/3315343302/
Lightning Photo: Photo by Fort Photo - http://www.flickr.com/photos/70619288@N00/3638881022
Electric Circuit Pictures: Screen shots while working in PhET simulation: http://phet.colorado.edu/simulations/sims.php?sim=Circuit_Construction_Kit_DC_Only