The Circuit

• An electric circuit is electrical elements connected together. The three elements of a flashlight simple circuit are the battery (or power source), the lamp (or resistor) and the wires that connect it all together. Other types of sources and resistors can be used in place of the battery and lamp.
  
  

Elements

• There are two types of elements: passive and active. The resistor is the passive element in a simple circuit; the battery is the active circuit. Passive elements are ones that cannot generate energy for the circuits while active elements can. An inductor is another type of passive element, while a generator is another type of active.
  
  

Sources

• The circuit receives power from either a voltage or current source. A source is always an active element; these sources can either be independent or dependent. An independent source will never change its energy output no matter what resistor is being used, while a dependent source's output will change depending on the resistor.
  
  

Resistance

• A resistor is anything that resists the flow of current (even the wires are a type of resistor because flowing through the wires slows the current slightly). Resistors can be fixed or variable. Most are fixed, where resistance is constant. Variable resistors have an adjustable resistance.
  
  

Current and Electrons

• When wires are attached to the battery, electrons move (positive electrons in one direction and negative electrons in the other) and a current is created. The battery provides the electromotive force, or voltage. Current that remains the same all the time is direct current, while current that changes is called alternating current (alternating current is used in appliances). Most simple circuits are direct current (DC) circuits.