PS-6.9   Compare the functioning of simple series and parallel electrical circuits.
Key Concepts:
Series circuits: current, resistance, voltage
Parallel circuits: current, resistance, voltage
Batteries: cells  
In a simple series circuit there is only one path for the electrical current to flow. In direct current series circuits every electron that leaves the negative terminal of the power source must travel through every component in the circuit. Any opening in the circuit, such as a burned out light bulb or an open switch will cause the entire circuit to stop working. Watch this video and you will see how this works. In the video three light bulbs are arranged in series with a power supply and a switch. The removal of any of the three light bulbs will cause the others to go out.
In series circuits resistance is cumulative. Since the current must travel through every resistance in the circuit, adding another resistor ( or any other device) will cause the resistance of the circuit to increase. When determining the resistance of a series circuit, just add all the resistances up. Diagram "A" shows a series circuit with 3 resistances. The total resistance in the circuit is 10Ω + 10Ω + 10Ω=30Ω.
Diagram A
In series circuits, like the one shown in diagram "A above, the current will be the same throughout the entire circuit. Again, this is because every electron must move through the entire circuit and therefore must go through every load. The current in the circuit decreases when additional resistors are added. This is why lights will get dimmer when additional light bulbs are added to a series circuit as shown in diagram "B" and "C" below.
              Two lamps are very bright.                                              3 lamps are dimmer!
B.          C.    
What happens when we place chemical cells in series? Suppose we place 3 dry cells in series. If we were to measure the individual voltages of each cell, we might get 1.3V, 1.2V and 1.4V. The individual voltages would depend on if the cells had been used previously. We can place the three cells in series like this:
Diagram D 
When cells are placed in series their voltages are added. The voltmeter would read the sum of the voltages, 3.9V. However, if we place the same three cells in parallel as shown in diagram "E" below the voltage by the voltmeter is the average of the three cells. Even though the cells produce a lower voltage when placed in parallel, there is one real benefit, they last longer.
Diagram E