Exploring Series and Parallel Circuits

Pamela Schneider               Luther East High School
                               2750 Glenwood-Lansing Rd.
                               Lansing, Il. 60438
                               (708) 895-8441

Objectives:

1.  To arrange batteries, bulbs and wires into functioning series and parallel 
    circuits.   
2.  To represent simple circuits using schematic diagrams.
3.  To explain and compare the effects of series and parallel circuits on bulb 
    brightness, relating the phenomena to the potential differences, current,
    and resistances throughout the respective circuits. 

Materials needed:

for each group of 2-4 students:       for demonstration purposes:
   2 size-D dry cells (batteries)        one large scale series circuit
   6 pieces of bare copper wire          one large scale parallel circuit
   3 flashlight bulbs                    light bulbs of various wattages
   3 bulb holders                        (optional:  logic circuit)
   1 multi-meter

Strategy:

1.  Try to arrange one bulb (without holder), one battery and wire in as many 
    ways as possible to make the bulb emit light.  Sketch each arrangement, 
    including failures as well as successes.  Similarities among the successful 
    trials should be discussed. 
 
    >In order for the light bulbs to light, there must be direct connections 
     from one battery terminal to the metal side of the bulb and from the metal 
     bottom of the bulb to the other terminal.  Review the concepts of potential 
     difference, current and resistance.  Introduce a "circuit" as a complete 
     path along which a charge can flow from the negative terminal of a power 
     source to the positive terminal of the source.  Electrons flow continuously 
     in a closed circuit. 
   
2.  Repeat step 1 with the bulb placed inside a holder.  Have the students note 
    which two parts of the bulb the holder makes contact with. 

    >Contact is made with the metal side and the metal bottom of the bulb.

3.  Using one battery, light as many bulbs in as many holders as possible.  
    Sketch each arrangement, noting the ones that work.  Compare results among 
    the different student groups.  

    >Ask the students which arrangements made the most bulbs glow.  When more 
     than one bulb is introduced into a circuit, the possible arrangements 
     include both series and parallel circuits as well as various combinations 
     of the two.  The parallel arrangements should make the most bulbs glow. 
     Introduce schematic diagramming (using symbols to represent electric 
     circuits) for wires, batteries and resistances.

4.   [Series] Wire two circuits in series.  One should have one bulb, while the 
     other should have two bulbs in series.  Do the bulbs light in each of these 
     series circuits?  Compare brightness. 

    >The circuit with two bulbs should be less bright.

5.  In the circuit with two bulbs, unscrew one of the bulbs.  Note what happens 
    to the other bulb. 

    >The other bulb goes out.

6.  [Parallel] Set up a parallel circuit with two bulbs.  Do both bulbs light in 
    this parallel circuit? 

    >Both bulbs should light.

7.  Unscrew one of the bulbs in the parallel circuit.  Note what happens to the 
    other bulb. 

    >The other bulb should remain lit.  Have the students describe in their own 
     words the differences between series and parallel circuits.  Guide them in 
     making a descriptive list of the two types of circuits on the chalkboard. 
     Use larger scaled series and parallel circuits with larger light bulbs as 
     part of a demonstration to help develop the concepts.  Have multi-meters 
     available to test the current, potential difference and resistance at 
     various points along each circuit. 

     Series Circuits
      >A single path is allowed for electron flow.
      >A break anywhere along the path stops the electron flow in the entire
       circuit.  (Devices in series act dependently.)
      >The total resistance in a circuit is equal to the sum of the individual
       resistances along the current path. RT = R1 + R2 + R3 ...
      >The current anywhere along the circuit is equal to the voltage supplied 
       by the source divided by the total resistance of the circuit. (Ohm's Law)
      >The potential difference, or voltage, is decreased over each resistance.
       The sum of the "voltage drops" should be equal to the amount of voltage 
       supplied.  VT = V1 + V2 + V3 + ...
      >The voltage drop across each device is proportional to its resistance.

     Parallel Circuits
      >Branches are formed providing separate paths for the flow of electrons.
      >Since current branches into separate pathways, a break in one or more of 
       those pathways does not interrupt the flow in the other paths.  (Devices
       act independently.)
      >The total equivalent resistance is less than the value of any individual
       resistor.  1/RT = 1/R1 + 1/R2 + 1/R3 + ...
      >Each device connects the same two points of the circuit; therefore, the
       voltage is the same across each device.
      >The amount of current in each branch is inversely proportional to the 
       resistance of the branch.
      >The total current is equal to the sum of the currents in each branch.
       IT = I1 + I2 + I3 + ...
      
References:
Hewitt, Paul, Conceptual Physics, Addison-Wesley, Menlo Park, CA, 1987