Return to Physics IndexExploring Series and Parallel Circuits

Pamela Schneider Luther East High School

2750 Glenwood-Lansing Rd.

Lansing, Il. 60438

(708) 895-8441Objectives:

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-meterStrategy:

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. R_{T}= R_{1}+ R_{2}+ R_{3}...

>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. V_{T}= V_{1}+ V_{2}+ V_{3}+ ...

>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/R_{T}= 1/R_{1}+ 1/R_{2}+ 1/R_{3}+ ...

>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.

I_{T}= I_{1}+ I_{2}+ I_{3}+ ...

References:

Hewitt, Paul,Conceptual Physics, Addison-Wesley, Menlo Park, CA, 1987