There are two kinds of circuits. One is a single pathway circuit called a series circuit. The second is called a parallel circuit. The two should be shown in separate lessons and demonstrations of two types can be done by the students during student directed labs.
Lesson X: WHAT IS A SERIES CIRCUIT?
This lesson will take a short time because the concept is an easy one. The next lesson, schematics, actually deals with the same material as it lets the students do the designs and the implementation of the series circuit. Thus, this lesson and a good part of the next should be taught during the same period.
Definition
A SERIES CIRCUIT = a single pathway through which electricity can flow
As we saw in the demonstration of the lemon and the compass as well as in our circuit that tested materials for their ability to conduct electricity, a series circuit is made up of
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1) a power source (a dry cell, battery, or outlet)
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2) a load ( a light, a bell, any kind of a motor, etc.)
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3) and two wires between the source and the load.
A series circuit can have more parts in it but its definitive characteristic is that all its parts are in a single loop, one thing lined up after the other, with the last wire returning to the power source.
Lesson XI: WHAT IS A CIRCUIT DRAWING OR A SCHEMATIC?
Discussion
Physicists and electricians use several symbols to draw circuits. Circuits are drawn before they are actually built in the same way that recipes are made before bakers bake, or plans are made before architects, or designers of buildings, have their buildings built. The symbols are easy to draw. Once rules for putting the symbols together are learned, circuits can be designed easily and quickly, and the actual circuits can be built so that we can get our hoped for and expected results.
DEFINITION: A CIRCUIT DRAWING OR A SCHEMATIC = a drawing or blueprint for an electric circuit
Method: The teacher should be sure to practice the symbols from the Designing Circuits Handout (which is at the end of this paper) before putting them on the board. Simply use the handout to introduce each of the symbols separately. Have the children practice each symbol on the board after they doodle a few times on a blank sheet of paper. The handouts should only be distributed and worked on when the children begin to feel that they have associated the symbols with their terms. These symbols should be the alphabet of circuit drawing. Like the printed and the cursive alphabet, the students have to develop ease in using the “letters” or the writing of “sentences,” that is, the designing of circuits.
(figure available in print form)
As soon as the children have been introduced to the symbols, they should practice drawing at their desks and on the board, the simplest circuit: source—wire—load—wire. The simple exercise in drawing the design will reinforce the concepts and the appropriate vocabulary (circuit, series circuit, source, load).
(figure available in print form)
The next step is that the children should work in groups of two or three to design other possible circuits and to try to predict what will happen. The handout has the suggested components for some basic designs and asks leading questions so that other designs can be designed and then tried.
Further discussion about circuits and circuit drawing: The following questions will suggest possible designs to the children. (The results can be discovered by the students as they implement their circuit designs).
Does it matter where you place the buzzer? the bell? the switch? What happens if you wire two lights, one after the other? What happens if you wire a light and a buzzer? What would happen if one light in the series circuit were to burn out or if one didn’t light because it was not screwed in correctly?
Demonstration of some simple circuits:
Materials
Each group should be given
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three batteries
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three sockets
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three light bulbs
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electric tape
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8 wires with the plastic stripped from the ends
Once each team or small group of students has put some basic designs on paper, it should be given supplies to actually build each circuit. No boards are needed because as soon as a light lights or a buzzer sound a different circuit should be designed and implemented.
Discussion
As the children wire up their circuits they will be surprised by some of the results. When they see that each of two lights in a series circuit burn half as bright as a single light they will be ready to calculate the voltages in the circuit. For example, if you use two 1.5 volt batteries in series, meaning the source provides 3.0 volts, each bulb will use 1.5 volts. Review lessons V and Vll. If one of the bulbs is disconnected or burns out there would be a break in the circuit and it would be an open circuit.
Lesson XII: WHAT ARE PARALLEL CIRCUITS?
As usual, review all the vocabulary cards taught so far in the described way.
None of the children in my class came upon a parallel circuit in their playing with their drawings or with their actual building of circuits. Nor were they able to figure out why all the lights in the classroom don’t go out when one light bulb is burned out; but it did get them into an excited discussion about the need for a different method of wiring. So the next step was the introduce parallel circuits.
The reason that electricity works separately on the loads of a parallel circuit is that in a parallel circuit the voltage across each path is the same. Since the voltage across each path is the same, the current splits at its forking pathways. It splits in proportion to the resistance in each path. On the other hand, in a series circuit, the current through each element is the same, while the voltage splits in proportion to the resistance of each element.
Discussion
As we have discussed and discovered, if lights are lined up one after the other in a circuit in a single path we have a series circuit. A series circuit is useless when we need to light more than one light that has a single source of power and is controlled by a single switch because if one light blows the circuit is no longer closed, and none of the lights will light.
Instead of series circuits, we use parallel circuits when there is more than one load. Strings of Christmas lights or the rows of lights around a make-up mirror are two examples that clearly illustrate the need for circuits that can still work if a single light goes out.
Definition
A PARALLEL CIRCUIT = a circuit in which there is more than one pathway through which electricity can flow
If one part of the circuit doesn’t work, the electricity can still follow a circular path because there are other ways through to different loads and back to the source. In other words, in a parallel circuit there is a source and wires leading away from it that fork to separate loads. It is at the
fork
that the current splits. The current that would pass through one load never reaches the other loads, so a failure of one load doesn’t create a break in the circuit. (In a series circuit, each load in the single pathway is part of the circuit of conductors for each other load).
Analogy to a parallel circuit: A good way to make this concept clear to the students through analogy is to compare a parallel circuit to a day’s chores done by a working mother who has to drop her children off at day-care, drop off her suits at the cleaner, go to work, pick up the children and return home. If there is a detour (a part of her normal route is closed), so that the cleaner’s is unreachable, she can still make it to her other destinations because there are alternative, other possible, roads that can take her there, and allow her to return home too.
Demonstration and Experimentation: At this point a couple parallel circuits should be drawn on the board by the teacher. Then a few children should try to reproduce them once they are erased. As before the children should work in small groups to make circuit drawings and then to implement their designs.
(figure available in print form)
CONCLUSIONS: Electricity is becoming a more important factor in our lives every day as more and better inventions are manufactured. We are presently teaching students who have been growing up with computerized toys, innumerable electric appliances and user friendly computers everywhere in their lives. All of these products use the above seeds of circuitry as their cores. We must be able to understand the fundamental principles of electricity so that we don’t feel like complete incompetents in a rapidly changing world. We must be able to teach our children at least these basics so that they can take over from where we leave off.
