Parallel and Series Circuits

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Students determine the rules that govern voltages and currents of series and parallel circuits.

Big Idea

In a series circuit, the current remains constant and voltage-drops add together and in a parallel circuit the currents add together and voltage-drops are constant.

Context and Supplies

This is a computer simulation laboratory where students determine how voltage and current change based on the type of circuit, series or parallel. This is a guided inquiry activity; the values of the electrical components students are to test are given, so their goal is to collect data and analyze them. This lesson builds off the previous lesson where students learned the definitions for voltage, current and resistance.

An engineering NGSS is applied here, HS-ETS1-4: Use a computer simulation to model the impact of proposed solutions to a complex real-world problem with numerous criteria. Science Practice 4: Analyzing and interpreting data, Science Practice 5: Using mathematics and computational thinking and Science Practice 7: Engaging in argument from evidence. Since voltage is an electric potential due to electric fields, the performance standards HS-PS3-5: Develop and use a model of two objects interacting through electric or magnetic fields to illustrate the forces between objects and the changes in energy of the objects due to the interaction. Also, students build virtual circuits which applies HS-PS3-3: Design, build, and refine a device that works within given constraints to convert one form of energy into another form of energy.

CCSS Math Practice 2: Reason abstractly and quantitatively and Math Practice 3: Construct viable arguments and critique the reasoning of others as they develop the "rules" of how the voltage and current behave in the different types of circuits.  

Students need access to computers and the internet to perform this activity. I provide classroom laptops and some students have their own laptops which they can use. They use a simulator called PhET: Circuit Construction Kit (DC Only) which is provided by the University of Colorado.


Two Types of Circuits with PhET Simulator

40 minutes

I tell students that today they are to determine the rules that govern the different voltage values and currents in two different types of circuits: parallel and series. They must construct the circuits, collect the data and analyze them to come up with these rules. I project a sample simple circuit from the PhET simulation Circuit Construction Kit (DC Only) on the board as a visual for the students. (In order to do this, within the PhET simulation, click the Load button on the top right of the screen and select one of the files, series and parallel, to load. Download the files which are in the resources of this section.)

I hand out Two Types of Circuits worksheet. Every student receives one and I expect each student to fill out his or her own copy. However, students work cooperatively in groups of 2-3, which gives them a chance to talk through the assignment and to help each other. I don't like to have groups bigger than 3 as the smaller group makes it more likely that all students make significant contributions to the assignment. While students work through this activity, I walk around the room and monitor their progress. I also employ the Colored Cups which informs me of when students are in need of support. 

Students begin the handout with a picture of a series circuit and a parallel circuit. They identify which is which and provide justification for their choices. This is a good starting activity because students develop their own understanding of how parallel and series circuits appear.

Then they construct a series circuit in the simulator and measure voltage and current values throughout the circuit. They place their data (colored red) in a table. I use this table format for all series and parallel circuits where the voltage, current and resistance are the column headings and the rows are the individual resistors in the circuit with the total value of each column at the bottom. The benefit of this format is that Ohm's Law (V=IR) works for each row of the table. This is important and I stress that to the the groups several times throughout the activity.

As students fill in the blank values on the series table, they see the pattern that the voltage changes across each of the resistors and that the voltage across each resistor adds up to the voltage of the circuit. They also see that the resistors add to give a total resistance and the current remains constant through a series circuit. 

The worksheet has them repeat this process for a parallel circuit. As students fill in the blank values on the parallel table, they see the pattern that the voltage remains constant across all resistors and that it is the current through each resistor that adds to the total current coming out of the battery. The total resistance in the parallel circuit is found with Ohm's Law (R = V/I). They see that the total resistance of the circuit is smaller than any of the individual resistors. 

The worksheet has students summarize the patterns or rules for each type of circuit and then finishes with two more circuits where students have to fill in the blanks for some missing values.

Types of Circuits Debrief

10 minutes

With 10 minutes left in the the lesson, students should be on the end of the activity sheet where they fill in the missing values for the two types of circuits. I collect the sheets from the students for later grading and we review the solutions. I display Two Types of Circuits - Front on my document camera and call on random students to identify the two types of circuits. I ask them to explain why they believe they have the name parallel and series. 

Then we review the data and conclude that in a series circuit, the voltage drops across individual resistors must equal the total voltage provided by the power supply and that the current remains constant. I point out that the table that is constructed has the variables V, I and R and that Ohm's Law always applies for each row.

The we review Two Types of Circuits - Back and conclude that in a parallel circuit, the voltage remains constant across all the resistors and that the individual currents through each resister must equal the total current coming out of the power supply. Again, we confirm that Ohm's Law applies across every row. In the next lesson, students practice these rules in what I call the Circuit Sudoku. The Ohm's Law across every row is an essential part of success on that assignment.