Ohm's Law

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Students will be able to define and apply Ohm's Law.

Big Idea

Today, students read all about Ohm's Law before applying it in an online simulation.


Now that students are able to define current from the previous lesson, we are ready to define Ohm's Law and apply it to simple circuits (HS-PS3-5). The lesson starts with a fun and simple video that makes students laugh. Then, students review the components of a circuit and are introduced to Ohm's Law in a paired reading activity (SP5 & SP8). Finally, students get to apply their new circuitry knowledge on an Ohm's Law simulation (SP2 & SP4).

Introduction with a Circuitry Video

5 minutes

Because there are so many resources on the internet, I decide to use this video to introduce Ohm's Law to my students. We just finished a lesson on current and drift speed, so today I want to capture students' attention with a simple video that appeals to their inner child. It is the goal of this video to show students that the flow of electrons create current and how current, voltage, and resistance combine to form a circuit.

Before I start the video I make sure my expectations are clear: students need to be sitting quietly, listening and watching the video, and taking notes on meaningful material. To me, meaningful material includes any reference to previously learned concepts, vocabulary, and the results of the different demos. While I do not collect their notes, these are an essential resource as students work through the paired reading activity and the simulation. I am telling my students these expectations as I'm on my way to start the video.

Ohm's Law Paired Reading Activity

30 minutes

To define Ohm's Law and relate it to power, students engage in a paired reading activity. I introduce this activity by passing out the paired reading document to each student and explaining that I have already chosen their partners. Partners work best for this activity, and I have a list of who will be working together based on their current grade in the class. I don't tell the students how I've paired them, but I ranked the class by overall grade, split the list in half, and then matched the first names on each list. To save class time, I print this list and organize the pairs prior to class starting. Because there are an odd number of students, I make an exception and have one group of three. Pairing students forces them to work with someone different and ensures that ability levels are somewhat equal.

Students understand they don't need to scramble for a partner and have the document in front of them. I next share how this paired reading activity works. I expect that the students read one page at a time individually while annotating the text with information they deem important. Students should stop reading when they get to the end of the page. Once both partners have come to the end of page 1, they exchange their annotations and copy down onto their document any ideas that they don't already have. For example, if one student thinks the diagram of a battery is important and her partner does not identify that, her partner needs to write the diagram down on her own paper. The students then repeat this process until they have completed all seven pages of the document. They have approximately 30 minutes to work through this packet, so they should be reading and discussing each page about every 4 minutes.

I share with students that this activity has three purposes. The first is to practice reading detailed information in a short amount of time. The AP Physics 1 exam has been redesigned to include more reading, so I want students to be able to practice reading and pulling out important information under a time constraint. This parallels the second goal, which is to build students' stamina for reading physics material. At first glance the reading might seem a bit long, but it is intentionally selected to help students students prepare for the 3 hours they will spend taking the AP exam. The final goal is that students must grasp an understanding of the concepts, vocabulary, and equations used to describe Ohm's Law.

After I'm done giving instructions, I reveal the pairs by simply reading them from the organized list. I have students move so they are sitting with each other, but since they are AP students I let them organize themselves and choose their own seat locations. Once everyone is settled I write the end time of the activity on the front board and begin to circulate the room. My circulation lets me know if students are on task and allows me to redirect students if I hear misconceptions or off-task conversations. 

The students often enjoy learning about Ohm's Law and dive right into the reading. They use electricity on a daily basis, so the topics of current, voltage, and resistance are exciting, yet familiar. The atmosphere of the room oscillates from nearly silent (when students are individually reading) to animated (when students are discussing their annotations). It is during this last animated discussion time that I instruct students to move into the next part of the lesson.

Ohm's Law Simulation

15 minutes

Now that students understand Ohm's Law, students use a computer simulation that allows them to be more active in their learning. The simulation allows students to adjust the resistivity in a wire and observe how it affects the resistance of that wire. Students are also able to analyze how current changes (according to Ohm's Law) when the voltage and resistance of a simple circuit is manipulated. 

Before we start the activity, I assign partners using the random student generator that already has my students' names loaded. By displaying the random generator on the front board, there is an element of suspense as the partners are assigned. Partners work best for this activity so that each student can be actively engaged throughout the work time.

We use MacBook Pro's in my district, so my students are familiar with how the computer and cart organization works. After each pair has been assigned, the students move to sit near each other, push their desks together, and grab a computer. As the computers are booting I pass out the resistance & Ohm's Law instructions so that each student receives a copy. This document directs them to PhET where they will be using the resistance in a wire & Ohm's Law simulations. My activity sheet is also meant to direct the students in their learning so that they are confident in what material needs to be understood.

While students are working, I walk around to ensure they are actively engaged in the learning process. This means that they are on the proper website, reading or discussing some component of the simulation, and writing down appropriate information from the simulation. When I walk around, I'm spot checking their written work and engaging students in questions such as "What observations have you made about the resistivity in wire?" or "What is the maximum current you can get on the Ohm's Law sim?" 

When there is approximately 5 minutes prior to the end of class, I ask students to put the computers back on the cart and return to their seats. I also tell them at this point that the lab is due at the start of the next class meeting. When the bell rings, students should then be well organized and ready to leave class.