Circuit Power

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Students perform power calculations on simple circuits and relate this concept to electrical devices they use at home and at school.

Big Idea

Power is the rate at which energy is transferred and it is measured in a circuit with the equation P=VI.

Context and Supplies

Building off Ohm's Law from lesson Ohm on the Range, students expand their knowledge of circuits as they learn about the rate at which energy is used in a circuit, a.k.a. power. Students learn how the concept of power applies to circuits and that there is a power equation, power = voltage x current, that applies to circuits. Students practice this concept by solving a series of power work problems.

CCSS Math Practice 2Reason abstractly and quantitatively and NGSS Science Practice 5: Using mathematics and computational thinking are applied in this lesson. 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. 

Circuit Quiz Review

5 minutes

In the last lesson, Household Circuits, students get a quiz on circuits. At this time, I hand back the quiz to the students and review a few of the problems where most of the class has struggles as identified in the Reflection. Specifically question 3 showed three valid forms of Ohm's Law (V=IR, I=V/R and R=V/I). As seen on Circuit Summary - Data, 40% of the class did not recognize that this was the same equation written in three different forms. This is worth reviewing; later in this lesson student use these to derive different forms of the power equation, P=VI. I put V=IR on the board and call on randoms students to solve for I and then solve for R. 

Introducing Power in Circuits

20 minutes

After a short review of some quiz questions, I bring up the Circuits and Power Power Point. Students get out their notebooks and take notes on the concept of power and how it applies to an electric circuit. 

A short Think-Pair-Share is done after the definition of power (the rate at which energy is transferred / transformed) is given and students are given a few examples on the power point of electrical devices that transform electrical energy into some other form of energy, such as a blender (electrical energy to mechanical energy) or a hair drier (electrical to thermal energy). 

Students think of a single device that converts electrical energy into all three forms of energy

  • Mechanical
  • Thermal
  • Light

They have 1 minute to think of such a device and write it in their notebook. Then, they pair with someone and share their idea. Then as a class, I have students share out some of their devices and what part of their device has the energy transfer. Answers include a computer (fan, display, processor that heats up), a smart phone (display, vibration indicator and processor that heats up), and a coffee maker (pump to move water, display or LED and heating coil to heat water). There are easily hundreds of examples.

To finish the lecture time, students learn about the power equation (P=VI) and how it relates to the equation they already know for power (P=W/t). We do a sample word problem with power. I display the problem and give students a minute to work through the problem before I display the solution (download the power point to see the animations that do not work in preview mode).  

I display the power equation along with Ohm's Law and ask students to substitute the V in the power equation in their notebooks. After 30 seconds, I display the result (P=RI^2). I have them do the same but get rid of the I in the power equation (P=V^2/R). We conclude with a brief summary of power concepts that apply to commercial electronic devises:

  • All electrical devices have a power rating.
  • All electrical devices transfer or convert electrical energy into some other form of energy, this can be measured via the power rating.
Then I give a glimpse of tomorrow's topic: for students to calculate the cost of powering our appliances and electronic devices.

Students Work on Power Problems

25 minutes

Next, I give students the Power Questions worksheet. They spend the rest of the period on these word problems. While students work individually on these word problems, I do small group instruction based on the Circuit Summary - Data. After analyzing the quiz data , I recognize that there are a handful of students who need additional support with definitions and with the application of Ohm's Law.

While the whole class is working on the power problems, I will do small group instruction. This is a strategy where I help students who struggle with a specific concept of idea by gathering them together in a small group setting and giving them direct instruction. The struggling students must be identified using data which is used the Circuit Summary - Data. I have identified 11 students who need additional practice on definitions and another 6 students who need support with using Ohm's Law. In groups of 4-5 I call them up and give some direct instruction focused on their area of need. I then give them the Ohms Law - Small Group Instruction worksheet and let them know that if they complete this sheet for homework and give it to me tomorrow, they'll receive bonus points on the quiz. 

Before the bell rings, I instruct students to finish the Power Questions for homework if they did not manage to finish it during class. I also give the following assignment:

Homework – Circuits and Power

Pick an electronic device in your home that you use and for which you can find the power rating (many devices have the power given on them or on their charger and will be in units of watts, often abbreviated W).  Make sure that it is an appliance that you plug in so that we know it requires 120 V.

Device name:_________________

Power:______________ watts

Estimated monthly usage:________________ hours

This assignment is in preparation for tomorrows topic, Electric Energy Calculation, which is to calculate the cost of running their chosen device.