Electricity

# Law-abiding systems

Does a hydroelectric dam satisfy Kirchhoff’s laws? NEW 11 Duration
40 mins Type
Group work

## Overview

Provided with a list of different systems, students are asked to identify which systems would/would not satisfy Kirchhoff’s laws. A set of required for systems that satisfy Kirchhoff’s laws and an interpretation of hydroelectric dams as “circuits” are offered to students before they are invited to create illustrations that show how a hydroelectric dam would satisfy Kirchhoff’s laws.

### Instructions

#### What you'll need

• "Kirchhoff’s law-abiding systems" handout, one for each group
• "Illustrating Kirchhoff’s laws" worksheet, one for each student
• "Teacher reference" handout

1. Begin the activity by organizing your students into small groups and providing each group with "Kirchhoff’s law-abiding systems" handout. Each student should also have a copy of the "Illustrating Kirchhoff’s laws" worksheet.
2. Introduce the students to the requirements for a system that satisfies both of Kirchhoff’s laws. Ensure students are clear on the implications of these requirements—for example, that a “closed-loop path” is one without a distinct beginning and end.
3. Ask groups to decide which of the examples would satisfy the requirements of Kirchhoff’s laws.
4. Encourage groups to share their decisions and thinking with the class.
5. Invite groups to decide if a hydroelectric dam would satisfy the requirements of Kirchhoff’s laws. Encourage groups to share their decisions and thinking with the class.
6. Briefly explain how thinking about the similarities between an electrical circuit and a hydroelectric dam can help clarify why a dam can meet the requirements of Kirchhoff’s laws.
7. Ask students to use this information to create a draft diagram of how a hydroelectric dam would satisfy the requirements of Kirchhoff’s laws.
8. Provide each group with a copy of the handout and ask each group to identify any parallels that might be drawn between these two types of systems.
9. Prompt students to use information from the handout to refine their draft diagrams to accurately show how a hydroelectric dam would satisfy the requirements of Kirchhoff’s laws ("Teacher reference" handout offers an example of how this might be visually represented).
10.  Conclude the activity by inviting students to reflect on how the interpretation of a hydroelectric dam as a type of circuit that satisfies Kirchhoff’s laws might have influenced or challenged their conception of circuits in general.

### Modify or extend this activity

• Students could be invited to identify other situations where Kirchhoff’s laws could be observed.
• Prior to creating the illustration of how a hydroelectric dam satisfies the requirements of Kirchhoff’s laws, students could be invited to create simple illustrations of circuits that meet the requirements of Kirchhoff’s laws.

### Curriculum Fit

#### Big idea

• Energy is found in different forms, is conserved, and has the ability to do work

#### Content

• Electric circuits (DC), Ohm’s law, and Kirchhoff’s laws

#### Curriculum competencies

##### Processing and analyzing data and information
• Use knowledge of scientific concepts to draw conclusions that are consistent with evidence
##### Evaluating
• Evaluate the validity and limitations of a model or analogy in relation to the phenomenon modelled
##### Communicating
• Formulate physical or mental theoretical models to describe a phenomenon

### Teaching Notes

• Kirchhoff’s first law (current law) would say that the amount of water flowing past each point in the dam is the same. This is actually true, assuming a simplified, ideal view of a dam where the all the water that enters the dam does not exit until the end point, and water cannot enter from anywhere other than the entry point.
• Kirchhoff’s second law (voltage law) would say that the energy “delivered” by the water to the load (the turbine) is equal to the energy difference between water at the entry point of the dam (the elevated area) and at the exit point (the river downstream). This is also true, assuming a simplified, ideal view of a dam where the water doesn’t regain energy from anything other than the water cycle (the battery) which returns the water to a more elevated area. Once the water transfers its energy to the turbine (the load), that energy is gone and the water must be ‘recharged’ before travelling through the dam again.

### Assessment

Throughout the activity, consider how well students:

• Demonstrate an understanding of the function and purpose of electric circuits as well as the characteristics described by Kirchhoff’s laws.
• Apply their understanding of the relevant terminology (current, load, battery, charged particle, potential difference) to electric circuits and other analogous systems.
• Consider the criteria provided for a system that satisfies both of Kirchhoff’s laws.
• Can make connections across multiple examples that help them generalize the characteristics that define a “circuit.”