Teacher Guide

# Dim the Lights

## Experimenting with circuits and resistance.

This resource was originally published in PhysicsQuest 2016: Currents.

This is the teacher guide for this lesson. A student-focused guide to assist learners as they perform the activity is available.

View the student guide: Dim the Lights

How does the amount of graphite affect how much current flows through it?

• Pencil
• Coin battery
• Red LED
• Dimly lit room
• White paper
• Pencil sharpener
• Ruler

Students discuss the benefits of dimming lights, then experiment by creating a circuit and changing the resistance. After they collect data and analyze it, they have a chance to refine their thinking about how resistance works and what it is.

• Total time
45 - 60 Minutes
• Education level
• Content Area
Resistance
• Educational topic
energy, circuits, resistance

In general when talking about circuits, things are divided into insulators—which won’t allow current to flow—and conductors, which allow current to flow easily. In reality, most things are resistors. Resistors, which are very smartly named, resist the flow of current. They allow some current to flow but not infinite current. Most electronics need to prevent too much current from flowing so they use resistors to regulate it. Most of the resistors in your electronics are made from ceramic. Pencil lead, or graphite, is also a great resistor. For an LED to light up, it must have current flowing through it. The more current there is, the brighter the LED will light. In a circuit, the amount of current flowing depends on a few things. It depends on the voltage of the battery—the higher the voltage, the more current. That’s why the AA battery can’t power something meant for a 9v battery: because the AA battery can’t create enough current. Current also depends on the amount of resistance in the circuit. Things like lights and buzzers have resistance and they slow down the current. The more resistance, the less current. The relationship between current, voltage, and resistance is called Ohm’s Law. You can find more information about Ohm’s Law in the first link under “suggested resources.” Ohm’s law describes a linear relationship. This means that when resistance is increased, the current decreases by the same amount. Your students will see this when they make a graph of the distance from the battery versus the brightness of the LED. In this experiment, students are creating a circuit with a battery, an LED, and a whole lot of graphite. Graphite is not very conductive so you will need to get quite a few layers of it on the paper for the experiment to work. If there isn’t enough graphite, there will be too much resistance and current won’t be able to flow at all. The more graphite there is in a given area, the less resistance there is in the circuit. Current can flow through more easily.

The more graphite there is in a given length, the easier it is for current to flow. At the same time, however, the longer the distance the current has to travel through the graphite, the more resistance it will encounter. In the circuit your students have created, they’ll be looking at what happens to a light as they change the length of graphite that the current must flow through. When the length of graphite in the circuit is longer, there is more resistance, so current doesn’t flow as easily. As a result, the light is dimmer. But what happens if there is more graphite in a given length? What happens if we draw the graphite “wire” a bit thicker on the page? Because there is now more graphite in a given length, current will be able to flow more easily and the light will be brighter. This idea of changing resistance by increasing or decreasing the length or size of a resistor is exactly how dimmer light switches and volume control knobs work. As you slide the dimmer light switch up and down, it’s exactly like sliding the leg of the LED down the length of the graphite and causing it to dim. In a volume control knob, as the volume is turned down the resistance is increased; as it is turned up, the resistance is decreased and the music gets louder.

Key terms: Key Terms

These are the key terms that students should know by the END of the two lessons. They do not need to be front loaded. In fact, studies show that presenting key terms to students before the lesson may not be as effective as having students observe and witness the phenomenon the key terms illustrate beforehand and learn the formalized words afterwards. For this reason, we recommend allowing students to grapple with the experiments without knowing these words and then exposing them to the formalized definitions afterwards in the context of what they learned.

However, if these words are helpful for students on an IEP, ELL students, or anyone else that may need more support, please use at your discretion.

• Circuit: A circuit is a closed loop through which charges move.
• Current: Flow of positive charges. When a complete circuit is created with a battery, current flows.
• Resistance: Resistance slows current down. The more resistance something has, the less current flows through it.
• Rheostat: A rheostat is a circuit element that can change its resistance. Things like volume control knobs and light dimmer switches are examples of rheostats.
Objective

Students will experiment to understand how dimming lights work.

Before the Experiment

Can you think of places you might use changing resistance such as dimmer light switches?

• Turn & Talk protocol
1. Pair students up
2. Give them a minute to think quietly
3. Give students 2 minutes to discuss their thinking
4. Have students record their answers or share out to the whole group
Setting Up
• Dim the lights

This should be done in a dimly lit room.

• Sharpen both ends of your pencil

Sharpen both ends of your pencil.

• Make a circuit

Make a circuit with the wire, the pencil, the battery, and the LED.

1. Does the pencil conduct?
2. How do you know?
• Draw two dark rectangles

On the white sheet of paper, use the pencil to draw two very dark rectangles using the pencil. One rectangle should be 1” x 4” and the other should be 0.5” x 4.”

• Place the battery with the “+” side down on one end of the larger rectangle

Place the battery with the “+” side down on one end of the larger rectangle.

• Put the shorter leg of the LED on the side of the battery that is facing up

Put the shorter leg of the LED on the side of the battery that is facing up and tape it in place.

• Put the longer leg of the LED about half an inch down on the rectangle

Put the longer leg of the LED about half an inch down on the rectangle. Does it light up? If not, get out your pencil and make the rectangle even darker. It might takes several layers of pencil lead to make this work.

• Put the longer leg of the LED about half an inch down on the rectangle

Put the longer leg of the LED about half an inch down on the rectangle. Does it light up? If not, get out your pencil and make the rectangle even darker. It might takes several layers of pencil lead to make this work.

During the Experiment
Collecting Data
• Make sure students are put into intentional groups

Make sure students are put into intentional groups. See above.

• Students will complete the experiment using the Student Guide

Students will complete the experiment using the Student Guide where we have outlined the experiment for students and along the way, they record results and answer questions.

Analyzing Data
• In the student guide, they will answer questions that help them understand circuits

In the student guide, they will answer questions that help them understand circuits.

• Continue to listen in on each group’s discussion

Continue to listen in on each group’s discussion, answer as few questions as possible. Even if a group is off a little, they will have a chance to work out these stuck points later.

Teacher Tips
• Suggested STEP UP Everyday Actions to incorporate into activity

Suggested STEP UP Everyday Actions to incorporate into activity

• When pairing students, try to have male/female partners and invite female students to share their ideas first
• As you put students into groups, consider having female or minority students take the leadership role.
• Take note of female participation. If they seem to be taking direction and following along, elevate their voice by asking them a question about their experiment.
• Consider using white boards

Consider using white boards so students have time to work through their ideas and brainstorms before saying them out loud.

• Roam around the room to listen in on discussion and notice experiment techniques

As students experiment, roam around the room to listen in on discussion and notice experiment techniques. If needed, stop the class and call over to a certain group that has hit on an important concept.

• RIP protocol

Consider using the RIP protocol (Research, Instruct, Plan) for lab group visits and conferring.

• Consider culturally responsive tools and strategies

Consider culturally responsive tools and strategies and/or open ended reflection questions to help push student thinking, evidence tracking, and connections to their lives. Look for *** below to find suggested places to add.

Conclusion
• Claim-pass protocol to have students share and refine their thinking

Claim-pass protocol to have students share and refine their thinking.

• One person in each group writes a claim (can be provided by the teacher or not) at the top of a paper about the following: From looking at your graphs and your answers to the previous questions, what can you say about how resistance changes as you have more pencil graphite in the circuit made by the battery, LED, and graphite?
• the pencil in response to a recent observation of a phenomenon.
• The student who wrote passes the paper and the pencil to the left.
• The person with the paper writes one piece of evidence that supports the claim under the claim
• The paper and pencil are passed to the left.
• Clarify and give concise definitions

After students have had a chance to discuss key ideas from the lesson and complete their student guides, you can now clarify and give concise definitions to the forces they experimented with.

• Real world connections -
• In this lab we talked about how resistance causes light to dim. What are other household objects that you control that might exhibit similar behavior due to the relationship between resistance and voltage?
• Ex: changing speed of ceiling fan, powering the amount of heat from an electric heater,
• Have students build the following circuits and determine which is brighter and explain why: A light in a series circuit with a 5 ohm resistor or a light in a series circuit with a 15 ohm resistor?
• Suggestions for drawing, illustrating, presenting content in creative ways
• Play this Quizziz game with class about Ohm's Law, resistance, conductors and insulators.
• After playing the game students can write short paragraphs about something they learned through the game that they might not have learned from the activity.
• Engineering and design challenges connected to the content
• Have students construct new circuits using different objects of their choosing. Have students classify which object is a conductor and an insulator.
• Example
• Example objects: coin, paper, salt water, play dough etc.

### Voltage, Current, Resistance, and Ohm's Law

When beginning to explore the world of electricity and electronics, it is vital to start by understanding the basics of voltage, current, and resistance. These are the three basic building blocks required to manipulate and utilize electricity. At first, these concepts can be difficult to understand because we cannot "see" them. One cannot see with the naked eye the energy flowing through a wire or the voltage of a battery sitting on a table. Even the lightning in the sky, while visible, is not truly the energy exchange happening from the clouds to the earth, but a reaction in the air to the energy passing through it. In order to detect this energy transfer, we must use measurement tools such as multimeters, spectrum analyzers, and oscilloscopes to visualize what is happening with the charge in a system. Fear not, however, this tutorial will give you the basic understanding of voltage, current, and resistance and how the three relate to each other.

• MS-PS4-3
Integrate qualitative scientific and technical information to support the claim that digitized signals are a more reliable way to encode and transmit information than analog signals.
• MS-PS4-1
Use mathematical representations to describe a simple model for waves that includes how the amplitude of a wave is related to the energy in a wave.

### Credits

Created by Rebecca Thompson, Ph.D., David Ellis, Amanda J Ellis

Activity layout by Donna Giachetti

Images by James Roche

Updated in 2023 by Sierra Crandell, M.Ed. partially funded by Eucalyptus Foundation

Extension by Jenna Tempkin with Society of Physics Students (SPS)