How is it possible to lower a switch and dim your lights without turning them off? The reason is resistance. This activity will look at how resistance can be changed. If you want to make a volume control, you can try this experiment with a buzzer instead of an LED!
Created by Daniel Falk from the Noun Project
Student Guide
Dim the Lights
Experimenting with circuits and resistance.
This resource was originally published in PhysicsQuest 2016: Currents.
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
Intro
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 with the wire, the pencil, the battery, and the LED
Make a circuit with the wire, the pencil, the battery, and the LED.
- Does the pencil conduct?
- How do you know?
- Use the pencil to draw two very 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
- Turn off the room’s lights
Turn off the room’s lights.
- Start with the larger rectangle
Start with the larger rectangle. Light up the LED as you did in the previous section.
- Slide the shorter leg along the rectangle
Slide the shorter leg along the rectangle.
- What happens to the LED?
- Do the same for the smaller rectangle
Do the same for the smaller rectangle.
- What happens?
- Again, make the LED light up on the big square
Again, make the LED light up on the big square. The shorter leg should start 0.5” from the battery.
- How bright is the LED?
On a scale from 1-10, with 1 being the dimmest and 10 being the brightest, how bright is the LED? Record it in the chart.
- Move the leg 0.5” down the square, so the leg is now 1” away from the battery
Move the leg 0.5” down the square, so the leg is now 1” away from the battery. On a scale from 1-10, how bright is the LED now? Record it in the chart.
- Continue steps 1-3 until the entire chart is filled
Continue steps 1-3 until the entire chart is filled.
- Repeat for the smaller rectangle
Repeat for the smaller rectangle.
Analyzing Data
- What happened when the leg of the LED got farther away from the battery?
What happened to the brightness of the LED when the leg of the LED got farther away from the battery?
- Graph the brightness of the distance from the battery versus the brightness of the LED
In the space below, graph the brightness of the distance from the battery versus the brightness of the LED for both the large rectangle and the small rectangle on the same graph
- What does the graph look like?
What does the graph look like?
- How does the graph of the small rectangle compare to the graph of the large rectangle?
How does the graph of the small rectangle compare to the graph of the large rectangle?
- Do they change at the same rate?
- Do they end at the same brightness level?
Conclusion
- What can you say about how resistance changes?
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?