Dim the Lights

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

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.

• 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.

• 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.

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