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Energy Transformations in a Dark Detector
A Carolina Essentials™ Activity
Grade & Discipline
9-12
Physical Science-Physics
Recommended for grades 9-12.
Time Requirements
Prep15 min
Activity30-45 min
Teacher Prep time: 15 min
Student Activity: 30-45 min
Safety Requirements
No PPE is required for the activity.
ViewMaterial List
Building a Dark Detector Kit
Piece of heavy construction paper
Alternative:
Mini breadboard
100 kΩ resistor
330 kΩ resistor
Light-dependent resistor (LDR)
2N3904 NPN transistor
Light-emitting diode (LED)
Piece of heavy construction paper
Phenomenon
Ask students to explain how street lights “know” when to come on and turn off? Have they seen street lights come on during the day? During storms? Going off during bright lightning storms?
Essential Question
What is required to design and build a device that can convert one form of energy to another?
Activity Objectives
- Design and build a simple circuit that converts light (radiant) energy to electrical energy.
- Explain the energy transformations occurring in the dark detector.
Next Generation Science Standards* (NGSS)
HS-PS3-3. Design, build, and refine a device that works within given constraints to convert one form of energy into another form of energy.
Science and Engineering Practices
Constructing Explanations and Designing Solutions
- Design, evaluate, and/or refine a solution to a complex real-world problem based on scientific knowledge, student-generated sources of evidence, prioritized criteria, and tradeoff considerations.
Disciplinary Core Ideas
PS3.A: Definitions of Energy
- At the macroscopic scale, energy manifests itself in multiple ways, such as in motion, sound, light, and in thermal energy.
Crosscutting Concepts
Energy and Matter
- Changes of energy and matter in a system can be described in terms of energy and matter flows into, out of and within that system.
Safety Procedures and Precautions
Work on a clean, dry surface. Check batteries for signs of corrosion. If corrosion is apparent, dispose or recycle the batteries properly.
Teacher Preparation and Disposal
If kits are not purchased, sort the necessary components for the dark detector into group baskets.
All materials can be reused.
- Identify all the parts in the kit package.
- If you are using individual components and not the kit, sort the components into a basket for students to ensure they have all the correct parts.
- Build the circuit using the schematic and wire diagram shown below:
- Emphasize placing all components into the bread board securely and in the proper location.
- Check the circuit by placing your hand over the light dependent resistor. If the LED did not come on, check that each component is securely in the bread board and in the current location.
- Place the flashlight on the construction paper and draw a circle around the light end. Cut out the circle ¼ in larger than you drew.
- You can pre-cut flashlight covers prior to the activity to save time.
- Turn on the flashlight. Dim or turn off the lights in the room.
- Working in a dimly lit to dark room enhances the effect of the LED intensity and ensures accurate observations.
- Shine the flashlight without the paper circle cover and record the intensity of light from the LED bulb.
- Use the paper cover to block ¼ of the flashlight and repeat the observation.
- Use the paper cover to block ½ of the flashlight and repeat the observation.
- Students observing the experiment should record the data in their notebooks.
You may choose to assign additional students to record data on a whiteboard or laptop, or to use a probe or video recorder.
- Use the paper cover to block ¾ of the flashlight and repeat the observation.
- Use the paper cover to block all the flashlight and repeat the observation.
Data and Observations
Student answers will vary depending on the ambient light in the room. The LED will be on when no light is present. It may be on if there is very dim, ambient light in the room or when 75% of the flashlight is covered. Maximum flashlight light will not result in the LED turning on.
Analysis & Discussion
1. Using the circuit diagram, label each point at which an energy transformation takes place and identify the specific transformation. Begin with the batteries.
Refer to the web resource “Demonstrating a Dark Detector.”
2. Using your observations, explain why light levels affected when the circuit LED came on. How can these results be used to explain street light behavior?
The LED activates when current flows through the transistor’s collector to the emitter. This happens when sufficient current flows through the transistor’s base to the emitter. Current flows into the base when the resistance of the light dependent resistor, the LDR, increases, which happens when the amount of light shining on the LDR decreases.
Street lights must work the same way. For example, on a stormy night, lightning produces enough light to increase resistance, turning off the street lights. Also, as the amount of day light changes throughout the year, lights come on at different times but always when the light intensity reaches a minimum threshold.
*Next Generation Science Standards® is a registered trademark of Achieve. Neither Achieve nor the lead states and partners that developed the Next Generation Science Standards were involved in the production of, and do not endorse, these products.
