Demonstration of the Seasons with Photocells and Propellers
Topic/Concept: Seasons
Type of Activity: Hands on demonstration
Prerequisite knowledge required: None
Resources required: Globe, light, photocells, propellers
Learning Objectives: Reinforce the reason for seasons in students
Common misconceptions: Seasons are caused by the Earth being farther away from the Sun during the winter and closer during the summer.
Detailed description of activity:
The best demo I have seen for explaining the seasons is one I got from AAS Education Specialist Gina Brissenden. Unfortunately, we don’t have all the materials in the department right now, but it would be pretty easy to make. It consisted of two photocells of the same area, each connected to a little propeller. The photocells and propellers all had little Velcro squares on the back so you could stick them onto a globe.
The idea is that you put the photocells on opposite hemispheres of the globe, about the same distance from the equator. When a light (a bulb mounted on a stand works well) shines on the cells, the propellers will spin. First, make sure that the students understand the connection between the speed of the propellers and the light received over the area of the photocell, and that they realize the seasons in the N. Hemisphere are opposite those in the S. Hemisphere. Once these things are established, you can use the model to investigate various hypotheses for the origin of the seasons. For example, it’s easy to show that simply moving the globe toward and away from the light (with no axis tilt) doesn’t cause a difference in propeller speed between the two hemispheres. Tilting the globe, of course, does produce the observed difference, and you can even demonstrate the varying lengths of daytime by rotating the tilted globe (one propeller spins longer than the other).
Assessment: You could have students fill out a worksheet along with the experiment.
Associated files and images: None
Comments: I’ve found this to be a lot easier to understand for most people than the oft-confusing diagrams in textbooks. Gina has experimented with having groups of students actually build these models themselves with very little instruction—I haven’t tried this, but she had very good results.
By Jennifer Hoffman (5/99)
Where can I get the little propellers for the photocells to use in demonstrating the seasons?
Amy Hines | 27 February, 04:13 PM | #