25_PV-InverseSquare

# 25_PV-InverseSquare - Dependence of Light Intensity on...

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Dependence of Light Intensity on Distance SPN LESSON #25 TEACHER INFORMATION LEARNING OUTCOME: After determining through measurement the relationship between the intensity of a point light source and the distance from the light source, students are able to make predictions on the basis of the inverse square law and relate that law to other phenomena such as gravitational and electrical forces. LESSON OVERVIEW: The purpose of this lesson is for students to determine the relationship between the intensity of a point light source and the distance from the light source. Measuring this relationship gives students experience with the “inverse square” relationship that also characterizes the gravitational force between two point masses and the electric force between two point charges. Data from SPN lesson #24, Efficiency of Energy Conversion , can be used in this lesson. Two versions of this lesson are provided in the student section; which one you use depends on the equipment you have available. Version One uses TI-83+/LabPro and Version Two uses TI-83/CBL. GRADE-LEVEL APPROPRIATENESS: This Level III Physical Setting lesson is designed for use with students enrolled in a high school physics or physical science class. MATERIALS Version One: Student handout (Version One), meterstick, 40 W incandescent light bulb and fixture, TI-83+ graphing calculator, Vernier LabPro unit Version Two: Student handout (Version Two), meterstick, 40 W incandescent light bulb and fixture, TI-83 graphing calculator, TI CBL unit, TI light probe SAFETY: Students need to take the usual precautions for handling electric appliances in setting up the bulb. TEACHING THE LESSON: Display a lighted light bulb in a fixture on a lab table. Given the supposition that the intensity of the light is “ x ” at a distance of 1 meter, ask students to predict what the intensity of the light will be at a distance of 2 meters (as a fraction of x ). Explain to students how they will be carrying out an investigation to test their prediction. If it is appropriate, in conjunction with previous learning about the dependence of gravitational and electric force on distance between point masses and/or charges, point out that the same relationship applies between those forces and the distance between objects. www.SchoolPowerNaturally.org

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ACCEPTABLE RESPONSES FOR DEVELOP YOUR UNDERSTANDING SECTION: The following data and calculations were obtained with a TI-83+/LabPro system: Version One: Distance from center of bulb (m) Intensity (W/m^2) 0.4 4.5265 0.5 4.4366 0.6 2.7715 0.7 2.2618 0.8 1.7302 0.9 1.3297 1.0 1.0554 These data fit a power regression on the TI-83+ of the following form: Intensity = 1.15*distance^-1.686. If the exponent equaled –2, the relationship would have been exactly that of intensity varying as
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25_PV-InverseSquare - Dependence of Light Intensity on...

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