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Activity to Teach the Inverse-Square Law for Radiation Flux

This activity teaches students about the inverse-square law for radiation flux and how to apply it to calculate energy per unit area. It includes a hands-on experiment and a discussion on planetary radiation flux. Suitable for Earth System and Environmental Science courses.

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Activity to Teach the Inverse-Square Law for Radiation Flux

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  1. Activity to Teach the Inverse-Square Law for Radiation Flux • Context: • This activity is used in a 300-level course on Earth Systems and Environmental Science (using Kump, Kasting, and Crane text), but could be used in an intro class, as well. • This is used at the beginning of a section on Energy Balance. • We just covered electromagnetic radiation and now are beginning a section on radiation flux. • Objectives: • Students will be able to demonstrate how the inverse-square law for radiation flux works. • Students will be able to define terms in the inverse-square law and apply this law to an example problem.

  2. We cover what “radiation flux” is after covering the electromagnetic spectrum: Radiation Flux • What do I mean by the term “flux” ???? • rate of energy or material that passes through a given area over a period of time • flux of water in a reservoir may be given in m3/day (volume per unit time)

  3. Getting to the concept of flux per unit area: Radiation Flux figure 3-4 DEMO...

  4. Radiation Flux How much flux does Venus get versus Earth? Mars? • To answer this, how might you go about estimating radiation flux to these different planets (assume you can measure flux on Earth, which we can!)? • Which planet do you think will have greater radiation flux? Why? • What parameters might be critical to understand from this concept? • Can you write a functional equation that captures this concept [e.g., y = f(x)]?

  5. Activity: • With the flashlight, ruler, and a piece of paper, do the following: • Shine the flashlight on your paper from 5cm distance (call this r0). Measure the diameter of the resulting spot of light. • Do the same using the light source 10cm distance (call this r). • Assume that the energy from the light is 100 units. Calculate how much ‘energy’ per unit area is received at the paper for each distance (call the value for 5cm distance, S0 and the value for 10cm distance S). • Take the ratio of these numbers . • Next, play with different ratios of your distance from the paper (r0 and r) and see if you get a number that is close to the ratio of energy.

  6. Inverse-Square Law Rearrange this equation to put the different ratios on opposite sides. Is this close to your equation? NOTE: Energy from the Sun is released in a spherical pattern, so the geometry of the system is 3-dimensional while yours is 2-dimensional. So= energy at reference Surface area of sphere = 4πr2

  7. Radiation Flux How much flux does Venus get versus Earth? Mars? Inverse-Square Law Distance from Sun: Venus: 0.72 AU Earth: 1.00 AU Mars: 1.52 AU Earth Flux: 1370 W/m2 What is flux on Venus and Mars? So= energy at reference Surface area of sphere = 4πr2

  8. Radiation Flux How much flux does Venus get versus Earth? Mars? Distance from Sun: Venus: 0.72 AU Earth: 1.00 AU Mars: 1.52 AU Earth Flux: 1370 W/m2 What is flux on Venus and Mars? Venus: Mars:

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