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# Difficult AS Physics Question - Intensity watch

1. Almost finished a homework sheet, but got stuck on the last question:

A student stands at a distance of 5.0m from a point source of sound, which is radiating uniformly in all directions. The intensity of the sound wave at her ear is 6.3 x 10^-6 Wm-2.

a. The receiving area of the student's ear canal is 1.5 cm2. Calculate how much energy passes into her ear in one minute.
b. The student moves to a point 1.8m from the source. Calculate the new intensity of the sound.

Already figured out part a: 6.3 x 10^-6 Wm-2 x 0.15 m = 9.5 x 10^-7 Jm-1

However, I'm not so sure about part b. I know that it will be a higher intensity since it is closer, but I'm not sure how to form the relationship between the intensity and the distance in a formula. Anyone?
2. Umm... 1.5cm2 doesnt equal 0.15m2. You have to multiply by 10^-4 instead of 10^-1 to convert to m^2 because area is a squared function of length if you know what I mean.
3. It gives you the energy per unit area for 5m from the source, you should know formula for sphere surface area. What is the total energy emitted per second?
4. For part b) remember that there is an inverse square relationship between the intensity and the distance from the source. [eg half the distance and the intensity is 4 times greater etc]

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Updated: September 28, 2010
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