INVERSE SQUARE LAW P 4 5 D Explain

INVERSE SQUARE LAW

P 4. 5 D: Explain how waves propagate from vibrating sources and why the intensity decreases with the square of the distance from a point source P 4. 5 E: Explain why everyone in a classroom can hear one person speaking, but why an amplification system is often used in the rear of a large concert auditorium.

Propagate (scientific definition): 1 : to cause to spread out and affect a greater number or greater area : extend 2 : to transmit (as sound or light) through a medium Source: http: //www. merriam-webster. com/dictionary/propagate

How do waves propagate? Recap 2 types of waves:

The inverse square law applies to: Gravity Electrostatics Light (and all electromagnetic waves) sound

The mathematical relationship between the intensity (I) of the source gravity/sound/light, and the intensity measured at distance (d) I = 1/d 2 *this is interchangeable with I = 1/r 2 where r= “radius”

Why does this occur? Spray Paint Applet: http: //www. mrwaynesclass. com/teacher/Gravity/Inverse. Square/inverse%20 square%20 law. swf As waves propagate in a spherical motion from the source, the energy is spread over a greater area. Bulb breaking: http: //www. acoustics. salford. ac. uk/feschools/waves/bulbvideo. htm

Balloon Example Surface of the balloon: wave front radiating out from a wave source – like a loudspeaker. Depth of color of the balloon: the intensity of the wave. As the energy (or total available amount of red rubber) has to spread out over a larger and larger area, it gets more thinly spread (sound gets quieter / balloon looks paler). Applet: http: //www. acoustics. salford. ac. uk/feschools/waves/propagation. htm

Balloon Example Applet: http: //www. acoustics. salford. ac. uk/feschools/waves/propagation. htm

Gravity Gravity’s strength abides by the inverse square law. A person at the top of a mountain weighs less than they do at the bottom by a small amount.

Light Like sound, as light leaves its source, the intensity decreases according to the inverse square law. Video: https: //www. youtube. com/watch? v=F-x. NMd. IXJIs

Example 1: the sun

Example 2: Flashlights

In addition, the inverse square law applies to all forms of electromagnetic radiation.

Q: Why do you think flashlights are designed with a mirror-like surface behind the bulb? Draw a diagram to support your response:

Sound As a sound wave propagates spherically, the sound energy is distributed over the everincreasing surface diameter of the wave front. Source: http: //www. acousticalsurfaces. com/acoustic_IOI/101_5. htm

Q: Why does a megaphone make sound project further?

A: It increases the amplitude (intensity) of the wave. The inverse square law is still in effect, but the initial energy is larger, so the wave travels further.