Power and Intensity

How do Power and Intensity relate?

Power is energy per time unit while Intensity is power per area unit.

What variables do Power and

Intensity depend on?

Power or Intensity

depending on which equation you are using

¼ or 4 a constant used

in both equations

1/π or Π another

constant used in both

equations

1/r^2 or r^2 this takes the

size of the object

(particularly the radius) into consideration

Equations

Using the variables we discovered in the previous slide, we can equate the equations needed for Power and Intensity.

Power = I4πr^2

Intensity = P(1/(4πr^2))

Inverse Square Law explained

Intensity

The intensity of a sound source that radiates equally in all directions, then is evenly radiated outward from a point sources in which it reflects off. The surface area of a sphere (4πr^2) is proportional to the square of the radius therefore as the intensity of the sound get radiated off the point source, it is spreading over and area that is increasing in proportion to the square distance from the source.

Sample Question

A canon from a ship explodes. Suppose person A is sitting on a boat 2 km from where the canon is blown and person B is scuba diving 500 m below water. Which person hears the canon explode first? And at what intensity do each of them hear it at?

Sample Question Solution

We will used the inverse square law for intensity in this problem, I=(1/r^2).

For person A, the intensity equates to I=1/(2000m)^2, I=2.5e-7.

For person B, the intensity equates to I=1/(500m)^2, I=4e-6.

Therefore, Person B has a high intensity and will be able to hear the explosion first, at intensity 2.5e-7, compared to Person A who will hear it second at intensity 4e-6. The difference in intensities is related to the distance the person is located from the canon when it explodes.

Thanks for watching!

Work Cited

- Physics 101 Textbook - equations

- Webassign Reading Assignment 6 – sample question

- http://en.wikipedia.org/wiki/Inverse-square_law