Physics 101 - Power and Intensity

2. How do Power and Intensity relate?
Power is energy per time unit while Intensity is power
per area unit.

3. 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

4. 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))

5. 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.

6. 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?

7. 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.

8. Thanks for watching!
Work Cited
- Physics 101 Textbook - equations
- Webassign Reading Assignment 6 – sample
question
- http://en.wikipedia.org/wiki/Inverse-square_law