Inverse Square Law and physics

In summary, the Inverse Square Law states that the amount of energy per unit area (in terms of light, sound, or other forms of energy) decreases as the square of the distance from the point of origin.
  • #1
kroniic
2
0
Hello all, I am currently in Year 11 (Australia),

1st time posting but I really need help, my Physics teacher did not explain how to use the Inverse Square law when dealing with the intensity of light and to make things worse, he would not help me I have a theory why but that is another story, what I need help with is how to put the Law to use for a report that is due tomorrow. :(

Many Thanks,

Anthony
 
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  • #2
Most forms of energy from a point source travel away from the point source in the shape of a sphere, or part of a sphere, such as a sound wave, or a beam of light. Since the area of the sphere is related to the square of the radius of the sphere, which is the distance from the point source, the amount of energy per unit area decreases by the square of the distance from the point source.

For some forces like gravity that effectively eminate from a point source (the center of mass of the object creating the gravitational field if the distance is suffieciently outside the object, depending on how spherical the object is), the amount of force decreases with the square of the distance. The reasoning could be similar to the energy case I stated above, but not all forces follow the inverse square law, for example the strong force that holds the the nucleus of atoms together has a finite distance of effectiveness, while forces that follow inverse square law extend to infinity (they are non-zero forces at any finite distance from the point source).

On a side note, if the energy is eminated from an infinitely long line, the energy per unit area decreases linearly with the distance from the line, and if the energy is eminated from an infinitely large plane, the energy per unit area is constant, the same at any distance from the plane. Gravity would follow the same rules, but the model would be an approximation, such as a very long cylinder, or very large plane with the target point relatively close enough to the line or plane shaped object producing the gravity to act similar to an infinitely long line or infinitely large plane.
 
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  • #3
Jeff Reid said:
Most forms of energy from a point source travel away from the point source in the shape of a sphere, or part of a sphere. Since the area of the sphere is related to the square of the radius of the sphere, which is the distance from the point source, the amount of energy per unit area decreases by the square of the distance.

On a side note, if the energy is eminated from an infinitely long line, the energy per unit area decreases linearly with the distance from the line, and if the energy is eminated from an infinitely large plane, the energy per unit area is the same at any distance from the plane.

I sort of understand it, like how it will be 1/4 of the intesity, there was something my teacher was saying about being directly proportional, indirectly and such, also said something about when plotting it, it will be a straight like rather then a curve.
 
  • #4
Directly proportional usually implies a linear, and not a quadratic relationship. Inverse square is a quadratic relationship, and the shape of the curve is a part of a parabola that extends along the x (distance) axis. (y = sqrt(x), or x = y^2). So if the distance doubles, the intensity at that doubled distance decreases by a factor of 4.
 
  • #5
kroniic said:
… there was something my teacher was saying about being directly proportional, indirectly and such, also said something about when plotting it, it will be a straight like rather then a curve.

(I don't think there's any such thing as "indirectly proportional" - could it have been "inversely proportional"?)

I don't understand how the Inverse Square Law can be directly proportional, or how it can be plotted straight - unless the teacher was talking about logarithms. :frown:
 
  • #7
tiny-tim said:
(I don't think there's any such thing as "indirectly proportional" - could it have been "inversely proportional"?)

I don't understand how the Inverse Square Law can be directly proportional, or how it can be plotted straight - unless the teacher was talking about logarithms. :frown:
Power density plot can be straight if we put 1/r^2 on abscissa axis.
Or maybe the lecture was about amplitude of electric field? Amplitude if electric field is proportional to quare root of intensity, so amplitude is inversely proportional to r=proportional to 1/r.
 

1. What is the Inverse Square Law?

The Inverse Square Law is a principle in physics that states that the intensity of a physical quantity, such as light or gravity, is inversely proportional to the square of the distance from the source. This means that as the distance from the source increases, the intensity decreases by a factor of four.

2. How does the Inverse Square Law apply to light?

The Inverse Square Law applies to light by describing how the intensity of light decreases as distance from the source increases. This means that the further you move away from a light source, the dimmer the light will appear. It also explains why objects appear brighter when they are closer to a light source.

3. What is the mathematical formula for the Inverse Square Law?

The mathematical formula for the Inverse Square Law is I = P/4πr², where I is the intensity, P is the power of the source, and r is the distance from the source. This formula shows that as the distance from the source increases, the intensity decreases in an inverse relationship to the square of the distance.

4. How does the Inverse Square Law apply to gravity?

The Inverse Square Law also applies to gravity by describing how the force of gravity decreases as the distance between two objects increases. This means that the further apart two objects are, the weaker the force of gravity between them will be. This law is essential in understanding the behavior of celestial bodies, such as planets and stars, in space.

5. What are some real-life applications of the Inverse Square Law?

The Inverse Square Law has several real-life applications, such as in photography, where it is used to calculate the correct exposure for a photo based on the distance from the light source. It is also used in satellite communication and GPS systems to determine the strength of the signal. In addition, this law is crucial in understanding radiation and how it dissipates over distance, which is essential in nuclear power and medical imaging.

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