Inverse Square Law Lab
Background: In physics, many phenomenon follow what is known as an inverse square law. An inverse
square law states that a physical quantity (such as the force of universal gravity or the electric force) is
inversely proportional to the square of the distance from the source. This is also true for other physical
quantities such as sound intensity and light intensity. Physical quantities that follow an inverse square law
can be mathematically/graphically expressed as:
Q

1
r2
where Q is the physical quantity (such as the electric force, universal gravitational force, sound intensity or
light intensity) and r is the distance between the source of the physical quantity and where you are
measuring it. For the force of universal gravity, this would be the distance between the two masses. For the
electrical force, this would be the force between two charges. For sound or light intensity this would be the
distance between the sound or light source and where the sound or light meter is placed.
What you will be doing in this lab is measuring the intensity of sound and light and then comparing those
results with calculations made about the electric force and universal gravitational force.
Procedure:
Universal Gravitational Force: Using the Law of Universal Gravitation, calculate the force of gravity
between two 1 kg masses if they are 2 meters apart, 1.5 meters apart, 1 meter apart, 0.5 meters apart, and
0.25 meters apart. Take this information and make a graph of the distance (on the x axis) versus the force
(on the y axis).
Electrical Force: Using the Coulomb’s Law, calculate the force of gravity between two 1 C charges if
they are 2 meters apart, 1.5 meters apart, 1 meter apart, 0.5 meters apart, and 0.25 meters apart. Take this
information and make a graph of the distance (on the x axis) versus the force (on the y axis).
Sound Intensity: You will be using a sound meter to measure the sound intensity coming from a tuning
fork at various distances. First, turn on the sound meter. The meter measures the sound intensity in decibels
(dB). You should record the initial background noise in the room. Start the meter 2 meters away from the
tuning fork. Strike the tuning fork on a surface. Record the sound intensity at 2 meters. Silence the tuning
fork. Move the meter to 1.5 meters from the tuning fork, strike the tuning fork, and record the sound
intensity. Repeat this process for distances of 1 meter, 0.5 meters, and 0.25 meters. For all of the values,
subtract the background noise. Take this information and make a graph of the distance (on the x axis)
versus the intensity (on the y axis).
Light Intensity: You will be using a CRL (calculator based lab) device to measure light intensity. First,
start the CBL. Hit the MODE button. Turn on the light source. Turn off the lights in the room. Start the
CBL 2 meters away from the light source. Record the light intensity at 2 meters. Repeat this process for
distances of 1.5 meters, 1 meter, 0.5 meters, and 0.25 meters. Take this information and make a graph of
the distance (on the x axis) versus the intensity (on the y axis).
Electrical Fields: You will be using the general setup that is shown in the figure below to measure the
intensity of the electric field.
QuickTime™ and a
decompressor
are needed to see this picture.
QuickTime™ and a
decompressor
are needed to see this picture.
For the charge configuration (shown above and to the right) place the paper on top of the foam board,
holding it down with the pushpins. Attach the alligator clips to the pushpins. You will be using the probes
on the multimeter in order to map the electric field. Place the positive alligator clip/pin (red) combination at
the center of the charge. Place the negative alligator clip/pin (black) combination on the ring. Place the
black probe on the alligator clip of the ring (seen on the picture above). Using the red probe, determine the
strength for 2 cm from the center, 4 cm from the center, 6 cm from the center, and 8 cm from the center.
Do not draw on the sheets of conductive paper.
Analysis Questions: Either in a separate questions section or your conclusion you need to compare the
graphs that you made. How are they similar and different? What could cause the differences? You should
also compare the graphs with the inverse square graph shown above. How are they similar and different?
What could have caused the differences with the ideal inverse square graph?