Coulomb’s Law Lab



Coulomb’s Law Lab

Name: Date:

Theory: Coulomb’s law states that the force between two charged objects varies directly with the product of their charges and inversely with the square of the distance between them.

Formula:

During this lab we will be unable to measure the size of the charge on the objects, therefore, we will charge them and consider it to be constant and only be investigating the inverse square portion of Coulomb’s Law.

Objective: To investigate Coulomb’s Law to the extent that the force between two charges varies inversely with the square of the distance between them.

Apparatus:

[pic]

When the ball is suspended from the dowel the only force acting on it is gravity. When the two pith ball are charged and brought near each other (r) the suspended ball is repelled a distance (d). The amount of deflection is directly proportional to the force acting on the ball (through resolution of forces). By measuring d and r and then plotting them we can prove the inverse square portion of Coulomb’s Law

Procedure:

1) Arrange the apparatus so that the hanging ball and mounted ball are between a light source and a piece of paper (so they cast distinct shadows). Mark the ZERO position of the hanging ball with one of your markers.

2) Charge the rubber rod with the fur and touch the rod to the hanging ball and then the mounted ball so that the two balls have like charges and will repel.

3) Move the mounted ball closer to the suspended ball until there is a small deflection noticed on the screen.

4) Mark the edge of the shadow of the suspended ball with one color marker and the edge of the shadow of the mounted ball with a different color marker.

a. NOTE: always make all measurements from the same edges of the two shadows

5) Move the mounted ball again until another noticeable deflection occurs. Mark the position of the suspended and mounted ball again.

6) Repeat step 5 for as many additional measurements you can (the more measurements the better your data)

7) Remove the paper and measure d and r for every deflection. Record the data in the table below.

8) Repeat steps 1-7 for two more trials.

Data:

Trial 1 Trial 2 Trial 3

|r |d |r |d |r |d |

| | | | | | |

| | | | | | |

| | | | | | |

| | | | | | |

| | | | | | |

| | | | | | |

Interpretations:

1) Put all your data onto an Excel spreadsheet, then for each trial determine r2 and 1/r2.

a. Note: Set up your data table as | r | r2 | 1/r2 | d |

2) Using Excel, plot the graph d vs. r.

a. Note: d must be on the y-axis and r must be on the x-axis

3) Using Excel, plot the graph d vs. 1/r2.

a. Note: once again d must be on the y-axis and 1/r2 must be on the x-axis.

Questions:

1) According to Coulomb’s Law, d and r should be an inverse square proportionality which, when graphed, is a half hyperbola. Do any of your graphs resemble this half hyperbola?

2) When plotted, d vs. 1/r2 should be a direct relationship. Do any of your graphs show this?

3) One or more of your trials may not be particularly good in this investigation. (This lab has a myriad of problems.) Suggest some possible causes of error.

Conclusion:

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