# Coulomb's Law

### Objectives

1. Determine the mathematical relationship between charge and electrostatic force.
2. Determine the mathematical relationship between distance of separation and electrostatic force.

Created with Geogebra

### Procedure

#### Setup

1. Set the electrostatic constant, k, to 9•109 Nm2/kg2.
2. Put a charge on the red circle at the origin (q1) of 3 mC.
3. Put a charge on the green circle of 3 mC, and set its position 20 meters away from the red circle.

#### Part I

1. Record the electrostatic force on each circle.
2. Change the separation distance to 40m (doubling), 60m (tripling), 80m (quadrupling), and 100m. Record the results of each trial.
3. Calculate the "fraction of original force" for each trial and record in the space provided.
4. Construct a graph of electrostatic force Fe (y) vs. separation distance r (x) using Excel. Be sure to label each graph and axis. Print out your graph.
5. Determine the mathematical relationship represented by the data.

#### Part IIA

1. Return to a separation distance of 60m and keep it constant.
2. Change the charge on one of the circles to 6 mC (doubling), 9 mC (tripling), and 12 mC (quadrupling). Record the force on each circle.
3. Use Excel to create an appropriate graph of the data. Be sure to label each graph and axis. Print out your graph.
4. Determine a mathematical relationship between the variables.

#### Part IIB

1. Change the charge on both circles to 0.006 C. Record the force on each circle.

#### Part III

1. Set a separation distance of 60 m.
2. Place a charge of 3 mC on one circle and -0.003 C on the other circle. Record your observations.

### Data Tables

##### Part I
Separation Distance (m) Force on 1 (N) Force on 2 (N) Fraction of Original Force
20
40
60
80
100

##### Part IIA
Charge on 1 (C) Charge on 2 (C) Force on 1 (N) Force on 2 (N)
0.003 0.003
0.003 0.006
0.003 0.009
0.003 0.012

### Questions

1. In Part 2B, what happened to the force when both charges were doubled?
2. Describe the forces between positive and negatively charged objects.
3. In each trial, how does the force on circle 1 compare to the force on circle 2? How would you describe the nature of electrostatic forces?
4. According to the graph produced in part 1, what is the relationship between separation distance and electrostatic force?
5. The magnitude of the electrostatic force between two charged objects is 10N when they are separated by a distance of 10 m. If the same charges were separated by a distance of 5m, what will be the magnitude of the electrostatic force between them?
6. According to the graph produced in part IIA, what appears to be the relationship between charge and electrostatic force?
7. The magnitude of the electrostatic force between two charged objects is 10N.
• If the charge strength of one of the objects is doubled, what happens to the magnitude of the electrostatic force between them?
• Suppose the charge on both objects is doubled. What happens to the magnitude of the electrostatic force between the two objects?
8. According to your observations of part III, how does the sign of the charge (+ or -) affect the electrostatic force between two objects?