# Coulomb's Law

### Objectives

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

### Procedure

#### Setup

- Set the electrostatic constant, k, to 9•10
^{9}Nm^{2}/kg^{2}. - Put a charge on the red circle at the origin (q
_{1}) of 3 mC^{}. - Put a charge on the green circle of 3 mC, and set its position 20 meters away from the red circle.

#### Part I

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

#### Part IIA

- Return to a separation distance of 60m and keep it constant.
- 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.
- Use Excel to create an appropriate graph of the data. Be sure to label each graph and axis. Print out your graph.
- Determine a mathematical relationship between the variables.

#### Part IIB

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

#### Part III

- Set a separation distance of 60 m.
- 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

- In Part 2B, what happened to the force when both charges were doubled?
- Describe the forces between positive and negatively charged objects.
- 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?
- According to the graph produced in part 1, what is the relationship between separation distance and electrostatic force?
- 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?
- According to the graph produced in part IIA, what appears to be the relationship between charge and electrostatic force?
- 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?

- According to your observations of part III, how does the sign of the charge (+ or -) affect the electrostatic force between two objects?