Coulomb’s Law Lab

Purpose

To use Coulomb’s law to demonstrate electric force between two charged objects.

Background

Any objects that are charged will exert forces on other charged objects. The charge is due to an excess or deficiency in electrons. One electron has an elementary charge, so every excess or deficient electron has a charge of 1.60 x 10-19 C. Coulomb’s law states that the electric force between two charged objects increases as their distance from one another decreases. Electric force can be calculated using Coulomb’s equation below.

If the charges are the same (positive or negative, regardless of magnitude), there is a repulsive force. If the charges are different (again, positive or negative, regardless of magnitude), there is an attractive force.

Problem

What is the electric force between two charged balloons?

Materials

• two small balloons
• meter stick
• small ruler
• balance
• piece of thread
• fur or wool (optional)

Procedure

1. Inflate both balloons to the same size. Find the mass (kg) of one balloon and record it in the data table below. Calculate the weight of the balloon (Fg = m x g).
2. Tie a 30-50 cm piece of string to the balloon with a known mass. Tie the other end of the string to the ruler.
3. Charge each balloon by brushing it against your head ( or the fur or wool if available) three times. Do this exactly the same for each balloon.
4. Hold the ruler at arm’s length so there are no other objects near the balloon.
5. Hold the second balloon by the knot and position it below the first balloon, as shown in the diagram to the right.
6. Slowly bring the second balloon upwards until the suspended balloon moves upward and the string slackens.
7. Measure the distance between the two balloons and record it in the data table below.
8. Repeat steps 3 - 7, but this time charge the balloon by brushing it against your hair or the fur by rubbing it 10 times.

Data and Analysis

1. Compare the distance between the balloons for each trial. Explain your observations.

2. When the string slackened and the balloon moved upwards, what two forces were equal?

3. Use Coulomb’s law to calculate the charge on each balloon for each trial. For this experiment, both balloons were charged in the same way (rubbed 3 or 10 times on the same material), so we will assume the same charge on each balloon.

4. Using elementary charge, calculate how many excess electrons there were on each balloon for each trial.

Discussion:

1. In terms of electron movement, explain the interaction between the balloons.

2. What would happen if you charged a balloon and touched it to an electroscope? Explain.

3. What errors might have occurred in the experiment?