Physics Core/AP 1 and 2

Electrical Force

# Lab: Electrical Force

#### Materials

1. Balloons
2. Scale
3. Ruler
4. Weights (uniform objects such as aspirin pills; coins are probably too heavy).
6. Container big enough to hold weights.(lightest possible plastic cup)
7. Piece of silk, wool, or other fabric.

#### Procedure

This lab is best carried out when humidity is low!

• Weigh 5, 10, and 15 units of your weights, divide by the number of units weighed, and determine the average weight of each unit. If not weighing in grams, convert your measurements to grams.
• Blow up your balloon and weight it again; it should be slightly more massive since you have added air under pressure. This is the minimum supported mass.
• Measure the diameter of the balloon. The radius of the balloon will be the average distance r separating charges on the surface of the balloon (q1) from charges on the "ground" surface (q2).
• Tie your container to the knot in the balloon using the string (you may need to pierce your container to attache the thread to it.
• Charge the balloon by rubbing it with your cloth. Let the ballon be attracted to a vertical surface and time how long it takes to discharge and fall. If possible, determine whether a painted drywall surface, wooden door, or glass mirror works best. Record the number of strokes required to charge the balloon sufficiently to hold it to the wall for at least 10 seconds.
• Recharge your balloon and stick it to the wall with one of your weight units in the container. Record how long it takes the balloon to discharge and fall.
• Repeat your trial run, adding only one more weight to the balloon each time, until the balloon falls immediately when you attempt to stick it to the wall.

#### Data Handling

1. Determine the gravitational force required to overcome the electrical attraction between the wall and the balloon. ${F}_{e}=\frac{k{q}_{1}{q}_{2}}{{r}^{2}}=\mathrm{mg}$
2. Assume that at the instant the balloon touches the wall, the induced charge in the wall attracting the balloon is equal to the charge on the balloon: q2 = q1. Using the radius of the balloon as r, calculate the values of q2 and q.
3. Using the amount of charge per electron, estimate the number of excess electrons at the instant the balloon touches the wall.

#### Report

1. Describe your materials, equipment, and procedures in sufficient detail that your fellow students could repeat your experiment.
2. Report your data. Be sure to indicate the amount of error in your measurements. For example, if you can only measure a mass of 25 gms within 1 gm, your error would be 25 ± 1, or 1/25 = 4%.
3. Present your data in an organized form, preferably in a table, in such a way it is easy to compare results as you repeate trials or vary a specific contributing factor.
4. Show a sample calculation, if you have calculated values.
5. If you did a series of experiments, varying something by increasing or decreasing a factor, try to plot your data (y-axis) as a function of the factor (x-axis).