Physics 212 Day-2

Interactive Video (IV-2)

Quantitative Investigation of Electric Forces:

· Create a Word file in which to store your answers and data. Be sure to include the names of all the members of your team and put IV-2 at the end of the names, e.g. bradannephil-IV-2. Enter your responses to all of the numbered exercises into this file to submit at the end of class.

A videotape was made of an experiment where two electrically charged spheres (a Ping-Pong ball and a fishing bobber each coated with graphite paint) were given identical amounts of electric charge. One was fixed on a stand and the other was hung at the end of a long string. The one that was fixed was brought close to the one hanging and a series of pictures was taken. These pictures are now contained in the file Charged spheres.vpt in the Physics 212 folder. Open this file and view all of the video images. You can step through them by using the arrow keys on the keyboard.

1. Describe carefully what you see in the video images.

2. Explain why you think the ping-pong ball moves away from the bobber. What different forces are acting on the Ping-Pong ball?

3. Draw a free-body diagram of the Ping-Pong ball and label all forces.

4. If the object at the end of the pendulum is stationary as pictured, then determine an expression for the horizontal electric force, FElectric between the two charged objects. Be sure to show your work and carefully define any variables you use. Try to determine an expression that only uses variales which you could measure easily (such as lengths).

If you were doing this experiment in the lab room today, you would need a meter stick to measure the relative positions of each of the two spheres. Luckily for you, the computer's software is able to help you make position measurements.

If you can't see the full image, then select "Fill Screen" under the "Movie" menu. This will cause the images to fit neatly on the screen. Place the cursor on a position of interest. You do not need to click the mouse! You can read the position of the cursor in the lower left corner of the screen. Note that these values are given in meters.

5. Take data from all of the images. Make a table to record the position of centers of both the ball and the bobber in each video image.

Helpful data:

Mass of Ping-Pong ball = 2.3 g

Diameter of Ping-Pong ball = 3.7 cm

Pendulum string length = 2.0 m

6. From your data and your earlier free body diagram of the forces on the Ping-Pong ball, compute the electric force (Felectric) on the Ping-Pong ball and the separation distance between the centers of the two spheres for each video image.

7. Once you have your completed the computations to obtain force and separation data, ask your instructor for a sheet of log-log graph paper and use your knowledge of graphical analysis to determine a mathematical expression for the relationship between the electric force and the separation distance between two charged objects.

8. Once you have your mathematical expression, write a brief description of the electric forces between two charged objects.