Electrostatic Foces

Interactions of Scotch Tape Strips

In this activity we observe the electrostatic forces in charged pieces of scotch tape. 

In this part, we placed two pieces of tape, sticky side down, on the table and pulled them off. We then brought the non-sticky sides of tape together. The two pieces of tape repelled each other. The closer the tapes got to each other, the more the repulsion force was.

For this part, we placed two new pieces of tape on the table. Next we placed two other pieces of tape on top of these ones. We pulled the pairs of tape of the table and then pulled the tapes apart. When the two top strips were brought toward one another, they repelled. The same thing happened when the two bottom strips were brought together. However, when a top and bottom strip came together they had an attractive force.

From our observations, we can conclude that there are two different types of charges. Two objects with the same charge will have a repulsive force, just as witnessed when the two "top" or two "bottom" tapes were brought into close proximity to one another in our experiment. Also, we observed that two objects with opposite forces will attract each other, such as when a "top" tape came into proximity with a "bottom" tape.

Electric Force Law Video Analysis Activity

In this activity, we observe the repulsive electric force of two like-charged balls. One ball is suspended by a string while another is secured to a stick and brought into proximity of the hanging ball as seen in the video below.

We drew a free body diagram to show the forces acting on the ball in the video. There is a tension from the string (T), the force due to the mass of the ball and acceleration of gravity (mg), and an electrical force (F_e). We were able to separate the forces into their components and consequently isolate the electric force in terms of mass (M), earth's gravitational constant (g), separation distance (X), and length of the string (L). Since M, g, and L were given, our variable X was easily obtained using the video and LoggerPro software.

We used LoggerPro software to track the motion of both balls from the video. We also used the software to create a graph showing Force vs. Separation Distance. Separation distance (X) was calculated as the difference of the position of the two balls. We can see that the points on the graph fit closely to the power trendline. The equation of the curve was y = (2.085x10^-5)x^(-1.842).

Here we answered some questions in conclusion to our experiment. We were able to show that  the electric force was roughly inversely proportional to the square of the distance (x^2) between the charges. We saw this when we added a power trendline to our graph. From the trendline curve equation, we found the power to be -1.842 which is very close to the -2.000 result we would like to have.

The percent difference was obtained by using the equation shown in the picture (2a). We took the absolute value of the difference between the true value and experimental value. The whole quantity was divided by the true value and then multiplied by 100% to obtain a percentage. It is important to note that the picture contains mistakes. The 1.793 above was obtained from an earlier incorrect trendline. The trendline was corrected and the percent error was actually only 5%. 

From this experiment, it is not possible to tell what the sign of the charge on either ball is. We know that both repel each other which means that they have the same charge. The only way to discover the charge on the ball is to introduce another object with a known charge (for example, + charge). If the new objects would repel, then the ball is of the same charge (+) and if they attract then the ball is (-).

We noticed that there is some uncertainty in our experiment. The uncertainty comes from various sources. However, we believe that the main sources came from the tracking of the objects on LoggerPro. The initial scaling of the video ("x" inches on the screen = 1 m) also had some level of uncertainty. Our points did not all fit within the trendline but the percent error was still within a reasonable range therefore we feel comfortable with our results.