Question

Activity 1: Tape Electrostatics – Home Experiment

You need the following material for this home activity: Transparent Tape, Aluminum Foil, String, Plastic ruler or Hair Comb, Piece of Wool, or fuzzy fabric.

You may have noticed that when you pull a long strip of transparent tape off a roll, it curls up. You will take advantage of this property to investigate the nature of the charge and electric forces.

• To make the tape strips easy to handle, fold the last 1/2 inch or so over to make a non-sticky tab that you can use as a handle.
• Place an approximately 10-inch long tab strip on a table surface sticky side down and smooth it out flat. This will be the base on which to make a pair of tab strips.

Place two more strips on top of this base, both sticky sides down. Label one tape T for top and the other B for the bottom. Now very slowly peel off both strips as one unit (to minimize charging).

• You will probably find that the pair of strips is mildly attracted to your hand or any nearby surface. If so, lightly and slowly drag the strips across a metal surface several times. This should eliminate the attraction (mostly).

Question 1. Why might bringing the tape in contact with metal make the tape become less attractive?

• Now take the two tabs and quickly rip the two strips apart. With a minimum of handling, hang the two strips about a foot apart off the edge of the table as shown

• Repeat the entire process just described to get two more tape strips labeled T and B. Hang them from the table as well for a total of four strips. Note, if at any time your strips seemed to have “lost” or “changed” their charge, make new T and B strips.

Question 2. How do you think the two T strips will interact if you bring one close to the other; will they attract or repulse each other or do nothing? For now, you can determine if the charged strips have similar or unlike charges. But you cannot determine which one is positive and which one is negative.

Prediction and explanation

Observation (was your prediction correct? If not, why?)

Question 3. How do you think the two B strips will interact if you bring one close to the other?

Prediction and explanation

Observation

Question 4. Using the two B strips, observe the force the strips exert on each other as a function of how far apart they are. You should be able to judge how big the force is by how much the hanging strip is deflected. Does that confirm what Coulomb's law says about force and distance? Explain.

Question 5. How do you think a B and a T strip will interact if you bring one close to the other?

Prediction and explanation   

Observation

Question 6. Now rub the plastic ruler (or hair comb) vigorously with the wool, fur, or fuzzy fabric. How can you tell if the plastic ruler now has a "B-type" charge or a "T-type" charge? Explain your reasoning, what you observe and what you conclude.

Question 7. Suspend a metal (aluminum foil) ball from a string (insulator). Bring the neutral metal ball close to a T strip (without touching it). What do you observe?

Question 8. Bring the neutral metal ball close to a B strip. What do you observe?

Question 9. How is the interaction of the metal ball with the tab strips different from the interaction of the glass or plastic rods with the tab strips?

To explain the basic observation described in questions 7 and 8, consider the following words and pictures. In a metal conductor, the electrons are relatively free to move while the protons are not. Assume that the metal ball starts out neutral, with positive and negative charge evenly distributed, with a net charge of zero.

Question 10. What will the distribution of charges in the ball look like if you bring the ball close to a positively charged tape? Sketch it on the figure.

Question 11. Would the interaction between the tape and ball result in an attractive force, a repulsive force, or no force at all? Carefully explain.

Question 12. How about if instead, you bring the ball close to a negatively charged tape? Sketch the distribution of charges in the ball on the figure.

Question 13. Would the interaction between the tape and ball result in an attractive force, a repulsive force, or no force at all? Carefully explain.

Answer 1

1) masking tape becomes less attractive when passed over a metal as excess electrons are discharged to the metal.

2) In this case, the two strips T will have the same electric charge, therefore when bringing both strips closer they will repel each other.

3) likewise, as with the T strips, the B strips repel each other when they approach.

4) Indeed, when the strips are far apart, they both hang vertically, but when they are made closer to each other, the strips are tilting outwards, which indicates that the repulsive force increases as they approach each other.

5) between a strip T and a strip B, an attractive force will be exerted, which will bring both strips closer to each other.

7) when it gets closer to the strip T, it is attracted by the metal sphere very weakly.

8) It is the same as for the T strip, there is a weak attraction between the strip and the sphere.

9) 10) 11) 12) 13) The difference is that in the case of metal or plastic rods they have an established excess charge, either negative or positive, while in the metal ball the attraction is made by the redistribution of the electrons within the metal sphere. . If the strip has a positive charge, the electrons are attracted by the strip and accumulate on the side closest to the strip, as the strip is lighter it is the one that makes the movement towards the metal sphere. The same happens if the strip has a negative charge, the electrons of the sphere are repelled and accumulate towards the other end, leaving the other side positively charged.