StaticCling - Pennsylvania State University



Physics 212P-1

Introductory Lab:

Exploring Static Electricity with Sticky Tape & A Video

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Physics Lab 212P-1

Equipment List (all items marked with * are in the student kit, others are supplied at the time of the lab)

Roll of scotch tape*

Scissors

Measuring scale

Glass rod + rubber rod

Silk cloth + wool/fur

Computer Software List

Microsoft Excel

Excel data file: twoball.xls

Videopoint movie file: twoball.mov.

Lab Activity 1: That darn static cling!

(A Qualitative Study of Electrostatics)

We all experience static electricity in everyday life. Amongst the many examples are: static cling, a bad hair day, getting zapped when you reach for a door knob, destroying a computer chip when replacing a board on your PC, the magic of Saran Wrap. This lab activity uses another piece of our everyday lives to explore static electricity. In your experimental “kit,” you will find a roll of scotch tape. This modest and unassuming piece of “equipment” will help you understand:

• the qualitative features of Coulomb’s Law;

• the existence of two types of charge;

• the conservation of charge;

• the behavior of insulators and conductors .

Note that inconsistent results will be obtained if you are not careful while handling the materials in the activity – so please follow the directions as faithfully as possible! The following experiments will need you to use strips of scotch tape that are about 20 cm long. Each time you are asked to use a strip of tape, fold over one end to form a non-sticky handle for easy handling.

A. Making a “T” Tape: Stick a 20 cm strip of tape on the lab table, sticky side down. This tape forms a standard base for making a “T” (for “top”) tape. Stick another strip of tape on top of this one, smoothing it down well with your thumb and fingers. Using a pen or marker, label the handle of this tape with a T. With a quick motion, peel off the T tape from the base tape. Test whether this T tape is attracted to your finger – if it is not, remake the tape. Once you know how to make a T tape, prepare two such tapes. Holding each one by the handle, bring the slick (non-sticky) sides of the two tapes towards each other. Observe what happens, noting how the behavior changes with the distance between the tapes.

Q1. Describe and explain what happens as the tapes are brought closer together.

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Q2. What QUALITATIVE conclusions can you draw about the relationship between electric force and the distance between charges? How do your observations relate to Coulomb’s Law?

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Q3. Can you tell from your experiment so far whether the tapes carry a positive charge or a negative charge? Briefly explain your answer.

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Q4. Using the additional equipment that has been supplied to you (a glass rod, a plastic rod, a piece of fur and a piece of silk), design and carry out an experiment that allows you to determine the sign of the charge on the tapes. Describe the experiment and its results, as well as any conventions that you may have followed for the sign of charge.

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B. Making a “B” tape: Place another base tape on the lab table as before. This time, use a marker to label the handle B (for "bottom"). Press another strip of tape on top of the B tape, sticky side down; label the handle T. Make sure the two tapes have stuck to each other smoothly. Now, VERY SLOWLY remove the pair of tapes from the table. Check whether the pair of tapes is attracted to your finger – if you do see an attraction, have one of your teammates rub the slick side of the tape with their fingers or thumb. Once you have made sure the pair of tapes is not charged, peel the pair apart to get a separate T tape and B tape. Let someone else in your team make another pair of B and T tapes. As before, your aim is to study the interaction between these different tapes by bringing the slick sides towards each other.

Q5. Describe the interaction between two T tapes, between two B tapes and between a T tape and a B tape.

T-T interaction: (a) no interaction (b) they attract (c) they repel

B-B interaction: (a) no interaction (b) they attract (c) they repel

T-B interaction: (a) no interaction (b) they attract (c) they repel

Q6. Explain why the two kinds of tapes acquired the charges that you observed. Clearly state what fundamental property of charge is demonstrated by this experiment. State any assumptions you had to make in answering this question and justify why these might be reasonable.

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Make another pair of T and B tapes. Suspend them from the edge of the table as shown in the figure below.

First, bring an insulator (e.g. a plastic pen) towards each tape and observe what happens. (Make sure that you do not allow the insulator to touch the tape.) Next, bring a conductor (e.g. a metallic object such as a key) towards each tape and observe what happens. (Make sure that you do not allow the conductor to touch the tape.)

Q7. Describe the interaction between the insulator and each tape.

Insulator-T tape: (a) no interaction (b) they attract (c) they repel

Insulator-B Tape: (a) no interaction (b) they attract (c) they repel

Q8. Using a simple atomic picture of an insulator, explain your observations. Use a few precise sentences and a clear sketch that shows how the different charges in the insulator (electrons and protons) are distributed when the insulator is far away from a tape and when it is close to a tape.

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Sketch:

Q9. Describe the interaction between the conductor and each of the tapes.

Conductor-T Tape: (a) no interaction (b) they attract (c) they repel

Conductor-B Tape: (a) no interaction (b) they attract (c) they repel

Q10. Using a few sentences and a clear sketch, explain your observations. Your sketch should show how the different charges (electrons & protons) in the conductor are distributed when the conductor is far away from a tape and when it is close to a tape. Was the interaction between the conductor and the two tapes stronger or weaker than that between the insulator and the two tapes? Explain your answer.

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Sketch:

Lab Activity 2: A Pithy[1] Experiment

(A Quantitative Study of Coulomb’s Law)

It’s now time for a “video experiment” that uses charged pith balls to illustrate quantitative aspects of Coulomb’s Law and acquaints you with the software (Microsoft Excel) that you will routinely use for analyzing and charting data measured in future experiments. The experiment is illustrated in the figure below (recall: pre-lab question #8): a charged pith ball held on a stick approaches another similarly charged pith ball suspended from a string. The balls repel each other so that the suspended ball is deflected as shown. The experiment may be viewed by opening the file “twoball.mov”. We have already measured the data for you by digitizing the movie and recording the (x,y) co-ordinates of the center-of-mass of each ball at different moments in time. To obtain the recorded data and begin the activity, open the Excel worksheet “twoball.xls" and follow the directions therein. From these co-ordinates, your goal is to:

• Deduce the angle θ and hence determine the electrostatic force F;

• Deduce the distance R between the spheres;

• Plot F versus 1/R2 and see if Coulomb’s Law is obeyed.

Note that the experiment involves several important assumptions:

• Even though the experiment is carried out dynamically (i.e. the masses involved are moving all the time), we assume static equilibrium at every instance in time;

• We assume that the charge is spread uniformly over the spheres, so that they may be treated as point charges.

Be sure to discuss these and any other necessary assumptions/approximations with your group to see how reasonable they are.

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[1] Precisely meaningful; cogent or terse (The American Heritage Dictionary)

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Slick side

sticky side

handle

F

qð

(x1, y1)

(x2, y2)

y

x

R

Mass of suspended sphere = 3 g

Length of string = 247.5 cm

θ

(x1, y1)

(x2, y2)

y

x

R

Mass of suspended sphere = 3 g

Length of string = 247.5 cm

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