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Charge and Electrostatics

By Carlos Hawkins,2014-07-09 07:30
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Charge and Electrostatics

    Charge and Electrostatics

    I. ELECTRICAL INTERACTIONS

    A. Press a piece of sticky tape, about 15-20 cm in length, firmly onto a smooth unpainted surface, for example, a

    notebook or an unpainted tabletop. (For ease in handling, make “handles” by folding each end of the tape to

    form portions that are not sticky.) Then peel the tape off the table and hang it from a support (a pen, your

    finger, or the edge of a table).

     Describe the behavior of the tape as you bring objects toward it (e.g., a hand, a pen).

    B. Make another piece of tape as described above. Bring the second tape toward the first. Describe your

    observations.

    ; It is important, as you perform the experiment above, that you keep your hands and other objects away

    from the tapes. Explain why this precaution is necessary.

    ; How does the distance between the tapes affect the interaction between them?

    C. Each member of your group should press a tape onto the table and write a “B” (for bottom) on it. Then press

    another tape on top of each B tape and label it “T” (for top).

    Pull each pair of tapes off the table as a unit. After they are off the table, separate the T and B tapes. Hang

    one of the T tapes and one of the B tapes from a support at your table.

    Describe the interaction between the following pairs of tape when they are brought near one another.

    ; two T tapes

    ; two B tapes

    ; a T and a B tape

    D. Obtain an acrylic rod and a piece of wool or fur. Rub the rod with the wool, and then hold the rod near newly

    made T and B tapes.

    ; Compare the interactions of the rod with the tapes to the interactions between the tapes in part C.

    Describe any similarities or differences.

    ; We say that the rod and tapes are electrically charged when they interact as you have observed.

    E. Base your answers to the following questions on the observations you have made thus far.

    1. Is it possible that there is only one type of charge? If not, what is the minimum number of different types

    of charge needed to account for your observations thus far? Explain.

    2. By convention, a glass rod is said to be “positively charged” when rubbed with silk. Obtain a glass rod

    and a piece of silk from your instructor.

    ; How do two objects that are positively charged interact? Explain how you can tell.

    ; Which tape, T or B, has a positive charge? Explain.

Discuss Part I with your instructor before continuing.

    Please remove all tape from the tabletop before continuing.

PART II. TESTING MATERIALS

    A. Obtain a green plastic strip and some rabbit fur. Hang the green strip by a thread from the metal rod, as

    shown in the front of the room. These two materials are your standards, which you will keep throughout this

    part of the experiment. Each time you test a different material, you’ll have to rub the green plastic strip with

    the fur.

    B. When a glass rod is rubbed with silk, the glass becomes positively charged. Rub the green plastic strip with

    the fur and rub a glass rod with silk. Working quickly, bring the glass rod near the green plastic strip and

    observe whether the plastic is attracted or repelled.

    1. If the green plastic is attracted to the positively charged glass rod, is the plastic charged positively or

    negatively?

    2. If the green plastic is repelled by the positively charged glass rod, is the plastic charged positively or

    negatively?

    3. Record your observations and the charge of your green plastic strip on your own paper.

    C. Now return the glass and silk so that others can use them, and get another type of rod and another type of

    cloth. Rub the new rod with the new cloth and use your standards to determine the type of charge on the

    new rod. Repeat for as many different combinations of materials as you can, remembering to re-charge your

    standards each time you test something new. Record your observations on your own paper in a table like the

    one shown below.

    This material< rubbed by this material< (attracts / repels) and is therefore charged

    the green plastic strip (positively / negatively)

PART III. ATTRACTION AND REPULSION

    A. Obtain an electrostatics box. (Be careful; they are fragile.) You’ll also need the green plastic strip and rabbit

    fur. Carefully remove the guide block and attached graphite sphere from inside the box.

B. Charge the sphere that is fastened to the guide block by following these steps:

    ; Vigorously rub your green plastic strip with the fur.

    ; Bring the sphere mounted on the guide block near the green plastic strip, but DO NOT touch the strip

    with the sphere.

    ; When the sphere is close to the plastic, briefly touch the sphere with your finger.

    ; After you’ve removed your finger from the sphere, slowly pull the coated sphere away from the charged

    plastic strip.

    C. Working quickly, insert the charged sphere into the chamber through one of the holes in the base. Gently

    slide the charged ball up to the suspended ball and bring them into contact. Describe what happens to the

    hanging sphere.

D. Answer the following questions based on your observations to this point. (Hint 1: The gray coating on the

    styrofoam spheres is graphite, a conducting material. This means charges can move around in it. Hint 2:

    Unless you see sparks, charges are NOT jumping through the air. Hint 3: “Uncharged” really means “having

    an equal number of positive and negative charges.”)

    1. Why does this strange “touching” ritual charge the graphite sphere? Draw a series of diagrams showing

    where charges are moving at each step of the way.

    2. Why does the hanging sphere (which is initially uncharged) attract to the charged sphere?

    3. Why does the hanging sphere then repel from the charged sphere?

PART IV. A MODEL FOR ELECTRIC CHARGE

    A. A small ball with zero net charge is positively charged

    on one side and equally negatively charged on the

    other side. The ball is placed near a positive point

    charge as shown.

    Would the ball be attracted toward, repelled from, or

    unaffected by the positive point charge? Explain.

    What does this tell you about how electric forces

    change with distance?

    B. Use the model suggested in part A above to help answer question D2 in part III. As part of your answer, draw

    sketches of the charge distribution on the two spheres both before and after they are brought near one

    another.

C. Newton’s Law of Universal Gravitation gave a way to describe the gravitational force between two masses.

    Using Newton’s Law as a model, write an expression that could possible describe the electric force between

    two charges. Be sure your expression is consistent with the observations you have made so far.

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