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Physics LibreTexts

1.2: Activities

  • Page ID
    23033
  • Things You Will Need

    [For the labs we do this quarter that involve activities at home, you are expected to be resourceful. If you don't have precisely the items in these lists, you need to think outside the box to get materials that will do the same job. You may have to test several possibilities to see what works best. In short, you need to think like an engineer. Channel your inner MacGyver!]

    • three chargeable insulators... some suggestions:
      • pvc pipe (or any hard plastic rod)
      • shot glass (a glass rod, if you have one, works even better)
      • plastic bottle
      • polystyrene (Styrofoam)
      • inflated latex or rubber balloon
    • various rubbing fabrics (wool, polyester, fur, etc.)
    • lightweight "testing" devices
      • drinking straw
      • tape (scotch tape is best, though electrical tape also works; masking tape does not work)
    • aluminum or tin can (empty)

    Creating a Standard

    We don't know offhand what the actual sign of the static charge we produce is, but we can determine the relative signs. Choose one of your three chargeable insulators to be your standard, rub it vigorously with a cloth (you may later want to try different kinds of cloth, to see which works best), and define the sign of the static charge created on the insulator to be positive. For example, you might choose the pvc pipe rubbed with wool to be your standard, thus defining it to be positively-charged. Note that this may not actually agree with our standard definition of positive or negative (as originally defined by Benjamin Franklin), but this is of no consequence.

    Creating Testing Devices

    We aim to detect forces between static charges, but we don't have a handy gauge that measures such small forces, so we will have to observe these forces by accelerating one of the charged objects with the force exerted on it by the other. But we will only observe this if the accelerated object has a very low mass. That's where the drinking straw and tape come in:

    drinking straw: Place a drinking straw into a small glass, such that one-half-to-two-thirds of the straw is beyond the lip of the glass. [If you are using a shot glass and the straw is too long, go ahead and cut it to the right size.] Remove the straw and rub one end of it vigorously with a cloth, and place it back into the glass with the charged end sticking out. Note you may find that the straw sticks to your fingers. If you handle it only by the uncharged end, you should be able to escape its fondness for you.

    tape: While the drinking straw is somewhat easier to work with, because you don't have to hold it, tape has the nice property that you can charge it either positively or negatively. Take a piece of tape about 7 or 8 centimeters (~3 inches) long, and stick most of it to a clean surface – leave a small tab unstuck to the surface, because you charge the tape by using the tab by suddenly pulling the tape from the surface. Try to avoid having the tape curl back to your fingers. The experiment is not ruined if it does this, but you would prefer to handle the charged end of the tape as little as possible.

    Both the straw and the dangling tape will be light enough to react (accelerate) when static charge is brought nearby. Once the signs of charge on these testing devices is determined from the standard, they can be used to determine the signs of many other statically-charged items. Before moving on to this, however, you should try charging tape with the opposite sign as the first piece of tape. This consists of trying different insulating surfaces: tile, stone, laminate, etc (conductors will not work very well). [If you try several surfaces with the same piece of tape, be sure to discharge the tape between tests.] If you can't find a surface that makes the tape respond differently, try this: Stick an uncharged piece of tape to a surface, leaving no "tab." Now stick a second uncharged piece of tape to the top of the first piece, and quickly pull it off. With two oppositely-charged tape testers, results of signs of charges for the many systems can be confirmed.

    [Note: Tape essentially plays the same role as the cloth that rubs the other surfaces. Since the type of surface defines the sign of the charge left on it after the cloth rubs it, and since charge conservation demands that the cloth is also charged (oppositely), the surface off of which the tape is pulled determines the sign of the charge on the tape.]

    Cataloging Charge Signs

    Your Task: Use the properties of static electric attraction and repulsion to bootstrap from your standard to create a catalog the signs of many different statically-charged systems. You should not only determine the signs of the charges with one experiment, but you should corroborate these results across different experiments. That is, every observation should result in the same conclusion about the sign of a charge, and every sign of a charge should have at least two corroborating results, preferably including both an attraction and a repulsion. You can (and should) even check testers (like two pieces of tape) against each other. Tabulate your results. Considering how many combinations are possible you should easily have a dozen or more entries.

    Effect on a Stream of Water

    If you hold one of your charged insulators close to a thin stream of water pouring from a faucet, you will see that it is deflected. Make a note of the direction of the deflection (attractive or repulsive). Make a prediction (before doing the test – it is okay to be wrong!) about the result of placing one of your charged insulators with the opposite charge near the stream. Report the result and discuss what you think is happening. You may want to google "water is a polar molecule," and review properties of dipoles to help you with this discussion.

    Effect on a Conductor

    Lay the empty aluminum can on its side on a flat, level surface. Build up charge on your "standard" insulator (so this is positive charge), and hold it close to the can. Get as much of the surface of the insulator as close to as much of the surface of the can as possible, for the best effect. Note the reaction of the can (attraction or repulsion). As with the water, make a prediction about what will happen if you bring an insulator with the opposite charge close to the can. Report the result and discuss what you think is happening. You should especially explain how this is different from the case of the water stream.

    Lab Report

    The object of this lab is to start with very little information and extract as much as information as you can from a few basic principles. There are no quantitative elements or uncertainties to estimate or compute. There are no "incorrect" tests for you to perform – if you find you are curious about something, by all means, test it! You should find – and avail yourself of – plenty of opportunities to corroborate results with multiple tests.

    Download, print, and complete this document, then upload your lab report to Canvas. [If you don't have a printer, then two other options are to edit the pdf directly on a computer, or creating a facsimile of the lab report format by hand.]