lab 1: electrostatics in your home introductionscphys/courses/e1b/e1b_1.pdf · physics 1b lab 1:...

Physics 1b Lab 1: Electrostatics in Your Home Spring 2007

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Introduction Most everyone has been “shocked” by the ability of electrons to transfer from one object to another - particularly on dry winter days! In this lab, you will explore how electrons are transferred between common house hold objects such as pieces of tape, StyrofoamTM, computer monitors, and even your fingers. Along the way, you will show that the force between charged surfaces (those with an excess or deficit of electrons) decreases with distance. This lab is divided into three parts. In the first section, you will be performing investigations similar to those performed by Benjamin Franklin during the mid-1700's. Like Franklin, you strive to learn how charges interact, and you will be asked to speculate on the how's and why's of electrostactics. Your experiments might not go exactly as your prejudiced mind may anticipate. Don't worry! As Franklin wrote in a letter to Cadwallader Colden on April 23, 1752, “Frequently in a variety of experiments tho' we miss what we expect to find, yet something valuable turns out, something surprising, and instructing, tho' unthought of.”1 Set your preconceptions aside and get ready for perceptive and accurate observation. In the second part of this lab, you will be quantifying the electrostatic properties of charged packing peanuts. During the winter holiday you may have had one or more encounters with packing peanuts that seemed determined to stick to you, the floor, and everything else, while at the same time, they seemed equally determined to stay away from one another. This type of behavior is caused by a build up of electrons on the Styrofoam that causes them to simultaneously repel one another (like repels like) and cling to objects with a deficit of electrons (opposites attract). Using just two charged packing peanuts you can (and will!) measure how the amount of repulsion is related to the separation between the pieces of Styrofoam. In the final section of the lab, you will explore the difference between induction and conduction. By using these techniques to charge a standard aluminum pie plate, you will determine the sign of the “charge-transferred,” as well as the efficiency of each method. This lab offers you a chance to safely play with electrons using things that should be lurking about your home already. Even with the power off, electricity, in the form of static, is all around us. For your lab report please refer to the attached Post-lab questionnaire. The questionnaire must be completed and handed in to your Lab TF’s mailbox before 6 PM on February 20.

Equipment (bold items will be supplied) • Scotch Magic TapeTM • StyrofoamTM cup • TV or CRT computer monitor • 2 Styrofoam packing peanuts (puffs) • Needle, thread and scissors • Ruler

• Lamp • 2 Rods /chopsticks / mixing spoons • 1 sheet of graph paper • Aluminum pie-plate • Styrofoam plate

Readings If you are interested in learning more about Benjamin Franklin and his scientific activities, you may enjoy reading Benjamin Franklin's Science by I. Bernard Cohen (Harvard University Press, 1990). Online information on Franklin can be found at http://sln.fi.edu/franklin/ 1 John Bigelow, The Works of Benjamin Franklin, (Knickerbocker Press, NY, 1904) Vol II, p. 370


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I. Sticky Electrostatics Rubbing materials together can generate static electricity. You can test this by scuffing your feet across carpeting on a dry day and then touching a doorknob. ZAP! But is it the friction of scuffing your feet across the floor that causes the charge buildup, or is something else going on? When two surfaces touch (like your socks on a carpet) chemical bonds can temporarily form between surfaces, as neighboring atoms share electrons.∗ When the surfaces are made of two different materials, the atoms in one surface often exert a stronger pull on the electrons than does the other surface. As a result, when the surfaces pull apart, electrons are stripped out of the weaker atoms by the stronger. These stolen electrons create a negative charge on one material, leaving positive “charge” (actually, a lack of charge/electrons) on the other surface. It is strictly the act of one surface touching and then not touching another surface that causes the charge transfer. So why does scuffing your feet cause charge to buildup? Neither your socks nor the carpet are perfectly smooth surfaces. When you scuff your feet across the floor you increase the number of atoms in your socks that will come in contact with the carpet. The more atoms “touch” and pull apart, the more electrons will get transferred. Similarly, when you rub a balloon through your hair, you increase the number of atoms in your hair that touch atoms on the balloon. The friction between your socks and the carpet or between your hair and the balloon has no effect on charge transfer. What matters is the amount of surface area that comes into contact. If you want to test this idea, try rubbing two balloons together. You'll find they have more friction between them than between one balloon and your hair, but because they are identical materials the atoms in each balloon have an equal hold on their electrons and no charge is transferred.

The likelihood of one atom latching onto another atom's electrons is something you deal with more often than you may think. You may have noticed the generation of static charge (and cling) when you take off your winter fleece (which is polyester). What's going on? Referring to the Triboelectric table at the right, you can predict which combinations of clothing are going to produce the greatest zap when separated.

∗ Neutral atoms will bond together to create complete shells of electrons. For a detailed

description, see: http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/bond.html

Rabbit's fur Lucite Bakelite Acetate Glass Quartz Mica Human hair Nylon Rayon Wool Cat's fur

Mos

t Pos

itive

(rea

dily

lose

ele

ctro

ns)

Silk Paper Cotton Wood Sealing wax Amber

Resins

Hard rubber Metals Polyester Polystyrene (Styrofoam) Orlon Saran Wrap Polyurethane Polyethylene Polypropylene Sulfur Celluloid Vinyl (PVC)

Mos

t Neg

ativ

e

(rea

dily

ste

al e

lect

rons

) Teflon

Triboelectric Series: Experimenters have established lists, called tribo-electric series, of the relative affinities materials have for gaining and losing electrons. By studying these lists, you can learn that rubbing wool on Styrofoam leads to negatively charged Styrofoam (and positively charged wool). Materials with similar properties (e.g. hair, wool, fur) clump together on the list and don't interact strongly. In general, objects listed near one other, like cotton and amber, interact poorly. This list's author notes, the series is reproducible only in rare circum-stances. Cleanliness, humidity, and manufacturing differences affect ordering. Adapted from Electrostatics and its Applications, edited by A.D. Moore, (Wiley & Sons, NY, 1973).


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I. Procedure 1. Stick a piece of plastic adhesive tape (Scotch Magic tape works well) about 40 cm long onto a

table top. This is your base tape. 2. Cut two 12-20 cm long pieces of tape. Create a non-sticky handle on the end of each piece by

folding over a couple cm section. These are your working strips. 3. Stick your working strips firmly to your base tape. Make sure they are in full contact with the

base tape by pressing them down firmly with your fingers. 4. Grasping their handles, briskly pull your working stripes off of the base tape (imagine you are

removing a band-aid). Letting the strips dangle freely, slowly bring the strips together. Experiment with bringing the tape together with the like sides facing each other (non-sticky to non-sticky) and the opposite sides facing each (non-sticky to sticky). What happens? How does the orientation of the tape affect what you see? What do you think is causing this effect?

5. One at a time, pass each of the working strips lightly between your fingers. Try bringing the tape back together again. Is the behavior of the tape different?

6. Carefully stick the two strips of tape together (sticky to non-sticky) so that you have a double thick piece of tape, and run your fingers down the length of the working strips.

7. Grasping one tape tab in each hand, quickly pull the strips of tape apart, repeating step 4 from this new starting configuration. Do the strips behave differently this time? Is the behavior the same or different from step 4?

8. Create four new working strips that are all about 10-cm long. 9. Create two double thick pieces of tape using your 4 new working strips. Use a pen to mark the

tabs of the top and bottom stripes in each pair so you can track which strips started on the top and bottom. (The piece with the non-sticky side exposed is the top.)

10. Quickly pull the two pairs of tape apart and test all possible combinations of bottom and top strips as you tested the strips in step 4. What do you discover?

11. At this point you do not know which strips are positive and which are negative. Using two objects from the list on page 2 (like hair and Styrofoam), create a negatively charged object.

12. Test a top and bottom piece of tape with the negatively charged object. How are the top and bottom pieces of tape charged?

13. TV and CRT computer screens tend to charge up. What type of charge (positive or negative) do you think collects on the screen?

14. Test your hypothesis with your positively and negatively charged pieces of tape. What type of charge do you find on the screen?


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II. Electrostatic Forces The electrostatic or Coulomb force between electrically charged objects is one of the four fundamental interactions of matter. Like the gravitational interaction, it has an infinite range, but unlike the gravitational interaction (where there is only one kind of mass, and the interaction is always attractive), there are two kinds of electrical charge and the force can be either attractive or repulsive. The electrostatic force between point charges is proportional to the product of the charges and falls off inversely with the square of the separation between the charges. This is true for spherically symmetric distributions of charge when the separation between their centers is much larger than the radius of the charge distribution. This relationship was determined quantitatively by Charles Augustine de Coulomb in 1785 and is known as Coulomb's law:

F = kQ1Q2

r2

!"#

$%&

where k, the Coulomb constant, has a value of about 9x109 N⋅m2/C2. It takes 6.2 !1018 electrons to create a 1 Coulomb of charge! (1 electron has a charge of 1.6 !10"19

C .) In the next activity you will test the Coulomb inverse square law for two charged Styrofoam “puffs” (sometimes called packing peanuts) and calculate the amount of charge on these puffs. In your experimental setup, you will suspend a charged packing peanut from thread. Before you start the procedure, consider what should happen. Initially, the charged packing peanut dangles straight down due to gravity. If an object with the same charge is brought near the puff, it will swing around from the charged source until the Coulomb force and gravity are balanced. In this new configuration, the puff is balanced between tension (T) from the string holding it up, gravity (mg) pulling it down, and the electrostatic force (FCoulomb) pushing it sideways. The vertical, y-component forces balance to: mg=Tcos! [Eq. 1] and the horizontal, x-component forces balance to: F

Coulomb= Tsin! [Eq. 2]

You can solve for the Coulomb force by dividing Eq. 2 by Eq. 1:

FCoulomb

mg=Tsin!

Tcos!!!"!!FCoulomb = mg tan! [Eq. 3]

For small angles tan! " sin! =x

L, (since for small angles y ! L )

Equation 3 becomes FCoulomb = mgx

L [Eq. 4]

Now the Coulomb force is given by

�

Fc = kQ1Q2

r2

!

" #

$

% & , where r is the distance

to a second puff off to the right; thus

kQ1Q2

r2= mg

x

L [Eq. 5]

Equation 5 shows that the displacement of the puff, x, is inversely proportional to r2.


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II. Procedure 1. Cut two 80-cm lengths of thread and one 20-cm length of thread. 2. Using the needle, string an 80-cm piece of thread through each puff as

shown. The puffs should be centered on the thread. 3. Select one puff to be your test puff, and the other to be your charge source.

Pull the thread tight at the base of the test puff, and stab the needle through the puff so that it creates a straight pointer at the puff's bottom (Step 3a). Use the 20-cm piece of thread to tie scissors to the second puff (Step 3b).

4. Attach both puffs to long rods (chopsticks, kabob sticks, or mixing spoons are fine) to form bi-fiber suspensions as shown (Final Setup). The suspensions should be as identical as possible so that the puffs hang at the same height.

5. Tape your test puff+rod to the edge of a table or counter several inches from one edge. Place a bright light straight in front of the puff so that the puff casts a sharp shadow on the surface behind it. Tape a ruler to that surface such that the shadow from the needle touches the 0 on the ruler.

6. Tape a second ruler to the surface behind the puff so that its 0 end is lined up with the rod on the test puff+rod. (see Final Setup).

7. Use a heavy book to hold the source rod in place. Initially, separate the two rods by about 20 cm.

8. Charge the puffs by rubbing them with fur, hair, wool, or some other electron source.


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9. After you have charged both puffs, you should notice that the test puff's needle's shadow no longer lines up with 0 on the ruler. Record the shadow's new position as well as the separation between the two rods.

10. Move the charge source puff+rod progressively closer to the test puff+rod, and repeat step 9 after each move. Your two rods should be about 1-cm apart for your last measurement. Charge can discharge (“evaporate”) from your puffs (especially on humid days), so you will need to work quickly. Working with a partner is encouraged!

11. When you are done with the initial measurements, you may want to verify that your puffs haven't discharged too much. How can you do this?

12. During the previous several steps you qualitatively measured how one charged puff moves in response to a charged source. You can use the numbers your recorded to glean a quantitative understanding of how the two puffs repelled one another. From the earlier discussion, we know the square of the separation is related to the displacement of the test puff:

kQ1Q2

r2= mg

x

L!!!!!!!

1

r2" x [Eq. 5]

13. Plot x versus 1/r2. Include error bars. Note, plotting x versus 1/r2 means that x is plotted on the vertical axsis and 1/r2 is plotted on the horizontal axsis. Try to draw a straight line that passes through all of your data points (the line should pass through the error bar associated with each data point not the point itself).

14. Equation 5 can be rewritten into the standard form for a straight line, y = mx + b with b = 0,

x =L

mgkQ1Q2

1

r2 therefore the slope of the line, m, equals

L

mgkQ1Q2 .

Since you charged the two puffs the same way, it is reasonable to assume they have equal amounts of charge on them, or Q1 = Q2 so Q1Q2 = Q2,

slope =L

mgkQ2 [Eq. 5]

15. What is the value of the slope of your line? Be sure to include units. The slope of the line is related to the amount of charge on the puffs. You can measure L and look up g and k. We have measured a handful of puffs and needles and determined puffs have an average weight of 0.05 ± 0.01grams and sewing needles typically weigh 0.12±.02 grams. Filling these values into the equation above, solve for Q2. What is the charge on each puff? How many electrons did each puff take from the fur or hair you used as an electron source? Does this number seem surprisingly large or small to you, considering the effects you observed due to the electrostatic charge?

Do not disassemble your test puff+rod until completing the next section!


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III. Charging by Induction - The Electrophorus Alessandro Count Volta is credited with inventing the electrophorus perpetuum in 1775. This practical machine allowed the (apparent) perpetual generation of charge. The principle behind it is simple. Like charge repels like charge. When a neutral object is brought near a negatively charged dielectric, the free electrons in the neutral object flow as far from the charged dielectric as they can get. If the neutral object is than touched with a conductive object connected to ground, those electrons will actually flee the neutral object, leaving it positively charged. If the neutral object is actually touched to the charged source, the electrons on the charged object will flow onto the neutral object, making it negatively charged. In this final part of the lab, you will create your own electrophorus perpetuum in a manner similar to that used by Volta. A regular Styrofoam pie plate becomes a charged dielectric when it is rubbed against your hair or a wool sweater. Combine this with an aluminum pie plate with Styrofoam-cup handle, and you're ready to “create” charge!


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III Procedure 1. Tape an upside-down Styrofoam plate to a table or counter top. The tape should only touch the

edges of the plate. This is your dielectric. 2. Tape a Styrofoam cup to the inside of an aluminum pie plate. The cup will serve as an insulating

handle for moving the charged pie plate. 3. Charge the Styrofoam plate by rubbing it with fur, or wool. 4. Untape your test puff + rod from the table and negatively charge it as you did earlier. Bring it

close to the Styrofoam plate. Is it attracted or repelled? What type of charge is on the Styrofoam plate? When you are done, hang your puff back up on the side of the table.

5. Make sure the aluminum pie plate is neutral (uncharged). Touching it with your hands should work. However, you can verify its neutrality by touching it to a water faucet, which serves as an excellent “ground”.

6. Holding on to its Styrofoam handle, move the neutral aluminum plate as close to the Styrofoam dielectric as possible without letting them touch! While keeping the plates as close together as possible, momentarily touch a finger to the top surface of the aluminum pie plate, and then raise the aluminun plate.

7. Now while touching only the handle bring the aluminum pie plate near the test puff. Is the puff attracted or repelled by the aluminum plate? What sign is the charge on the aluminum plate? Is this the same or opposite of the charge on the Styrofoam plate? Was the process used to charge the aluminum plate induction or conduction?

8. You can recharge the aluminum plate as many times as you want, as long as you don't allow the two plates to touch. The process of charging the plate requires energy, which is introduced by the work done when the aluminum plate is separated from the charged Styrofoam. Try untaping the Styrofoam plate from the table and repeating steps 5 and 6. What happens? When you are done, retape the Styrofoam plate to the table.

9. Repeat steps 3, 5-7, but this time firmly touch the plates together but don't touch the pie plate yourself. How does the test puff react? What do you notice about the amount of charge transferred this time? Was the process used to charge the aluminum plate induction or conduction?

10. Which process do you think is more efficient at transferring charge, induction or conduction? 11. Draw figures to show the motion of charges throughout this section of the experiment. Where did

the charge start? Where did it move when you held the aluminum plate near the Styrofoam? What happened when you momentarily touched the aluminum plate with your finger? What happened when you raised the pie plate away from the dielectric?

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