Electrostatics In this set of experiments, we separate electric charges by friction and explore the two different kinds of electric charge and how they interact.

I. Theory

Electric Charge We find two kinds of electric charge in nature • Positive (protons in the atomic nucleus all have positive electric charge) • Negative (electrons all have negative electric charge of the same size as protons)

Atoms normally have equal numbers of protons and electrons and are thus electrically neutral, or uncharged. Likewise, larger objects usually have equal numbers of negative charges (electrons) and positive charges (protons) and are neutral. If there is a slight imbalance between the number of protons and the number of electrons, then the object is electrically charged.

Electrons (usually the “outer” electrons) can be removed from an atom. For example, rubbing two different materials, say rubber and cloth, against each other can cause some electrons to move from one material to the other. Electrons are held more firmly in rubber (or plastic) than in cloth. Thus when a rubber rod is rubbed with cloth, the rubber rod does not want to give up its electrons to the cloth, so electrons transfer from cloth to the rubber, making the rubber rod negatively charged (more electrons than protons). The cloth now has a deficiency of electrons, so it is left positively charged (due to more protons than electrons). A glass rod, on the other hand, loses electrons easily compared to the cloth and is willing to give up its electrons to the cloth. If you rub the glass rod with the cloth, the glass becomes positively charged and the cloth gains electrons and becomes negatively charged. Charge is conserved in all these cases; no charge is created or destroyed, but charged particles are simply transferred from one place to another. By convention we always assume that negative charges (electrons) are transferred while positive charges remain where they were.

Electric Force Objects that have a net electric charge exert an electric force on other charged objects. Like charges repel, and opposites attract. The magnitude of the electric force depends on the amount of charge on each object and inversely on the square of the distance between the objects, as given by Coulomb’s Law. Electrostatics is the study of the forces between charges that are static (not moving). We will explore the electric force in the experiments below.

Attraction by Polarization A charged object can exert an attractive electric force on an electrically neutral object. This comes about through a process called polarization in which the charged object causes a re- arrangement of the charges in the neutral object. Say the charged object has net positive charge; it will then repel the protons and attract the electrons in the neutral object. The protons and electrons in the neutral object will re-arrange themselves such that the electrons move a little closer to the charged positively-object, leading to an attractive force between the charged object and the neutral object.

electrostatics - 1 Electrical Conductors Electrons can move around easily on metal objects since most metals are good electric conductors. If you bring a charged object near a conducting surface, you will cause electrons to move on the conductor even though there is no physical contact.

Conservation of Charge In interpreting the effects observed in this experiment, we assume that charge is a conserved quantity, in other words, the total amount of charge on an electrically isolated, or insulated, object does not change. If positive charge is attracted to one side of a neutral body, negative charge + + + - must congregate on that side leaving + + + positive charge on the other, to conserve - + + + - charge (see Fig. 1b). You will make a lot of + + + drawings in this experiment, showing + + + - where the charge is. You can’t see it, so - you have to make hypotheses using (a) (b) conservation of charge. See figure 1 for an example of drawings you will be making. Figure 1. Pith balls with (a) a net positive charge, In this experiment we will measure the sign and (b) polarization but no net charge. of charge using an electrometer. However, the sign of the charge is based on an arbitrary convention. When the first scientific studies of electrostatic charge were made, the charge left on the rubber rod after rubbing it with cat fur was called negative. Negative charge is in fact an excess of electrons, but in this experiment we have no way of relating the charges that we see to elementary particles like the electron. We will use flannel in place of cat fur; the electrostatic charges that result are the same. II. Experimental Procedure

A. Charging by Friction and Electrostatic Forces. We will demonstrate the existence of two kinds of electric charge and explore their interactions. To “charge” a rubber (or plastic) or glass rod, rub it vigorously with your hair or a piece of flannel. 1. Charge the rubber/plastic rod and hang it from a string, using the rod holder. Then charge the glass rod and bring it near to one end of the rubber rod. Q1. Explain what happens. Does the strength of the electrostatic force appear to depend on the distance between the rods? 2. Re-charge the rubber/plastic rod and then rub the full length rod all over the hanging pith ball so as to transfer a good amount of charge to the pith ball. (Do not touch the ball with your finger, which could discharge the ball.) The ball now has the same sign of charge as the rod. (It is important to get a good amount of charge on the ball. If the experiments below don’t work out as you expect, try to get a stronger charge on the ball).

Q2. Is the pith ball attracted to or repelled from the rubber rod? Why?

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Bring a glass rod that has been rubbed with cloth near the same ball without touching.

Q3. Is the pith ball attracted to or repelled from the glass rod? Why? Let the ball contact the glass rod all over. It is now charged in the same way that the glass is charged. Next bring the charged rubber rod near the ball. Q4. Describe what the ball does now. Is the pith ball attracted to or repelled from the rubber rod? Why? Charge Measurement with the Electrometer. Electric charge can be measured directly using an electrometer along with a metal ball and housing. Figure 2 shows the electrometer controls, and Fig. 3 gives some details on how we use the electrometer to measure charge.

Fig. 2. Electrometer controls. Connect the electrometer’s red alligator clip to the metal strip inside the cylindrical housing. The strip is connected to a smooth ball on top which is used to collect charge. Connect the black alligator clip to the cylindrical housing. The electrometer plus ball and housing forms an electroscope, or instrument for measuring charge. Turn on the electrometer unit using the ON/OFF button, choose a suitable voltage range (try the 30V range first), and “zero” the reading by pressing the “zero” button. Bring the charged rubber rod near the electroscope ball. Q5. What sign of charge is indicated? Do the same with the charged glass rod; what sign of charge is indicated? By doing an electrometer test similar to those above, determine if there is electricity on the hair or cloth which was used to rub the rubber rod.

electrostatics - 3 Q6. What kind of charge is on the hair or flannel used to charge the rubber rod?

electrometer enclosure sphere for charge collection

Cint ≈ 25 pF Cext V ≈ 100 pF

grounded shield

Figure 3. Diagram showing the PASCO electrometer connected to the shell and center post of an old electroscope. The two capacitances shown represent "stray capacitance" between the wire measuring charge and nearby grounded surfaces. The electrometer actually measures the voltage required to force the charge being measured onto these capacitances. (Capacitance will be treated in more detail later in the course.)

B. Attraction by Polarization Discharge the pith ball by rubbing it with your finger and thumb. (You can bring the pith ball near the electrometer ball to check that there is no significant charge on the pith ball.) Now, bring a charged rubber rod close to (but not touching!) the hanging pith ball. Make assumptions about the location of the charges. Sketch or describe what happened. Q7. What is the net charge on the pith ball? What is the sign of the net charge on the rubber rod? Is the pith ball attracted to or repelled from the rubber rod? Why? Put a charged rubber/plastic rod near a very thin running stream of water (the flow should be thin enough that it is breaking up into droplets.

Q8. What happens? Explain why this happens.

C. Charging by Induction.

1. Discharge the electroscope by pressing the zero button on the electrometer. 2. Bring a charged rubber rod near the electroscope. The meter will indicate a voltage. Why? 3. While holding the rod near the electroscope, touch the metal strip inside the cylindrical housing with your finger. The electrometer should indicate zero. 4. Now, remove your finger. 5. Finally, remove the rod. Q9. Making sketches in your lab book, indicate the location of the charges in each step of the process you just completed. Q10. What sign of charge is left on the electrometer if you switch steps 4 and 5? Why?

electrostatics - 4 Note that the electroscope is left with a net charge when the rod is removed after removing your finger. We call this process "charging by induction." Electrons are pulled in from your hand (or pushed out to your hand) by the influence of the charged rod, then become trapped on the electroscope when your finger is removed.

D. The Electrophorous.

This is a plastic square and a metal disk with an insulated handle used to illustrate charging by induction. Note that the plastic square tends to hold on firmly to any charge it acquires. 1. Rub the plastic plate vigorously with flannel (or hair). 2. Place the metal disk on top of the plate. 3. Ground (or discharge) the metal disk by touching it with your fingers (or use the neon bulb by holding one leg with your fingers and touching the other leg to the disk; look for the flash). 4. Remove your fingers (or bulb), then remove the metal disk from the plastic BY MEANS OF THE INSULATED HANDLE. 5. Discharge the disk again. 6. Repeat 2 through 5 as many times as you can note a spark or for as long as your patience lasts (don’t be too patient!). Q11. Making sketches in your lab book, indicate the location of the charges in steps 2-5 of the process you just completed: 7. Devise and carry out a method to determine the sign of the charge on the plastic and on the metal (when it is charged). Q12. The bulb will flash in steps 3 and 5 an infinite number of times. These flashes are times when energy leaves the system. When is energy put back into the system? (Remember, you can’t ever have an infinite supply of energy!)

III. Equipment Stand and one pith ball Rubber and glass rods, silk, flannel and fur Electroscope, with PASCO Basic Electrometer (ES-9078) in place of gold foil. Electrophorus Neon bulb Rod holder and string

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