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Static Electricity - Lesson 2

Methods of Charging

Grounding - the Removal of a Charge

The previous three sections of Lesson 2 discussed the three common methods ofcharging - charging by friction , charging by induction , and charging by conduction .A discussion of charging would not be complete without a discussion of uncharging .Objects with an excess of charge - either positive or negative - can have thischarge removed by a process known as grounding. Grounding is the process ofremoving the excess charge on an object by means of the transfer of electronsbetween it and another object of substantial size. When a charged object isgrounded, the excess charge is balanced by the transfer of electrons between thecharged object and a ground. A ground is simply an object that serves as aseemingly infinite reservoir of electrons; the ground is capable of transferringelectrons to or receiving electrons from a charged object in order to neutralizethat object. In this last section of Lesson 2, the process of grounding will bediscussed.

To begin our discussion of grounding, we will consider the grounding of a negativelycharged electroscope. Any negatively charged object has an excess of electrons.If it is to have its charge removed, then it will have to loseits excess electrons. Once the excess electrons areremoved from the object, there will be equal numbers ofprotons and electrons within the object and it will have abalance of charge. To remove the excess of electrons froma negatively charged electroscope, the electroscope willhave to be connected by a conducting pathway to another

object that is capable of receiving those electrons. Theother object is the ground. In typical electrostaticexperiments and demonstrations, this is simply done bytouching the electroscope with one's hand. Upon contact, the excess electronsleave the electroscope and enter the person who touches it. These excesselectrons subsequently spread about the surface of the person.

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This process of grounding works because excess electrons find each otherrepulsive. As is always the case, repulsive affects between like-charged electronsforces them to look for a means of spatially separating themselves from eachother. This spatial separation is achieved by moving to a larger object that allows a

greater surface area over which to spread. Because of the relative size of a personcompared to a typical electroscope, the excess electrons (nearly all of them) arecapable of reducing the repulsive forces by moving into the person (i.e., theground). Like contact charging discussed earlier , grounding is simply anotherexample of charge sharing between two objects. The extent to which an object iswilling to share excess charge is proportional to its size. So an effective ground issimply an object with significant enough size to share the overwhelming majority ofexcess charge.

The previous discussion describes the grounding of a negatively chargedelectroscope. Electrons were transferred from theelectroscope to the ground. But what if the electroscope ispositively charged? How does electron transfer allow anobject with an excess of protons to become neutralized? Toexplore these questions, we will consider the grounding of apositively charged electroscope. A positively chargedelectroscope must gain electrons in order to acquire anequal number of protons and electrons. By gaining electronsfrom the ground , the electroscope will have a balance ofcharge and therefore be neutral. Thus, the grounding of apositively charged electroscope involves the transfer of electrons from the ground into the electroscope. This process works because excess positive charge on theelectroscope attracts electrons from the ground (in this case, a person). While thismay disrupt any balance of charge present on the person, the significantly largersize of the person allows for the excess charge to distance itself further fromeach other. As in the case of grounding a negatively charged electroscope, thegrounding of a positively charged electroscope involves charge sharing. The excesspositive charge is shared between the electroscope and the ground. And onceagain, the extent to which an object is willing to share excess charge isproportional to its size. The person is an effective ground because it has enoughsize to share the overwhelming majority of excess positive charge.

The Need for a Conducting Pathway

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Any object can be grounded provided that the charged atoms of that object have aconducting pathway between the atoms and the ground. A common lab activityinvolves taping two straws to a charged aluminum plate. One straw is covered withaluminum foil and the other straw is bare plastic. When the aluminum-covered

straw is touched, the aluminum plate loses its charge. It is grounded by means ofthe movement of electrons from the ground to the aluminum plate. When theplastic straw is touched, grounding does not occur. The plastic serves as aninsulator and prevents the flow of electrons from the ground to the aluminumplate. Grounding requires a conducting pathway between the ground and the objectto be grounded. Electrons will travel along that pathway.

Lesson 2 of this unit of The Physics Classroom has been focused on methods ofcharging and discharging objects. One principle that continually arose was therelationship between force and distance. This relationship will be explored inLesson 3.