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# Electrostatics Notes

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This was from our Advanced Summer Class, Physics IV. It's about electrostatics.

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### Electrostatics Notes

1. 1. ELECTROSTATICS NOTESThere are two types of charges: positive and negative. Like charges repel and oppositecharges attract.There exist in nature two types of charges: positive and negative. The force between them is suchthat like charges repel and opposite charges attract. For example, two positive charges broughtnear each other will be pushed away from each other. On the other hand, one positive charge andone negative charge brought near each other will be pulled towards each other. It is this principleon which all electrostatics is based.In general, a material is either a conductor or an insulator. A conductor allows electriccharge to travel through it easily; an insulator does not.Each type of material has a different arrangement of atoms, electrons, and protons. The particulararrangement of a materials atoms can be very advantageous for electric charge to travel throughit. Metals, for instance, are very good conductors. Charges can flow through a metal with verylittle resistance. The particular arrangement of a materials atoms can by disadvantageous,however. Rubber, for instance, has its atoms arranged such that it is very difficult for electriccharge to flow through it. Rubber is considered to be an insulator.A persons body acts as a conductor.Surprisingly, a persons body allows electric charge to travel through it easily. It is for this reasonthat one must be careful not to plug in the radio while in the bathtub: electric charges from theoutlet can run through your body, electrocuting you.When certain types of materials are rubbed against other certain types, charge may betransferred from one to the other.Each object contains charge. For most objects, the number of negative charges equals the numberof positive charges, giving a net charge of zero. These charges are free to move, however. Whencertain types of materials are rubbed against each other, the materials atoms are arranged suchthat some of the charges from one material will be transferred to the other material. This givesboth objects a net charge. One object will be positively charged, the other will be negativelycharged. For example, when a teflon rod is rubbed against silk, charge is transferred.When an uncharged object is placed near a charged object its charges rearrangethemselves. Those charges attracted to the charged object move towards the charged objectand those charges repelled move away. This effect is known as polarization.As discussed before, most objects are uncharged. An uncharged object means that the totalnumber of positive charges equals the total number of negative charges. The net charge on the
2. 2. object is zero. When an uncharged object is polarized as described above, its net charge remainszero. No additional charge is put on the object. What does happen, however, is the charges insidethe object rearrange themselves. When a negatively charged object, for instance, is placed nearan uncharged object, the negative charges move away from the charged object and the positivecharges move towards the charged object. It is this effect that makes the uncharged object act asif it is charged.Charges on a conductor tend to gather at sharp points.For several complex reasons, the charges on a conductor will gather at sharp points. A metalcone, for instance, which has been charged will have a lot of charge at the point, and much lesscharge elsewhere.F = kq1q2/r2, where F = electrostatic force, k = Coloumbs constant, q1 =first charge, q2 = second charge, and r = the distance between the twocharges.As can be seen from the formula, the force between two charged objectsdepends on the amount of charge and the distance between the charges. Itshould be noted that a negative force using the equation above indicates thatthe force is attractive, and a positive force indicates that it is repulsive.Each type of material allows electric charge to flow through it to acertain extent. The measure of how receptive a material is to electriccurrent is called the conductivity. Those materials with a highconductivity are called conductors. Those with a low cuductivity arecalled insulators.All materials have a certain conductivity. The conductivity of a materialdepends on the materials arrangement of free electrons. Below is a chart ofsome common materials and their conductivities.Material Electrical Conductivity (1/(ohm*m))Silver 60 x 106Copper 60 x 106Lead 5 x 106Wood 10-8- 10-14Glass 10-10- 10-14Hard Rubber 10-13- 10-16Values taken from Tipler, Paul A. Physics, Third Edition. 1991.The human body acts as a conductor.
3. 3. The human body has a large enough conductivity so that it is considered to bea conductor. For this reason, it is ill-advised to plug in a radio while taking abath. Dr. Thornton, do you know a ballpark figure for a personsconductivity?When certain types of materials are rubbed against other certaintypes, charge may be transferred from one to the other. The amount andtype of charge is determined by the Triboelectric Series.The Triboelectric Series is a list of materials which determines whichmaterials will become positively charged and which will become negativelycharged when rubbed together. A sample portion of the Series is shownbelow. To read the chart, those materials nearer the top will becomepositively charged and those nearer the bottom will become negativelycharged. For instance, when Teflon is rubbed against silk, the Teflonbecomes negatively charged and the silk becomes positively charged.Positive ChargeHuman hairNylonWoolSilkPaperCottonWoodHard rubberTeflonNegative ChargeAn uncharged object placed near a charged object becomes polarized.As discussed before, most objects are uncharged. An uncharged object means that the totalnumber of positive charges equals the total number of negative charges. The net charge on theobject is zero. When an uncharged object is placed near a charged object, its net charge remainszero. No additional charge is put on the object. What does happen, however, is the charges insidethe object rearrange themselves. When a negatively charged object, for instance, is placed nearan uncharged object, the negative charges move away from the charged object and the positivecharges move towards the charged object. It is this effect that makes the uncharged object act asif it is charged.
4. 4. Charges on a conductor tend to gather at sharp points.Due to the properties of electric fields and conductors, the charges on a conductor will gather atsharp points. A metal cone, for instance, which has been charged will have a lot of charge at thepoint, and much less charge elsewhere.ElectrostaticsElectrostatics, as the name implies, is the study of stationary electric charges. A rod of plasticrubbed with fur or a rod of glass rubbed with silk will attract small pieces of paper and is said tobe electrically charged. The charge on plastic rubbed with fur is defined as negative, and thecharge on glass rubbed with silk is defined as positive.Electric chargeElectrically charged objects have several important characteristics:Like charges repel one another; that is, positive repels positive and negative repels negative.Unlike charges attract each another; that is, positive attracts negative.Charge is conserved. A neutral object has no net charge. If the plastic rod and fur are initiallyneutral, when the rod becomes charged by the fur, a negative charge is transferred from the furto the rod. The net negative charge on the rod is equal to the net positive charge on the fur.A conductor is a material through which electric charges can easily flow. An insulator is amaterial through which electric charges do not move easily, if at all. An electroscope is a simpledevice used to indicate the existence of charge. As shown in Figure 1, the electroscope consistsof a conducting knob and attached lightweight conducting leavesâ€”commonly made of gold foilor aluminum foil. When a charged object touches the knob, the like charges repel and force theleaves apart. The electroscope will indicate the presence of charge but does not directly indicatewhether the charge is positive or negative.