Physics 2020 Lab: Charges and Electrostatics page 1 of 8Lab: Charges and ElectrostaticsINTRODUCTION:Most modern applications of electricity involve electric charges in motion orcurrent electricity. Historically, however, the first studies of electricity involved staticcharges, or electrostatics. You certainly feel the effects of electrostatic chargesevery time you touch a doorknob in the wintertime and get zapped.In this lab you will be introduced to an instrument called an electroscope, which isused to measure electric charge. By the end of this lab you should understand howthe electroscope physically works, you should know how to use it to measure electriccharge, and you should be able to use it as a tool to discover some electricalproperties of various plastic materials.THE ELECTROSCOPE:The instrument that you will use in this lab is called anelectroscope, shown in Figure 1. The electroscope consistsof a metal ball connected by a metal rod (the stem) to a verythin leaf of gold foil. The fragile part of the instrument isenclosed in a protective case. Electrons can flow freelywithin the ball, stem, and gold leaf. The leaf is verylightweight and floppy, so that an electrical repulsive forceacting to lift it up can easily overcome the downward forceof gravity.PRECAUTIONS & NOTES:- Never tilt the electroscope or turn it upside down. This can tear the foil off. Just leave it on the table.- When you bring a charged object near the electroscope, do so from thetop. Do not bring any charged object near the glass sides, as this can twist thegold foil and rip it off.- In order to ground the electroscope (that is, remove all the net charge fromit so it’s neutral), touch the metal grounding plate to the metal ball on top of theelectroscope. Touching your hand to the top of the electroscope can sometimeswork, if you do not have any charge on your hands.- Before charging and testing the plastic rods, make sure that they areneutral first. You can ground the plastic rods by covering the metal groundingplate with a damp towel and wipe the rods across the cloth. Always test the rodwith the electroscope to make sure it is neutral first.- If you rub two objects together to test the charge on each, make sure youhold the side that was rubbed near to the metal ball on the electroscope.University of Colorado at Boulder, Department of Physicsball metal stem gold case glass side wall leaf Fig.1. ElectroscopePhysics 2020 Lab: Charges and Electrostatics page 2 of 8PART I: USING THE ELECTROSCOPEThe simplest way to obtain an electric charge is to take almost any two materialsand rub them together. Often, one of the materials becomes negatively charged andthe other positively. For example, if you scuff the rubber soles of your shoes along acarpet, you will acquire some of the electrons from the rug. Your body will then havean excess of electrons – to get rid of these, just touch the nearest doorknob or metalbookcase. A similar effect occurs if you rub a teflon rod with a woolen cloth; theteflon rod acquires a negative charge and the wool acquires an equal amount ofpositive charge.We know that all matter contains positive and negative charges. The positivecharge is deep inside each atom in the atomic nucleus, while the negative charge ison the electrons that orbit outside the nucleus. In normal, neutral matter the amountsof positive and negative charge are equal, and we are unaware of either. Given anytwo different materials, it usually happens that one has a greater attraction, oraffinity, for electrons than the other; when the two are rubbed together, the materialwith higher affinity captures some electrons from the material with lower affinity. Forexample, teflon has higher electron affinity than wool; when they are rubbed togetherthe teflon captures a few electrons from the wool; the teflon therefore acquires anexcess of electrons and is negatively charged, while the wool is left with a deficit ofelectrons and is therefore positively charged.It is important to note here the difference between insulators and conductors:electrons can move freely within conductors (such as metal), but don’t move veryeasily on insulators (like cloth or plastic rods). The preceding paragraph applies toinsulators in this lab.Make sure there is no charge on the electroscope so that its leaf hangs straightdown as in Fig. 2a. Now rub the teflon rod with the wool and bring the rod close to(but not touching) the ball of the electroscope, causing the leaf to rise. Using Fig. 2as a guide, explain what happens.University of Colorado at Boulder, Department of PhysicsPhysics 2020 Lab: Charges and Electrostatics page 3 of 8What forces are acting on the leaf?Fig. 2. a) If the electroscope is neutral and no charges are close by, the leaf willhang straight down. b) A charged rod close to the ball causes the leaf to rise.Take away the teflon rod, and bring the wool cloth close to (but not touching) the ball.(Because the charge is spread over the cloth, it is harder to get the leaf to rise thisway.) Draw a diagram similar to Fig. 2b and explain what happens.Using just a neutral electroscope as you did here, can you tell the sign of thecharge which is close to the ball? If so, how?Take a charged rod and touch it to the metal ball on the electroscope, then take therod away. Using one or more diagrams similar to Fig. 2b, explain what happens.University of Colorado at Boulder, Department of Physics+ + + + (a) (b)Physics 2020 Lab: Charges and Electrostatics page 4 of 8Charge the electroscope negatively by touching the ball with the negatively chargedrod. Find out and explain how you can increase the amount of charge left on theelectroscope. How can you tell that more charge has been left?Now (with the electroscope charged negatively) bring the negative rod close to, butnot touching, the ball. Do the same with the positive wool. In each case record andexplain clearly (with diagrams like Fig. 2) what happens.With the electroscope negatively charged, can you tell the sign of the charge whichis close to the ball? If so, how?University of Colorado at Boulder, Department of PhysicsPhysics 2020 Lab: Charges and Electrostatics page 5 of 8The electroscope can be charged positively by using a negatively charged teflon rod,as follows: Neutralize the
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