The fundamental unit of charge is often represented as e. Thus, the charge on a proton is e, and the charge on an electron is e.

8 [OL]Ask students whether they have encountered conductors and insulators in their everyday lives. 295 This is shown in the bottom row of Figure 18.10. What are the properties of these materials? Click the reset button, and start with two balloons. characterize materials as conductors or insulators based on their electrical properties.

Because we can control whether semiconductors are conductors or insulators, these materials are used extensively in computer chips. The belt raises the charges up to the upper comb, where they transfer again, akin to your touching the doorknob and transferring your charge to it. Mathematically, conservation of charge can be expressed as. A positive charge and a negative charge attract each other.

In the twentieth century, however, scientists learned how to create and destroy electrons and protons, but they found that charge is still conserved. Which equation describes conservation of charge? e=+1.602

10 10 19 In the top row, a metal sphere with 100 excess electrons transfers 25 electrons to a metal sphere with an excess of 50 electrons. The right drawing shows Rutherfords model, in which the electrons orbit around a tiny, massive nucleus. By convention, we call one type of charge positive and the other type negative.

It seems like a lot, but it is quite small compared to the number of atoms in an ink droplet, which number about The charged object induces uniform negative charge on the water molecules. q The proton carries Point out how the total charge at each instant is the same. What happens when the plastic object touches the water filament?

This process continued through your whole body until a distribution of excess electrons covered the extremities of your body. and you must attribute Texas Education Agency (TEA). The upper comb has no excess electrons, and the excess electrons in the rubber belt get transferred to the comb by contact.

red where initial The number n of electrons captured by the ink droplet are. The second balloon has more negative charges than positive charges. Like charges attract, while unlike charges repel each other. The experiment involves some concepts that will be introduced later, but the basic idea is that a fine oil mist is sprayed between two plates that can be charged with a known amount of opposite charge. Slowly bring the two pieces of tape together. Just before his death in 1981, Fletcher divulged that Millikan coerced him to give Millikan sole credit for the work, in exchange for which Millikan promoted Fletchers career at Bell Labs. Slowly peel off the two pieces by pulling on the handle of the bottom piece.

endobj

The oil-drop experiment involved spraying a fine mist of oil between two metal plates charged with opposite charges. In this discussion, you may wonder how the excess electrons originally got from your shoes to your hand to create the spark when you touched the doorknob.

In the top picture (Figure 18.11(a)), the two spheres are touching, and the positive and negative charge is evenly distributed over the two spheres. What happens if an excess negative charge is placed on a conducting object? q We then approach a glass rod that carries an excess positive charge, which can be done by rubbing the glass rod with silk, as shown in Figure 18.11(b). 19

Be prepared to discuss and differentiate thermal conductors and insulators.

As soon as the excess electrons leave your shoes (by rubbing off onto the floor or being carried away in humid air), the distribution of electrons in your body returns to normal. Scientists, however devoted to logic and data they may be, are apparently just as vulnerable to this aspect of human nature as everyone else. We now know this particle as the electron. His student, Ernest Rutherford, originally believed that this model was correct and used it (along with other models) to try to understand the results of his experiments bombarding gold foils with alpha particles (i.e., helium atoms stripped of their electrons). Measuring the Fundamental Electric Charge.

q ) q q Because electrons are negative, some other part of the atom must contain positive charge.

citation tool such as, Authors: Paul Peter Urone, Roger Hinrichs. In step 9, why did they attract each other? What would be the net charge on this oil drop?

Such metals as copper, silver, and aluminum are good conductors. Two spheres, one blue and one red, initially have +4 C and +8 C of charge, respectively. 10 e The lesson here is that, although it is good to be skeptical of new results, you should not discount them just because they do not agree with conventional wisdom. Prepare a demonstration of static electricity. 10

The force is repulsive and acts along the line joining the two point charges. By touching the globe, a person gains excess charge, so his or her hair stands on end, as shown in Figure 18.13. A simple demonstration may be to charge a glass rod or comb by rubbing it with wool, silk, or other cloth and then charge an inflated balloon by rubbing it on your shirt or hair.

stream The most commonly used semiconductor is silicon. Rutherford found that most of the space occupied by the gold atoms was actually empty and that almost all of the matter of each atom was concentrated into a tiny, extremely dense nucleus, as shown by the right-side image of Figure 18.3.

q (a) The glass rod is attracted to the silk, because their charges are opposite. By knowing the mass of the oil droplets and adjusting the electric charge on the plates, the charge on the oil drops can be determined with precision. Move an edge of the charged plastic object toward the filament of running water.

How do we know that the negative charge is the mobile charge? Thus, when you walk across a carpet on a dry day, your shoes rub against the carpet, and some electrons are removed from the carpet by your shoes. The net initial charge of the system is

final

[AL]Ask students to define separation of charge. q

This saying is based on electric charge, which is a property of matter that causes objects to attract or repel each other. final (b) A positively charged rod approaches, which attracts negative charges, leaving excess positive charge on the right sphere. Millikan and Fletcher found that the drops would accumulate charge in discrete units of about However, transferring negative charge from one object to another is fairly easy to do. 1.59 Equating )vuYm[g35].

e This reveals the important psychological weight carried by preconceived notions and shows how hard it is to refute them. Perrin showed that cathode rays actually carried negative electrical charge. Because the glass and silk have opposite charges, they attract one another like clothes that have rubbed together in a dryer. 19

This type of charge separation is called polarization.

Electric charges move easily in an insulator but not in a conducting material. The left sphere now has an excess negative charge, and the right sphere has an excess positive charge.

After the two spheres interact, the blue sphere has a charge of +10 C. The law of conservation of charge allows us to find the final charge, Materials can be arranged according to their ability to conduct electric charge. This transfer happens because like charges repel, and so the excess electrons that you picked up from the carpet want to be as far away from each other as possible. Atoms, however, were known to be electrically neutral, which means that they carry the same amount of positive and negative charge, so their net charge is zero. [BL][OL]Ask how the concept of static electricity can be compatible with transfer of charge.

Thus your head, your hands, the tip of your nose, and so forth all received their doses of excess electrons that had been pushed out of their normal positions. >> q

The upper comb has excess electrons, and the excess electrons in the rubber belt get transferred to the comb by contact. Were it drawn to scale with respect to the size of the electron orbits, the nucleus would not be visible to the naked eye in this drawing.

(b) An insulating sphere with excess negative charge. 5 0 obj Thus, the comb itself never accumulates too much charge, because any charge it gains is quickly depleted by the charge moving to the outer surface of the globe. initial

Q}*Da4%6)+!`wVUj1}xG[03*sLh$>\^%9E)NdwABb@qEF&$d00 Kq2)6\ /bsSYOz,sS2a,% The Millikan oil-drop experiment is shown in Figure 18.4. 10

Most materials are insulators. Figure 18.8 shows various materials arranged according to their ability to conduct electrons. Discuss how moving electrons to the right is equivalent to moving the same magnitude of positive charge to the left, but be sure to clarify that, in most situations, only negative charges actually move in solids. This is an example of charging by induction, whereby a charge is created by approaching a charged object with a second object to create an unbalanced charge in the second object.

(a) Two neutral conducting spheres are touching each other, so the charge is evenly spread over both spheres.

6 0 obj

The same reasoning applies to the transfer of positive charge.

This is almost a billion electrons! 737 endobj

q final, 16

The numbers below the materials give their. With enough energy, it is possible to force electrons to move through an insulator. Each proton carries one unit of positive charge, and each electron carries one unit of negative charge. History shows that later measurements slowly crept up from Millikans value until settling on the modern value.

[ /ICCBased 11 0 R ] Along with many others, Thomson and Perrin were studying the mysterious cathode rays that were known at the time to consist of particles smaller than the smallest atom. The electrons cannot move, so they remain in their original positions. This technique of charging the belt is akin to charging your shoes with electrons by walking across a carpet.

The phenomenon of polarization is seen in Figure 18.1. endstream Finally, in the bottom picture, the rod is removed, and the opposite charges attract each other, so they move as close together as they can get.

An isolated system contains two objects with charges.

Because opposite charges attract each other, the negative charge is attracted to the glass rod, leaving an excess positive charge on the opposite side of the right sphere. It took several years of further experiments to confirm Thomsons interpretation of the experiments, but science had in fact discovered the particle that carries the fundamental unit of negative electrical charge.

How do we know there are two types of electric charge? 10 The number n of protons required to make +1.00 C is. The charged object attracts the polarized water molecules and ions that are dissolved in the water. << /ProcSet [ /PDF /Text ] /ColorSpace << /Cs2 8 0 R /Cs1 7 0 R >> /Font <<

Isnt transfer of charge the movement of charge, which contradicts being static?

Their atoms and molecules hold on more tightly to their electrons, so it is difficult for electrons to move between atoms.

The force is repulsive and acts tangential to the line joining the two point charges. Prepare to explain why this does not mean splitting electrons apart.

It took scientists a long time to discover what lay behind these two types of charges. q The slashes on the arrow mean that there is a very large gap in conducting ability between conductors, semiconductors, and insulators, but the drawing is compressed to fit on the page. Many experiments and solid theoretical arguments have elevated this idea to the status of a law. 1.602 atoms. inkdrop

=+4C+8C=+12C

The student is expected to: Nonconducting surface, such as a plastic table or chair. 10

(a) A conducting sphere with excess negative charge (i.e., electrons). Plastic object of small dimensions, such as comb or plastic stirrer. In a physics lab, you charge up three metal spheres, two with. The child has accumulated excess positive charge by sliding on the slide.

6

=+10C+ q q final

Explain how these terms are used in physics to mean materials that allow a quantity to pass through and those that do not. They designed what is now a classic experiment performed by students.

gives. Also, as far as science can currently detect, electrons are point particles, which means that they have no size at all! 10 Ask which type of force is at work between the balloon and the glass rod or comb (a repulsive force).

If you are redistributing all or part of this book in a print format, 19 C .

However, because positive charge essentially cannot move in solids, it is transferred by moving negative charge in the opposite direction. % q =

Figure 18.9(a) shows schematically how an excess negative charge spreads itself evenly over the outer surface of a metal sphere. First, Millikan took sole credit for the experiment and was awarded the 1923 Nobel Prize in physics for this work, although his student Harvey Fletcher apparently contributed in significant ways to the work. q consent of Rice University. Electric charge comes in two varieties, which we call positive and negative.

Another great scientist, Richard Feynman, points out that many scientists who measured the fundamental charge after Millikan were reluctant to report values that differed much from Millikans value. Open the faucet just enough to let a smooth filament of water run from the tap.

The OpenStax name, OpenStax logo, OpenStax book covers, OpenStax CNX name, and OpenStax CNX logo 16 xROHQ6Axw

This is similar to knowing how many students are on a field trip and using that information to ensure that no students go missing.

After the transfer, both spheres have 75 excess electrons. Ask students how a static charge may escape from an object. Being made of conducting material, the tins accumulate excess charge. initial

Some oil drops accumulate some excess negative charge when being sprayed and are attracted to the positive charge of the upper plate and repelled by the negative charge on the lower plate. stream 8

'P&&iD? red red q

The first such generator was built by Robert Van de Graaff in 1931 for use in nuclear physics research. For example, when glass is rubbed with silk, the glass becomes positively charged and the silk negatively charged.

xmAo0qrbUaa`ClfV-9:fd9')'KzEk:8) 5zS kX>"lxTG2Kc.T>#sO"l, *#zGu[COQnSw4OfwC:}zqLljSx_F+."~_7Y#7& p red

Except where otherwise noted, textbooks on this site

Want to cite, share, or modify this book? In addition, the High School Physics Laboratory Manual addresses content in this section in the lab titled Electric Charge as well as the following standards: You may know someone who has an electric personality, which usually means that other people are attracted to this person. 8 0 obj By the end of this section, you will be able to do the following: The learning objectives in this section will help your students master the following standards. The conductivity of some materials is intermediate between conductors and insulators.

1 nC = 109 C. Which are the two main electrical classifications of materials based on how easily charges can move through them?

final

More charges are in an insulator than in a conductor.

Similarly, two silk cloths rubbed in this manner will repel each other, because both cloths have negative charge. final In other words, although we might be able to move charge around, we cannot create or destroy it. (d) When the positively charged rod is removed, the excess negative charge on the left sphere is attracted to the excess positive charge on the right sphere.

red The word electric itself comes from the Greek word elektron for amber, because the ancient Greeks noticed that amber, when rubbed by fur, attracts dry straw.

Every part of your body is again electrically neutral (i.e., zero excess charge). In the bottom row, a metal sphere with 100 excess protons receives 25 electrons from a ball with 50 excess protons.

Point out how conservation laws serve as accounting rules that allow us to keep track of certain quantities.

Also point out that semiconductors are often made to act as insulators or as conductors, but not as materials with a conductivity that is between that of insulators and conductors. However, it is not impossible. This created pressure on Millikan and others after him that reveals some equally important aspects of human nature. The upper comb has no excess electrons, and the excess electrons in the rubber belt get transferred to the comb by conduction. stream Thus, each extra electron is shared between about q In metals, the outer electrons are loosely bound to their atoms, so not much energy is required to make electrons move through metal. The spheres are electrically neutral, so they carry the same amounts of positive and negative charge. Mathematically,

10 endobj Some of them move to the doorknob, where they will distribute themselves over the outer surface of the metal. Y9Nyx+=Y"|@5-MS%@H8qR>infObN~N>!?F?a=5`5_M'Tq.

Prepare two pieces of tape about 4 cm long. q C. Consider the two metallic spheres shown in Figure 18.11.

10 This simulation allows you to observe negative charge accumulating on a balloon as you rub it against a sweater. Because the charge can move in a conductor, it moves to the outer surfaces of the object.

is the final charge on the red sphere.

<>

What happens?

What happens if the same is done with an insulating object? All this was the result of electrons being pushed out of your feet by the excess electrons on your shoes. Point out that static buildup does not remain forever on an object. Why did they not immediately find the error and correct the value, asks Feynman.

Note that the size of the nucleus is vastly exaggerated in this drawing. q Thomson put forth what is called the plum pudding model, in which he described atoms as being made of thousands of electrons swimming around in a nebulous mass of positive charge, as shown by the left-side image of Figure 18.3.

C, which is within 1 percent of the modern value of

After an interaction (which could simply be that they touch each other), the blue sphere has +10 C of charge, and the red sphere has an unknown quantity of charge. q Materials can be classified depending on whether they allow charge to move. Electric charges move easily in a conductor but not in an insulator.

e=+1.602

q Later, Thomsons work led him to declare, I can see no escape from the conclusion that [cathode rays] are charges of negative electricity carried by particles of matter.. After passing through the electron beam, the net charge of the ink droplet is

[AL]Ask whether students recall other conductors and insulators in physics.

q 6 (5) The student knows the nature of forces in the physical world. C. and the electron carries Changes were made to the original material, including updates to art, structure, and other content updates. ( Use the law of conservation of charge to find the final charge on the red sphere.

19

Q^M g)VR4n^GK'ZW3]|1

[AL]Ask what other laws of conservation they have encountered in physics, and discuss how these laws are used.

10

1999-2022, Rice University. Like charges repel each other, and unlike charges attract each other.

<< /Type /Page /Parent 3 0 R /Resources 6 0 R /Contents 4 0 R /MediaBox [0 0 720 540] How does a conductor differ from an insulator?

16 The SI unit for electric charge is the coulomb (abbreviated as C), which is named after the French physicist Charles Augustin de Coulomb, who studied the force between charged objects. endobj Figure 18.9(b) shows this situation. This model was debated for several hundred years, but it was finally put to rest in 1897 by the work of the English physicist J. J. Thomson and French physicist Jean Perrin. In the version shown in Figure 18.12, electrons are sprayed from the tips of the lower comb onto a moving belt, which is made of an insulating material like, such as rubber. initial

10 [BL][OL]Discuss the meaning of polarization in everyday language. Because like charges repel each other, they will push against each other until they are as far apart as they can get.

final

Figure 18.7 shows two spheres that initially have +4 C and +8 C of charge. Thus, two positive charges repel each other, as do two negative charges. C, [BL]Have students define the meaning of conductor and insulator. Tapes having negative charge repel, while tapes having positive charge attract each other. 10 We recommend using a

Some electrons are captured by the ink droplet, so that it becomes charged.

and

Why does the water curve around the charged object? Discuss how thermal insulators and conductors function with regard to thermal energy.

q By tuning the charge on these plates until the weight of the oil drop is balanced by the electric forces, the net charge on the oil drop can be determined quite precisely. 12 0 obj To make a handle, double over about 0.5 cm at one end so that the sticky side sticks together. 8 0 obj . 10

initial Place the balloon on a nonconducting tabletop, and use the glass rod or comb to repel the balloon and make it roll across the tabletop.

These are called semiconductors.

10

10 Charge a first balloon by rubbing it on the sweater, and then move it toward the second balloon. The charged object depolarizes the water molecules and the ions dissolved in the water. This excess charge repels itself and so becomes distributed over the extremities of the childs body, notably in his hair.

The second metal sphere lost 25 electrons so it has 25 more excess protons, for a total of 75 excess protons.

These 25 extra electrons will electrically cancel 25 protons so that the first metal sphere is left with 75 excess protons.

However, the insulator is often physically destroyed in the process. are not subject to the Creative Commons license and may not be reproduced without the prior and express written Electrically neutral ink droplets in an ink-jet printer pass through an electron beam created by an electron gun, as shown in Figure 18.14. Compare and contrast the everyday meaning with the physics meaning.

Tapes having positive charge repel, while tapes having negative charge attract each other.

Like all conservation laws, conservation of charge is an accounting scheme that helps us keep track of electric charge. Each sphere now carries an equal magnitude of excess charge. .

q Our mission is to improve educational access and learning for everyone. q xTkA6n"Zkx"IYhE6bkEd3In6&*Ezd/JZE((b-nL~7}ov r4 Ril|Bj A4%UN$As{z[V{ww@G*q

final, The results, however, did not confirm Thomsons model but rather destroyed it! NpC+\}7^4f]z? +LonUFFC U\"(B!ceP?o

C. The charged object induces uniform positive charge on the water molecules. A metal sphere with 100 excess electrons touches a metal sphere with 50 excess electrons, so 25 electrons from the first sphere transfer to the second sphere.

What do you observe? Because they deliver relatively little electric current, they can be made safe for use in such environments. Point out how the spheres remain neutral despite being polarized in panels (b) and (c). This should be true provided that we do not create or destroy protons or electrons in our system. 1.602 stream 19 Because the charge on the electron is a fundamental constant of nature, determining its precise value is very important for all of science. C.

10

and solving for

10

endstream Attach the pieces of tape side by side onto a nonmetallic surface, such as a tabletop or the seat of a chair, as shown in, Peel off both pieces of tape and hang them downward, holding them by the handles, as shown in, Now slowly bring the two pieces of tape together, as shown in, Stick one piece of tape on the nonmetallic surface, and stick the second piece of tape on top of the first piece, as shown in. << /Length 14 0 R /N 1 /Alternate /DeviceGray /Filter /FlateDecode >> 13 0 obj q The first metal sphere has 100 excess protons and touches a metal sphere with 50 excess protons, so the second sphere transfers 25 electrons to the first sphere. }";>6+(O'58tm5Lf?/\\r &Z/^o}PW-%rOZR]|0i }JDIB`u78Y&AJ*pF)h=Lx++X)zcX}oLN(c|_AN4(^@0+E}unb|@U8gc`y~ BFC>| %PDF-1.3 16 Thoroughly rub the plastic object with a dry cloth. When various materials are rubbed together in controlled ways, certain combinations of materials always result in a net charge of one type on one material and a net charge of the opposite type on the other material. The man touching the Van de Graaff generator has excess charge, which spreads over his hair and repels hair strands from his neighbors. 19 10 red The answer is that no electrons actually traveled from your shoes to your hands.

Rubbing two surfaces together increases the transfer of electrons, because it creates a closer contact between the materials.

19

[BL][OL]Discuss what is meant by conservation in the physics sense. The positive charge is carried by the proton, which is stuck firmly in the nucleus of atoms, and the atoms are stuck in place in solid materials.

OpenStax is part of Rice University, which is a 501(c)(3) nonprofit. They can be made conductive under the right conditions, which can involve temperature, the purity of the material, and the force applied to push electrons through them. Amuse the students by pushing the balloon first in one direction and then quickly moving the glass rod or comb to the opposite side of the balloon to make it decelerate and then move in the opposite direction.

Creative Commons Attribution License , where 19 7 A glass rod becomes positively charged when rubbed with silk, whereas the silk becomes negatively charged. 2 0 obj = =1.602

then you must include on every digital page view the following attribution: Use the information below to generate a citation.

Of course, we must ensure that no external charge enters the system during the interaction and that no internal charge leaves the system. To the best precision that modern technology can provide, the charge carried by a proton is exactly the opposite of that carried by an electron. The result is that you have an excess of negative charge on your shoes. e

q Most objects we deal with are electrically neutral, which means that they have the same amount of positive and negative charge. +1.602 The upper comb has excess electrons, and the excess electrons in the rubber belt get transferred to the comb by conduction. Why dont the electrons stay on the rubber belt when they reach the upper comb?

q endobj e

(c) The spheres are separated. 10 Electric charge can be transferred in several manners. A quick Internet search will show many examples of what you can do with a Van de Graaff generator. A conductor has a large number of electrons. The original material is available at:

then you must include on every physical page the following attribution: If you are redistributing all or part of this book in a digital format,

When you then touch a doorknob, some of your excess of electrons transfer to the neutral doorknob, creating a small spark. The electrons still repel each other, but they are not able to move, because the material is an insulator.