 # Preview Section 1 Electric Charge Section 2 Electric Force

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Preview Section 1 Electric Charge Section 2 Electric Force
Section 3 The Electric Field

The student is expected to:
TEKS 5E characterize materials as conductors or insulators based on their electrical properties

What do you think? Suppose that after this balloon is rubbed against the girl’s hair, it is held against the wall. It will be attracted to the wall and stick to it. Explain why the balloon is attracted to the wall. Why does it eventually fall? This question is a little trickier than those on the previous slide. Why would the balloon attract the wall? Point out that the wall was not rubbed by the balloon. Is it still a case of “opposites attract”? If so, how did the balloon and wall become “opposites”?

Electric Charge There are two types of charge, positive and negative.
Like charges repel. Positive and positive Negative and negative The two balloons Opposite charges attract. Positive and negative The balloon and the hair.

Transferring Electric Charge
Atoms have smaller particles called protons (+ charge), neutrons, and electrons (- charge). Number of protons = number of electrons Atoms are neutral (no net charge). Electrons are easily transferred from one atom to another. Protons and neutrons remain in nearly fixed positions. When rubbing a balloon on your hair, electrons are attracted to the balloon and transfer. The balloon is left with excess electrons (- charge). The hair is left with an equal excess of protons (+ charge). Stress the following: Atoms are neutral because they have equal numbers of protons (+) and electrons (-). Only electrons are free to move. Positive charge occurs because electrons are lost and the positive charges remain behind. The amount of + charge always equals the amount of - charge because protons and electrons have equal charges (just opposite signs). Some materials attract electrons more strongly than others, so when contact occurs, the electrons are transferred. In this case, the balloon attracts electrons more strongly than the hair.

Millikan Oil Drop Experiment
Millikan sprayed oil drops between charged metal plates. The oil drops were negatively charged by friction. By adjusting the voltage on the plates, he could make the drops rise and fall.

Millikan’s Results Millikan found that the amount of charge on objects was always a multiple of some fundamental charge (e). In other words, charge is quantized. e turned out to be the amount of charge on an electron. e =  coulombs Coulomb is the SI unit of charge. When doing the Millikan oil drop experiment, every oil drop has a charge that is some multiple of -1.6 x C. The drops might have a charge of -3.2 x C or -9.6 x C or -4.8 x C, but there is NEVER an oil drop with a charge of x C or -7.0 x C. This suggests that charge can only come in certain values (is quantized) and those values are all multiples of the basic charge of x C.

Millikan's Oil Drop Experiment
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Coulombs and Electrons
What amount of charge does a single electron carry? How many electrons are needed to produce an amount of charge equal to C? What is the mass of this number of electrons? -1.60  C/electron 6.25  1018 electrons/C 5.69  kg about 5 billionths of a gram Try to help students understand the size of these quantities. The electrons per coulomb is simply the inverse of the coulombs per electron. It is important for students to understand the small size of electrons as well.

Conductors and Insulators
What is meant by the term electrical conductor? Provide a few examples. What is meant by the term electrical insulator? Why do conductors and insulators behave differently? Conductors allow electrons to flow freely through them. Silver, copper, aluminum, and other metals Electrons do not flow freely though insulators. Plastic, rubber, glass Outer electrons in metals are loosely bound to the nucleus and relatively free to move.

Charging by Contact Both insulators and conductors can be charged by contact. Rubbing two materials together results in a transfer of electrons. When charging metal, the charge may move through your body into the ground. The metal and your body are conductors, so the charge moves through them. You must hold the conductor with an insulating material, such as rubber gloves, to keep the charge on the metal.

Charging by Induction A charged rod is held near a metal sphere. Why do the charges in the metal arrange themselves as shown? The metal sphere is connected to the ground with a conductor. Why did some of the electrons move off the sphere? The negative charges (electrons) are repelled and the positive charges (protons) are attracted. The negatives move because it is a metal and the electrons are loosely bound. The metal sphere is grounded (touching it with your finger will do nicely). Because they are repelled by the charged rubber rod and free to move, some of the electrons move off the sphere into the ground. (Because Earth is so large, it can absorb a large number of electrons without becoming locally charged to any degree.)

Charging by Induction The conductor connecting the sphere to ground is removed. What type of net charge does the sphere now possess? The negatively charged rod is removed. Why do the charges move into the positions shown? The sphere is now positively charged because it has excess protons (due to the loss of electrons). The electrons repel each other and are attracted to the protons, so they move back into stable positions. Emphasize that charging by induction leaves the object with an opposite charge, and no contact occurs between the rod and the object being charged. This can only occur with conductors because insulators do not allow the free movement of electrons to the other side or to the ground. Additional questions to ask: Would the sphere be charged if the rubber rod was removed before the ground was removed? Why or why not? (No, because the electrons would be attracted by the extra + charges and as a result they would move back into the sphere.) Have students sketch the four diagrams over with just one change: make the rod a positively-charged glass rod. Students should show some electrons moving to the left, extra electrons entering the sphere because they are attracted to the positive charges, the ground removed, and the sphere left with extra electrons spread around evenly. The sphere is now negatively charged.

Surface Charges Why does a charged balloon stick to the wall?
A positive surface charge is induced on the wall by the negatively-charged balloon. Electrons shift within atoms due to attraction or repulsion. The insulator does not have a net charge. The diagram shows the opposite case. Why can a charged comb pick up little pieces of paper? Be sure students understand that the diagram shows a positively-charged object inducing a surface charge on a neutral object. This is opposite the behavior of the balloon and the wall because the balloon is negatively charged. Note that the positively-charged object shown in the diagram will be attracted to the negatives in the wall and repelled by the positives in the wall. Ask the students why the net effect is one of attraction. (Because the negative charges in the wall are closer, the attractive force is stronger.) The comb picks up paper pieces by the same principle. The charged comb induces charges on the surfaces of the paper pieces, and the charged pieces are then attracted to the comb. The PhET web site has two interesting simulations: Choose “Go to Simulations,” then “Electricity and Magnets,” and then choose either “Balloons and Static Electricity” or “John Travoltage.” The “Balloons and Static Electricity” simulation allows you to rub the balloon on a sweater and pick up static charge. The balloon can then be released and it will go toward the sweater (opposites attract) or toward the wall (an induced charge on wall’s surface will attract the balloon as it gets close). Once the balloon sticks to the sweater or wall you can pull it away and release it again. The “John Travoltage” simulation simply shows the build up of charge from rubbing your feet against the carpet and then the discharge into a door handle.

Now what do you think? In the top picture, the girl has rubbed the balloon on her hair, and now there is a force of attraction between them. Normally, a balloon and hair would not attract each other. What happened to each to produce this force? In the lower picture, the two balloons are repelling each other. How was this force of repulsion produced? Students should be able to explain each by discussing transfer of charge and the laws of attraction and repulsion.

Now what do you think? Suppose that after this balloon is rubbed against the girl’s hair, it is held against the wall. It will be attracted to the wall and stick to it. Explain why the balloon is attracted to the wall. Why does it eventually fall? Students should be able to explain that an induced + charge occurs on the surface of the wall, so the balloon is attracted to this surface charge.

The student is expected to:
TEKS 5A research and describe the historical development of the concepts of gravitational, electromagnetic, weak nuclear, and strong nuclear forces 5C describe and calculate how the magnitude of the electrical force between two objects depends on their charges and the distance between them 5D identify examples of electric and magnetic forces in everyday life

Coulomb’s Law The force between two charged particles depends on the amount of charge and on the distance between them. Force has a direct relationship with both charges. Force has an inverse square relationship with distance. Discuss the similarity between this equation and the equation for gravitational force. Both are inverse square relationships with distance. The conversion factor, kC, is discussed on the next slide.

Coulomb’s Law Use the known units for q, r, and F to determine the units of kc. kc = 8.99  109 N•m2/C2 The distance (r) is measured from center to center for spherical charge distributions. The Coulomb constant is needed to convert the units on the right (C2/m2) into units for force, or newtons.

Classroom Practice Problem
The electron and proton in a hydrogen atom are separated, on the average, a distance of about 5.3  m. Find the magnitude of both the gravitational force and the electric force acting between them. Answer: Fe = 8.2  10-8 N, Fg = 3.6  N The electric force is more than 1039 times greater than the gravitational force. Atoms and molecules are held together by electric forces. Gravity has little effect.

Classroom Practice Problem
A balloon is rubbed against a small piece of wool and receives a charge of C while the wool receives an equal positive charge. Assume the charges are located at a single point on each object and they are 3.0 cm apart. What is the force between the balloon and wool? Answer: 3.6 N attractive Remind students to be careful with units. The must convert micro-coulombs and centimeters into units appropriate for kC.

Superposition Principle
The net force on a charged object is the sum of all of the forces due to other charged objects. Charge q3 shown has two forces acting on it. q2 pulls to the left. q1 pushes up and to the right. The vector sum is shown in the lower diagram.

Classroom Practice Problem
Two charges, q1 and q2, lie on the x-axis. The first charge is at the origin and the second charge is at x = 1.0 m. Determine the force on a third charge, q3, placed at x = 0.75 m. The charges are as follows: q1 = +10.0C , q2 = +7.5C, q3 = -5.0C Answer: Fleft = 0.80 N and Fright= 5.4 N, so Fnet = 4.6 N to the right

Electric Force Like gravity, the electric force is a field force.
Similarities Both forces are related to distance in the same way. Differences Two types of charge and only one type of mass Electric forces can attract or repel while gravity only attracts. Electric forces are far stronger than gravitational forces.

Coulomb’s Apparatus Coulomb developed his law using a torsion balance like that shown. He measured the force between the two charged spheres by the amount of twisting in the wire. Point out the similarities between this apparatus and that used by Cavendish to find the value for G (covered in the chapter “Circular Motion and Gravitation”).

Now what do you think? Electric forces and gravitational forces are both field forces. Two charged particles would feel the effects of both fields. Imagine two electrons attracting each other due to the gravitational force and repelling each other due to the electrostatic force. Which force is greater? Is one slightly greater or much greater than the other, or are they about the same? What evidence exists to support your answer? The electrical force of repulsion would completely overwhelm the gravitational force of attraction. It is significantly greater. Students may cite the Classroom Practice Problem (from slide 4) as evidence. The gravitational force is insignificant at the atomic level; atoms and molecules are held together by electric forces.

The student is expected to:
TEKS 5C describe and calculate how the magnitude of the electrical force between two objects depends on their charges and the distance between them 5E characterize materials as conductors or insulators based on their electrical properties

What do you think? In the chapter “Circular Motion and Gravitation,” you learned about the gravitational field (g). The diagram shows the “g” field around Earth. In this section, we will study the electric field (E) around charged particles. On the next slide are three different diagrams. Make a sketch of the “E” field for each charge or combination of charges. When asking students to express their ideas, you might try one of the following methods. (1) You could ask them to write their answers in their notebook and then discuss them. (2) You could ask them to first write their ideas and then share them with a small group of 3 or 4 students. At that time you can have each group present their consensus idea. This can be facilitated with the use of whiteboards for the groups. The most important aspect of eliciting student’s ideas is the acceptance of all ideas as valid. Do not correct or judge them. You might want to ask questions to help clarify their answers. You do not want to discourage students from thinking about these questions and just waiting for the correct answer from the teacher. Thank them for sharing their ideas. Misconceptions are common and can be dealt with if they are first expressed in writing and orally. Remind students that fields are representations of forces at a distance. The electric fields are not as simple as the gravitational fields because there are two types of charge. Let the students try to make their best predictions. They will probably struggle a little trying to decide how to make the E field around the negative object different from that around the positive object and then struggle even more with the + and - object combination.

What do you think? Make a sketch of the “E” field for each charge or combination of charges. How are your sketches similar? How are they different? Explain.

Electric Field Strength
Electric fields (E) have magnitude and direction. The direction is defined as the direction of the force on a small, positive test charge (q0) placed in the field caused by Q. The magnitude of the field is defined as the force per unit charge on q0. Point out that the E field is produced by Q and creates a force on q0. Point out the SI unit is N/C. It is not obvious in this equation, but the size of q0 does not change the size of E. That is because a larger value for q0 will increase the Felectric also, and the ratio of the two will remain unchanged. You can return to this concept when the equation for the electric field strength due to a point charge is introduced (slide 6).

Electric Fields and Test Charges
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Test Charges A small test charge will not significantly affect the field. If the test charge (q0) is large, it will affect the way the charges are distributed on the charged conductor. This would change the field around the conductor. Be sure students understand that in diagram (b) the charges in the conducting sphere causing the E field have rearranged because of the large test charge near it. Test charges will always be considered small enough to have no effect on the field.

Electric Field Strength
Combine Coulomb’s law with the definition of electric field to derive an equation for E due to a point charge. SI unit: N/C The field strength does not depend on the test charge. Allow students some time to work out the equation before showing it on the slide. They will see that the field strength does not depend on the test charge because this value cancels from the equation.

Sample Electric Field Strengths
The electric field in a fluorescent tube causes the electrons to move through the tube, and this causes the gas inside the tube to glow. The electric field under a thundercloud just before the lightning bolt is sometimes strong enough to induce a charge in your hair, which will cause your hair to stand on end.

Classroom Practice Problems
An electric field around a charged object is  106 N/C at a distance of m. Find the charge on the object. Answer: 6.62  10-6 C or 6.62 C Suppose a small test charge of C was placed at the point that is m from the charged object. What force would be exerted on the test charge and on the object? Answer: 1.19 N for both test charge and object For problems, it is a good idea to go through the steps on the overhead projector or board so students can see the process instead of just seeing the solution. Allow students some time to work on problems and then show them the proper solutions. Do not rush through the solutions. Discuss the importance of units at every step. Problem solving is a developed skill and good examples are very helpful. Students do not need to use Coulomb’s law to calculate the answer to the second question. Felectric = Eq0 is much simpler.

Classroom Practice Problems
A charge q1 = 4.50 C experiences an attractive force of 1.35 N at a distance of m from a charged object, q2. Find the strength of the electric field due to q2 at a distance of m from q2. Answer: 3.00  105 N/C Find the charge, q2. Answer: C For problems, it is a good idea to go through the steps on the overhead projector or board so students can see the process instead of just seeing the solution. Allow students some time to work on problems and then show them the proper solutions. Do not rush through the solutions. Discuss the importance of units at every step. Problem solving is a developed skill and good examples are very helpful. Help students with the two equations for electric field. There will likely be confusion about the charge on the object “causing” the field and the charge on the object “in the field.” Sometimes a sketch is helpful if students write the amount of charge next to each object. Point out that a field exists without a test charge. However, the force requires two or more charges.

Electric Field Lines - Rules
Apply the above rules and sketch the E field around the charge shown. After giving students some time to draw the field, show them the second diagram. Show how the rules were applied to create this diagram.

Electric Field Lines - Rules
Apply the above rules and sketch the E field around the charge shown. After giving students some time to draw the field, show them the second diagram. Show how the rules were applied to create this diagram.

Electric Field Lines - Rules
Apply the above rules and sketch the E field around the charge combination shown. After giving students some time to draw the field show them the next slide, which has a diagram of the field. Show how the rules were applied to create this diagram.

Electric Field Lines - Rules

Electric Field Lines - Rules
Apply the above rules and sketch the E field around the charge combination shown. After giving students some time to draw the field show them the next slide, which has a diagram of the field. Show how the rules were applied to create this diagram.

Electric Field Lines - Rules

Rules for Drawing Electric Field Lines
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Rules for Sketching Fields Created by Several Charges
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Electrostatic Equilibrium
Electrostatic equilibrium occurs in conductors when no net motion of charges exists within the conductor. Charges in a conductor are free to move, but are not moving when equilibrium exists. The rules below result from this fact. If the four rules stated were violated, the charges within the conductor would move.

Now what do you think? What is an electric field?
When sketching electric fields, what information is conveyed by the direction of the field lines? When sketching electric fields, what information is conveyed by the density of the field lines? Why must electric field lines just outside a conductor be perpendicular to the conductor? E fields show the direction of the force. The lines show the direction of the force on a positive test particle. The density of the lines represents the strength of the force. If the field lines were not perpendicular to the conductor, there would be a component along the surface. This would cause the charges to move until there was no component of the force along the surface.