1 AP Physics C E & M Electric Charge & Field 20151229 - PowerPoint PPT Presentation

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AP Physics C ­ E & M Electric Charge & Field 2015­12­29 www.njctl.org 2

Electric Charge and Field Click on the topic to go to that section • Electric Charge and Atomic Structure • Conductors and Insulators • Coulomb's Law • Electric Field of Point Charges • Electric Field of Continuous Charge Distributions • Electric Dipole 3

Electric Charge and Atomic Structure Return to Table of Contents 4

Electric Charge When you take two non metallic objects, such as a plastic ruler and animal fur and rub them together, you get an interesting effect. Before they are rubbed, the plastic ruler is held over bits of paper and nothing happens. After the rubbing, the plastic ruler is held over the bits of paper and they are accelerated towards the ruler. There must be a force that was created by the rubbing of the ruler and fur together. In ancient Greece, people noticed that when thread was spun over an amber spindle, the thread was attracted to the spindle. The Greek word for amber was "elektron," hence this force was called electric. 5

Electric Charge Further experimentation showed that dissimilar materials would attract each other after rubbing, while similar materials would repel each other. These effects would not happen before the contact, and would fade after a period of time. This led to the proposal that something was being exchanged between the materials ­ later named "charge." Because there was an attractive and a repulsive force, it was further proposed that the charge came in two types. 6

Electric Charge In the 18th century, Benjamin Franklin named the two types of charge when he observed that when a rubber rod was rubbed by animal fur, the objects attracted each other. Benjamin named the charge on the rod, negative, and the charge on the fur, positive. If he named them oppositely, we wouldn't have to deal with the difference between actual current flow and the defined "conventional current." More on this in the Current and Circuits unit. Like charges repel. Opposite charges attract. 7

Conservation of Charge No new charge was created when dissimilar objects are rubbed against each other ­ it is just separated. The positive charge acquired by one object is exactly equal in magnitude and opposite in sign to the charge lost by the other object. Electric charge is conserved ­ the total amount of electric charge in a closed system is neither created nor destroyed ­ just like energy, linear momentum and angular momentum. 8

1 A plastic rod is rubbed with a piece of wool. During the process the plastic rod acquires a positive charge and the wool: A acquires an equal positive charge. B acquires an equal negative charge. Answer C acquires a smaller positive charge. B D acquires a smaller negative charge. E remains neutral. 9

2 Two pith spheres covered with conducting paint are hanging from two insulating threads. When the spheres are brought close to each other, they attract each other. What type of charge is on the spheres? After they touch, will they separate or cling together? Discuss all Since the spheres attract each other, possibilities. they have opposite charges. If the spheres have equal amounts of Answer charge, they will neutralize after touching and hang from the threads vertically. If one sphere has a larger amount of charge, they will share the charge a fter touching (same charge on each) and repel each other. 10

Atomic Structure When we rub two objects together, why does one attain a positive charge and the other one a negative charge? The answer lies in the st ructure of the atom. As you already know, an atom is comprised of electrons, protons, and neutrons. What does it look like? 11

Atomic Structure Here's a look at a Helium atom. The nucleus is buried deep within the atom and is 1,000,000 times smaller than the atom. The two protons and two neutrons are shown in red and purple ­ the width of the nucleus is 1x10 ­7 nm. The diagram shows a magnified view of the nucleus. What is the significance of the dark circle surrounded by the lighter http://commons.wikimedia.org/wiki/File:Helium_atom_QM_rev1.svg shades of gray? 12

Atomic Structure You know that Helium has two electrons ­ yet they're not shown on this picture. That's because we don't know exactly where those electrons are. We only know a probability of where they might be. The darker the shade means that it is more probable that the electrons are found within that shape. For more information, refer to http://commons.wikimedia.org/wiki/File:Helium_atom_QM_rev1.svg the Quantum Physics and Atomic Models chapter of the AP Physics 2 class. 13

Atomic Structure It has been shown that electric charges move between objects. Because they are more lightly bound to the atom than the protons and the neutrons, the electrons are the particles that move between atoms and carry this charge . The electrons are fundamental particles ­ physicists have not found any underlying structure ­ yet. http://commons.wikimedia.org/wiki/File:Helium_atom_QM_rev1.svg 14

The Electron We'll focus on the electron in this chapter as it is the source of most of the physical phenomena in the Electricity and Magnetism course. J.J.Thomson discovered the electron in 1897, and concluded it was part of the atom ­ the first evidence that the atom had an underlying structure. In a series of experiments between 1909 and 1913, Robert Millikan and his graduate student, Harvey Fletcher, established the value of the charge, "e," on an electron. 15

Measurement of Charge Millikan and Fletcher's work and subsequent experiments have established the value of "e" as 1.602 x 10 ­19 Coulombs. The charge on an electron is ­e and the charge on a proton is +e. It has also been demonstrated that this is the smallest value of charge (with the exception of quarks which were covered in the AP Physics 2 course) and all larger charges are an integral multiple of this number. Later, the electron's mass was found to be 9.1 x 10 ­31 kg. 16

3 Which of these could be the charge on an object? A 0.80 x 10 ­19 C B 2.00 x 10 ­19 C C 3.2 x 10 ­19 C Answer D 4.0 x 10 ­19 C E All of the above 17

4 Which of these could be the charge on an object? A 0.40 m C B 4.8 mC C 5.0 C Answer B D 6.5 C E All of the above 18

Conductors and Insulators Return to Table of Contents 19

Conductors A conductor is an element or material which allows the free flow of charge. If a metal sphere is given a charge Q, that charge will be uniformly distributed over the surface of the sphere. This is because the charges repel one another and the material will not hinder the charge movement so they are able to move all the way to the surface. 20

Insulator A insulator is an element or material which prevents charge from freely flowing. If a charge Q is uniformly distributed throughout the spherical insulator, the charge will remain where it is because even though the charges repel one another, the insulator prevents them from moving very far. 21

Semi­Conductor Semi­Conductors have properties of both conductors and insulators. These properties can be altered by adding small amounts of other elements. They are crucial to the operation of transistors which are then used in cell phones, computers, recording equipment, music playback devices and many other applications. Silicon and Germanium are semi­conductors. 22

5 A conducting sphere is charged with a negative charge ­Q. Which statement about the charge distribution is correct? A Charge in concentrated at the center of the sphere. Charge is concentrated at the bottom part of the B sphere. Answer D Charge is evenly distributed throughout the entire C volume of the sphere. Charge is evenly distributed on the surface of the D sphere. E More information is required. 23

6 An insulated sphere is charged uniformly with a positive charge +Q. Which statement about the charge distribution is correct? A Charge in concentrated at the center of the sphere. Charge is concentrated at the bottom part of the B sphere. Charge is evenly distributed throughout the entire Answer C volume of the sphere. C Charge is evenly distributed on the surface of the D sphere. E More information is required. 24

7 A positively charged sphere is brought near end A of the uncharged metal bar shown below. Ends A and B of the metal bar will be charged: positive, negative A B negative, positive positive, positive C Answer B negative, negative D E neutral, neutral 25