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Lecture 1 Ch21 - PH 222-3A Spring 2007 222 3A ELECTRIC...

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PH 222-3A Spring 2007 PH 222 3A Spring 2007 ELECTRIC CHARGE Lecture 1 Chapter 21 (Halliday/Resnick/Walker, Fundamentals of Physics 8 th edition) 1
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Chapter 21 Electric Charge In this chapter we will introduce a new property of In this chapter we will introduce a new property of matter known as “electric charge” (symbol q ). We will explore the charge of atomic constituents. Moreover, we will describe the following properties of charge: - Types of electric charge -Forces among two charges (Coulomb’s law) Forces among two charges (Coulomb s law) - Charge quantization - Charge conservation 2
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Empirically it was known since ancient times that if amber is rubbed on cloth, it acquires the property of attracting light objects such as feathers. This phenomenon was attributed to a new property of matter called “electric charge.” (Electron is the Greek name for amber.) More experiments show th t th t di ti t t f l t i h that there are two distinct types of electric charge: positive (color code: red) and negative (color code: black). The names “positive” and “negative” were given by Benjamin Franklin were given by Benjamin Franklin. When we rub a glass rod with silk cloth, both objects acquire electric charge. The sign on the charge on the glass rod is defined as positive. In a similar fashion, when we rub a plastic rod with fur both objects acquire electric charge. The sign on the charge on the plastic rod is defined as negative. 3
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Q: Do we have enough information so as to be able to determine the sign of all other charges in t ? T thi ti d nature? To answer this question we need one more piece of information. Further experiments on charged objects showed that: 1. Charges of the same type (either both positive or both negative) repel each other (fig. a ). both negative) repel each other (fig. ). 2. Charges of opposite type on the other hand attract each other (fig. b ). The force direction allows us to determine the The force direction allows us to determine the sign of an unknown electric charge. Charges of the same sign repel each Charges of the same sign repel each other. Charges of opposite sign attract each other. 4
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