普通物理 General Physics 21 - Coulomb's Law 郭艷光 Yen-Kuang Kuo 國立彰化師大物理系暨光電科技研究所 電子郵件：ykuo@cc.ncue.edu.tw 網頁：http://ykuo.ncue.edu.tw

普通物理教材-21，國立彰化師範大學物理系/郭艷光教授 Outline 21-1 What Is Physics? 21-2 Electric Charge 21-3 Conductors and Insulators 21-4 Coulomb’s Law 21-5 Charge Is Quantized 21-6 Charge Is Conserved 2019/4/10 普通物理教材-21，國立彰化師範大學物理系/郭艷光教授

普通物理教材-21，國立彰化師範大學物理系/郭艷光教授 21-1 What Is Physics? We will introduce a new property of matter known as “electric charge” (symbol q). 2019/4/10 普通物理教材-21，國立彰化師範大學物理系/郭艷光教授

普通物理教材-21，國立彰化師範大學物理系/郭艷光教授 21-2 Electric Charge Charges of the same sign repel each other. Charges of opposite sign attract each other. 2019/4/10 普通物理教材-21，國立彰化師範大學物理系/郭艷光教授

21-3 Conductors and Insulators + Conductors are materials that allow charges to move freely through them. Examples : copper, aluminum, and mercury. Insulators are materials through which charges cannot move freely. Examples are: plastic, rubber, glass, and ceramics. 2019/4/10 普通物理教材-21，國立彰化師範大學物理系/郭艷光教授

普通物理教材-21，國立彰化師範大學物理系/郭艷光教授 21-4 Coulomb’s Law 2019/4/10 普通物理教材-21，國立彰化師範大學物理系/郭艷光教授

普通物理教材-21，國立彰化師範大學物理系/郭艷光教授 21-4 Coulomb’s Law 2019/4/10 普通物理教材-21，國立彰化師範大學物理系/郭艷光教授

普通物理教材-21，國立彰化師範大學物理系/郭艷光教授 21-4 Coulomb’s Law q2 q1 q1 q2 r F m1 m2 r F m2 m1 2019/4/10 普通物理教材-21，國立彰化師範大學物理系/郭艷光教授

普通物理教材-21，國立彰化師範大學物理系/郭艷光教授 Example 21-1 (a) Fig. a shows two positively charged particles fixed in place on an x axis. The charges are q1 = 1.60×10－19 C and q2=3.20×10－19 C, and the particle separation is R = 0.0200 m. What are the magnitude and direction of the electrostatic force on particle 1 from particle 2? 2019/4/10 普通物理教材-21，國立彰化師範大學物理系/郭艷光教授

普通物理教材-21，國立彰化師範大學物理系/郭艷光教授 Example 21-1 2019/4/10 普通物理教材-21，國立彰化師範大學物理系/郭艷光教授

普通物理教材-21，國立彰化師範大學物理系/郭艷光教授 Example 21-1 (b) Fig.c is identical to Figure (a) except that particle 3 now lies on the x axis between particles 1 and 2. Particle 3 has charge q3=3.20×10－19 C and is at a distance ¾ R from particle 1. What is the net electrostatic force on particle F1,net due to particles 2 and 3? 2019/4/10 普通物理教材-21，國立彰化師範大學物理系/郭艷光教授

普通物理教材-21，國立彰化師範大學物理系/郭艷光教授 21-4 Coulomb’s Law 2019/4/10 普通物理教材-21，國立彰化師範大學物理系/郭艷光教授

普通物理教材-21，國立彰化師範大學物理系/郭艷光教授 Example 21-1 (c) Fig.e is identical to Fig. a except that particle 4 is now included. It has charge q4=3.20×10－19 C, is at a distance ¾ R from particle 1, and lies on a line that makes an angle θ=60o with the x axis. What is the net electrostatic force on particle 1 due to particles 2 and 4? 2019/4/10 普通物理教材-21，國立彰化師範大學物理系/郭艷光教授

普通物理教材-21，國立彰化師範大學物理系/郭艷光教授 21-4 Coulomb’s Law 2019/4/10 普通物理教材-21，國立彰化師範大學物理系/郭艷光教授

普通物理教材-21，國立彰化師範大學物理系/郭艷光教授 Example 21-2 Fig. a shows two particles fixed in place: a particle of charge q1= 8q at the origin and a particle of charge q2 = 2q at x = L. At what point (other than infinitely far away) can a proton be placed so that it is in equilibrium (the net force on it is zero)? Is that equilibrium stable or unstable? 2019/4/10 普通物理教材-21，國立彰化師範大學物理系/郭艷光教授

普通物理教材-21，國立彰化師範大學物理系/郭艷光教授 21-4 Coulomb’s Law 2019/4/10 普通物理教材-21，國立彰化師範大學物理系/郭艷光教授

普通物理教材-21，國立彰化師範大學物理系/郭艷光教授 Example 21-3 In Fig. a , two identical, electrically isolated conducting spheres A and B are separated by a (center-to-center) distance a that is large compared to the spheres. Sphere A has a positive charge of＋Q, and sphere B is electrically neutral. Initially, there is no electrostatic force between the spheres. (Assume that there is no induced charge on the spheres because of their large separation.) 2019/4/10 普通物理教材-21，國立彰化師範大學物理系/郭艷光教授

普通物理教材-21，國立彰化師範大學物理系/郭艷光教授 Example 21-3 (a) Suppose the spheres are connected for a moment by a conducting wire. The wire is thin enough so that any net charge on it is negligible. What is the electrostatic force between the spheres after the wire is removed? 2019/4/10 普通物理教材-21，國立彰化師範大學物理系/郭艷光教授

普通物理教材-21，國立彰化師範大學物理系/郭艷光教授 Example 21-3 (b) Next, suppose sphere A is grounded momentarily, and then the ground connection is removed. What now is the electrostatic force between the spheres? 2019/4/10 普通物理教材-21，國立彰化師範大學物理系/郭艷光教授

普通物理教材-21，國立彰化師範大學物理系/郭艷光教授 21-5 Charge Is Quantized Mass and charge of atomic constituents Neutron (n): Mass m = 1.67510－27 kg ; 　　　　　　Charge q = 0 Note 1: We use the symbol “－e” and “＋e” for the electron and proton charge, respectively. This is known as the elementary charge. Notation: Z = 92 = number of protons/electrons A = 235 = number of protons + neutrons Ne Np Nn 2019/4/10 普通物理教材-21，國立彰化師範大學物理系/郭艷光教授

普通物理教材-21，國立彰化師範大學物理系/郭艷光教授 21-5 Charge Is Quantized Mass and charge of atomic constituents Proton (p): Mass m=1.67310－27 kg; Charge q=＋1.60210－19 C Note 2: Atoms are electrically neutral. The number of electrons is equal to the number of protons. This number is known as “ atomic number ” (symbol: Z). The chemical properties of atoms are determined exclusively by Z. Ne Np Nn 2019/4/10 普通物理教材-21，國立彰化師範大學物理系/郭艷光教授

普通物理教材-21，國立彰化師範大學物理系/郭艷光教授 21-5 Charge Is Quantized Mass and charge of atomic constituents Electron (e): Mass m = 9.1110－31 kg Charge q =－1.60210－19 C Note 3: The sum of the number of protons and the number of neutrons is known as the “ mass number ” (symbol: A) Ne Np Nn 2019/4/10 普通物理教材-21，國立彰化師範大學物理系/郭艷光教授

普通物理教材-21，國立彰化師範大學物理系/郭艷光教授 Example 21-4 The nucleus in an iron atom has a radius of about 4.0×10－15 m and contains 26 protons. (a) What is the magnitude of the repulsive electrostatic force between two of the protons that are separated by 4.0×10－15 m? 2019/4/10 普通物理教材-21，國立彰化師範大學物理系/郭艷光教授

普通物理教材-21，國立彰化師範大學物理系/郭艷光教授 21-5 Charge Is Quantized 2019/4/10 普通物理教材-21，國立彰化師範大學物理系/郭艷光教授

普通物理教材-21，國立彰化師範大學物理系/郭艷光教授 Example 21-4 The nucleus in an iron atom has a radius of about 4.0×10－15 m and contains 26 protons. (b) What is the magnitude of the gravitational force between those same two protons? 2019/4/10 普通物理教材-21，國立彰化師範大學物理系/郭艷光教授

普通物理教材-21，國立彰化師範大學物理系/郭艷光教授 21-6 Charge Is Conserved silk glass rod silk glass rod - + 2019/4/10 普通物理教材-21，國立彰化師範大學物理系/郭艷光教授

普通物理教材-21，國立彰化師範大學物理系/郭艷光教授 End of chapter 21! 2019/4/10 普通物理教材-21，國立彰化師範大學物理系/郭艷光教授