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Chapter 26. Electric Charges and Forces The electric force is one of the fundamental forces of nature. Controlled electricity is the cornerstone of our modern, technological society. Chapter Goal: To develop a basic understanding of electric phenomena in terms of charges, forces, and fields. Slide 2 Topics: Developing a Charge Model Charge Insulators and Conductors Coulombs Law The Field Model Chapter 26. Electric Charges and Forces Slide 3 Stop to think 26.1page 792 Stop to think 26.2page 795 Stop to think 26.3page 799 Stop to think 26.4 page 805 Stop to think 26.5page 808 Stop to think 26.6page 810 Slide 4 Charge Model, Part I Slide 5 Slide 6 Charges Quantization of Electric Charges The electric charge, q, is said to be quantized q is the standard symbol used for charge as a variable Electric charge exists as discrete packets q = Ne N is an integer e is the fundamental unit of charge |e| = 1.6 x 10-19 C Electron: q = -e Proton: q = +e Slide 7 Slide 8 Rank in order, from most positive to most negative, the charges q a to q e of these five systems. A. q a = q b > q e > q c > q d B. q a > q e > q d > q c > q b C. q e > q a > q d > q b > q c D. q d > q c > q e > q a = q b E. q d > q c > q e > q a > q b Slide 9 Conductors Electrical conductors are materials in which some of the electrons are free electrons Free electrons are not bound to the atoms These electrons can move relatively freely through the Material Examples of good conductors include copper, aluminum and silver When a good conductor is charged in a small region, the charge readily distributes itself over the entire surface of the material Slide 10 Slide 11 Insulators Electrical insulators are materials in which all of the electrons are bound to atoms These electrons can not move relatively freely through the material Examples of good insulators include glass, rubber and wood When a good insulator is charged in a small region, the charge is unable to move to other regions of the material Slide 12 Slide 13 Charging an electroscope Slide 14 Charge Polarization Slide 15 Slide 16 The Electric Dipole Slide 17 Slide 18 Charging by Induction, Step 1 Slide 19 Charging by Induction, Step 2 Slide 20 Charging by Induction, Step 3 Slide 21 Metal spheres A and B are initially neutral and are touching. A positively charge rod is brought near A, but not touching. Is A now positive, Negative or neutral. If keeping rod near the A, separate A and B, are they still neutral or charged? Slide 22 Coulombs Law Charles Coulomb measured the magnitudes of electric forces between two small charged spheres He found the force depended on the charges and the distance between them In SI units K = 8.99 10 9 N m 2 /C 2. K = 1/4 0, 0 = 8.85x10 -12 C 2 /Nm 2 Slide 23 Slide 24 Charges A and B exert repulsive forces on each other. q A = 4q B. Which statement is true? A. F A on B > F B on A B. F A on B < F B on A C. F A on B = F B on A Slide 25 Vector Nature of Electrical Forces Electrical forces obey Newtons Third Law The force on q1 is equal in magnitude and opposite in direction to the force on q2 F21 = -F12 With like signs for the charges, the product q1q2 is positive and the force is repulsive Slide 26 Slide 27 Example 26.4 The point of zero force. Two positively charged particles q1 and q2 = 3q1 are 10 cm apart. Where(other than at infinity) could a third charge q3 be placed so as to experience no net force. From the figure, you can see: At point A, above the axis, and at B, outside the charges, cannot possibly add to zero. However, at point C on the x-axis between the charges, the two forces are oppositely directed Slide 28 Important Concepts Slide 29 The Electric Field We begin our investigation of electric fields by postulating a field model that describes how charges interact: 1.Some charges, which we will call the source charges, alter the space around them by creating an electric field. 2.A separate charge in the electric field experiences a force exerted by the field. Suppose probe charge q experiences an electric force F on q due to other charges. The units of the electric field are N/C. The magnitude E of the electric field is called the electric field strength. Slide 30 Important Concepts Slide 31 An electron is placed at the position marked by the dot. The force on the electron is A.to the right. B.to the left. C.zero. D.Theres not enough information to tell. Slide 32 A. E 2 > E 4 > E 1 > E 3 B. E 1 = E 2 > E 3 = E 4 C. E 2 > E 1 = E 4 > E 3 D. E 2 > E 1 > E 4 > E 3 E. E 1 > E 2 > E 3 > E 4 Rank in order, from largest to smallest, the electric field strengths E 1 to E 4 at points 1 to 4.