Presentation on theme: "Chapter 26. Electric Charges and Forces"— Presentation transcript:
1 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.
2 Chapter 26. Electric Charges and Forces Topics:Developing a Charge ModelChargeInsulators and ConductorsCoulomb’s LawThe Field Model
3 Stop to think 26. 1. page 792 Stop to think 26. 2 Stop to think page 792 Stop to think page 795 Stop to think page 799 Stop to think page 805 Stop to think page 808 Stop to think page 810
6 Charges Quantization of Electric Charges The electric charge, q, is said to be quantizedq is the standard symbol used for charge as a variableElectric charge exists as discrete packetsq = NeN is an integere is the fundamental unit of charge|e| = 1.6 x CElectron: q = -eProton: q = +e
8 Rank in order, from most positive to most negative, the charges qa to qe of these five systems. qa = qb > qe > qc > qdqa > qe > qd > qc > qbqe > qa > qd > qb > qcqd > qc > qe > qa = qbqd > qc > qe > qa > qbSTT25.2
9 ConductorsElectrical conductors are materials in which some ofthe electrons are free electronsFree electrons are not bound to the atomsThese electrons can move relatively freely through theMaterialExamples of good conductors include copper, aluminumand silverWhen a good conductor is charged in a small region, thecharge readily distributes itself over the entire surface ofthe material
11 InsulatorsElectrical insulators are materials in which all of theelectrons are bound to atomsThese electrons can not move relatively freely through the materialExamples of good insulators include glass, rubber andwoodWhen a good insulator is charged in a small region, thecharge is unable to move to other regions of the material
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 theA, separate A and B, are they still neutral or charged?
22 Charles Coulomb measured the magnitudes of electric Coulomb’s LawCharles Coulomb measured the magnitudes of electricforces between two small charged spheresHe found the force depended on the charges and the distance between themIn SI units K = 8.99 × 109 N m2/C2. K = 1/4πε0 ,ε 0 = 8.85x C2 /Nm2
24 Charges A and B exert repulsive forces on each other. qA = 4qB Charges A and B exert repulsive forces on each other. qA = 4qB. Which statement is true?STT25.4FA on B > FB on AFA on B < FB on AFA on B = FB on A
25 Vector Nature of Electrical Forces Electrical forces obey Newton’s Third LawThe force on q1 is equal in magnitude andopposite in direction to the force on q2F21 = -F12With like signs for the charges, the productq1q2 is positive and the force is repulsive
27 Example 26. 4 The point of zero force Example 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
29 The Electric FieldWe begin our investigation of electric fields by postulating a field model that describes how charges interact:Some charges, which we will call the source charges, alter the space around them by creating an electric field.A separate charge in the electric field experiences a force exerted by the field.Suppose probe charge q experiences an electric forceFon 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.
31 An electron is placed at the position marked by the dot An electron is placed at the position marked by the dot. The force on the electron isto the right.to the left.zero.There’s not enough information to tell.STT25.5
32 Rank in order, from largest to smallest, the electric field strengths E1 to E4 at points 1 to 4. E2 > E4 > E1 > E3E1 = E2 > E3 = E4E2 > E1 = E4 > E3E2 > E1 > E4 > E3E1 > E2 > E3 > E4STT25.6