 # Chapter 26. Electric Charges and Forces

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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.

Chapter 26. Electric Charges and Forces
Topics: Developing a Charge Model Charge Insulators and Conductors Coulomb’s Law The Field Model

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

Charge Model, Part I

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 C Electron: q = -e Proton: q = +e

Rank in order, from most positive to most negative, the charges qa to qe of these five systems.
qa = qb > qe > qc > qd qa > qe > qd > qc > qb qe > qa > qd > qb > qc qd > qc > qe > qa = qb qd > qc > qe > qa > qb STT25.2

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

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

Charging an electroscope

Charge Polarization

Charge Polarization

The Electric Dipole

The Electric Dipole

Charging by Induction, Step 1

Charging by Induction, Step 2

Charging by Induction, Step 3

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?

Charles Coulomb measured the magnitudes of electric
Coulomb’s 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 × 109 N m2/C2. K = 1/4πε0 ,ε 0 = 8.85x C2 /Nm2

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? STT25.4 FA on B > FB on A FA on B < FB on A FA on B = FB on A

Vector Nature of Electrical Forces
Electrical forces obey Newton’s 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

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

Important Concepts

The Electric Field We 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 force Fon 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.

Important Concepts

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 is to the right. to the left. zero. There’s not enough information to tell. STT25.5

Rank in order, from largest to smallest, the electric field strengths E1 to E4 at points 1 to 4.
E2 > E4 > E1 > E3 E1 = E2 > E3 = E4 E2 > E1 = E4 > E3 E2 > E1 > E4 > E3 E1 > E2 > E3 > E4 STT25.6