Chapter 17: Electrostatics Methods of Charging Coulomb’s Law Electric Fields

Types of Electrical Charge: two kinds positive (+) negative (-) unlike attract like repel there is a force (FE) between electrically charged particles called the electrostatic force

the study of electrical charges at rest Electrostatics: the study of electrical charges at rest

Charged Objects: an atom is electrically neutral same number of protons (+) as electrons (-) objects are charged by adding/removing electrons charge is quantized: all charges occur as a multiple of the fundamental charge (e) e = 1.60210-19 C

Charge Is Conserved: no charge is created or destroyed negative charge a buildup of electrons more electrons than protons classical definition charge accumulated by a glass rod rubbed with silk or wool positive charge a loss of electrons fewer electrons than protons a hard, rubber rod rubbed with fur

Vocabulary: Conductor Insulator a substance that allows electrons to move easily throughout metals are good conductors because freely-flowing electrons metallic bonds Insulator a substance that does not allow electrons to move freely (electrons stay in one place) glass and plastic

Vocabulary: Semiconductor Superconductor Electroscope partially conductive, partially insulative most metalloids Superconductor as temperatures decreases, conductivity increases close to 0K, conductivity is infinite some metals do this closer to 100K ! Electroscope an instrument used to detect the presence of an electrostatic charge

Electroscope:

Methods of Charging: Conduction a charged object touches another object charge divides equally same sign

Methods of Charging: Induction a charged object is brought near, but not touching, another object opposites attract and likes repel with a ground, the other object acquires an opposite charge

Methods of Charging: Polarization a charged object is brought near an insulator if the compound is polar there will be an attraction/repulsion ex. water

Coulomb’s Law: describes the electrostatic force between two charged objects (another inverse square law relationship!)

Coulomb’s Law: , called the Coulomb constant q is the magnitude (NO sign) of each charge (C) r is the center-to-center distance (m) F is the electrostatic force (N)

Coulomb’s Law: attractive or repulsive your job to decide which direction toward/away “radially”

Charge: charges are small usually expressed in non-SI units microcoulombs 1C = 110-6C nanocoulombs 1 nC = 110-9 C picocoulombs 1 pC = 110-12 C convert for calculations

Elementary Charge (qo): the charge of either a proton or an electron charge on protons/electrons equal in magnitude opposite signs qo =  1.60210-19 C

Coulomb's Law Problems: don’t forget to convert… C and cm are not SI forces are vectors… Coulomb's Law only calculates the magnitude no +/- signs electrostatic force between two charges free body diagrams determine the direction opposites attract, likes repel

Coulomb's Law Problems:

More Than Two Charges: draw a force diagram add the vectors Law of Superposition draw a force diagram add the vectors calculate the resultant force

Law of Superposition: Step 1: Draw a picture. Example: A sphere with a charge of 6.0C is located near two other charged spheres. A –3.0C charge is located 4.00cm to the right and a 1.5C charge is located 3.00cm directly underneath. Determine the net force on the 6.0C sphere. Step 1: Draw a picture.

Law of Superposition: Step 2: Example: A sphere with a charge of 6.0C is located near two other charged spheres. A –3.0C charge is located 4.00cm to the right and a 1.5C charge is located 3.00cm directly underneath. Determine the net force on the 6.0C sphere. Step 2: Determine which forces are attractive and repulsive. 6.0C -3.0C 4.00cm 3.00cm 1.5C

Law of Superposition: Step 3: Example: A sphere with a charge of 6.0C is located near two other charged spheres. A –3.0C charge is located 4.00cm to the right and a 1.5C charge is located 3.00cm directly underneath. Determine the net force on the 6.0C sphere. Step 3: Determine each force using Coulomb's Law. 6.0C -3.0C 4.00cm 3.00cm 1.5C

Law of Superposition: Step 4: Example: A sphere with a charge of 6.0C is located near two other charged spheres. A –3.0C charge is located 4.00cm to the right and a 1.5C charge is located 3.00cm directly underneath. Determine the net force on the 6.0C sphere. Step 4: Draw a force diagram of the two forces and determine the resultant force.

More Practice Problems: A charge of 3.0C (q1) is near two charges, a -2.0C charge (q2) 0.050m to the north and a -4.0C charge (q3) 0.030m to the south. What total force is exerted on q1? Three particles are placed in a line. The left particle has a charge of –67C, the middle particle +45C, and the right particle -83C. The middle particle is 2cm from each of the others. Determine the net force acting on the middle particle. the right particle.

Electric Fields: exists in the space surrounding a charged object exerts a force on nearby charged objects force decreases as distance increases

Electric Fields: vector quantity magnitude given by equation at right force per unit charge SI unit is N/C also V/m direction given by direction of FE on a test charge positive test charge

Test Charge: a (+) charge of very small magnitude determines direction of effect on is negligible diagram on pg 643

Electric Field Strength: E is electric field (N/C) k is Coulomb’s constant (9x109 Nm2/C2) d is center-to-center distance (m)

Electric Field Direction: If q is positive field is “radially outward” If q is negative field is “radially inward” In other words… field lines travel FROM positive TO negative

Sample Problem: An electric field around a charged object is 5.95106 N/C at a distance of 10.0cm. Find the charge on the object.

Electric Field Lines: direction is that of a positive test charge The electric field can be represented with electric field lines. direction is that of a positive test charge from (+) to (-) density is a measure of the strength of # of lines 

Rules For Drawing: Electric field lines begin on (+) charges or  end on (-) charges or  The number of lines drawn is proportional to the magnitude of the charge. leaving a positive charge or approaching a negative charge Field lines may not cross. See pg 648-649

Properties of Conductors: Excess charge resides entirely on the conductor's outer surface. Charge accumulates where the radius of curvature of the surface is smallest (sharp points). irregularly shaped objects

Properties of Conductors: Inside a charged conductor, the electric field is zero. Just outside a charged conductor, the electric field is perpendicular to the surface. so that