S-113 Define these terms A.Charge B.Potential Difference (Voltage) C.Current (Amps) D.Resistance

Objective I can define electric field in terms of the force on the test charge I can calculate the magnitude and direction of the force on a positive or negative charge placed in a specified field

Electric Charge and Electric Field AP Physics Chapter 16

Electric Charge and Electric Field 16.1 Static Electricity

Static electricity – not moving Two types of charge positive (+) when electrons are lost negative (-) when electrons are gained Objects can gain charges by rubbing 16.1

16.1 Static Electricity Like charges repel Unlike charges attract Law of Conservation of electric charge – the net amount of electric charge produced in a process is zero 16.1

Electric Charge and Electric Field 16.2 Electric Charge in the Atom

Atoms are made of positive protons negative electrons Electrons move from one object to another when the objects are rubbed Eventually charges bleed off objects because water molecules are polar 16.2

Electric Charge and Electric Field 16.3 Insulators and Conductors

Conductors – outer electrons of atoms are free to move through the material Insulator – electrons tightly held, do not move 16.3

16.3 Insulators and Conductors Semiconductors – conduct electricity under some circumstances, don’t under other conditions 16.3

Electric Charge and Electric Field 16.4 Induced Charge; the Electroscope

Induction – charging without contact Object is brought near a charged object Electrons move Object is grounded An electroscope measures if an object has a charge on it 16.4

Electric Charge and Electric Field 16.5 Coulomb’s Law

Electric charges apply forces to each other From experiments Force is proportional to charge Inversely proportional to square of distance 16.5

16.5 Coulomb’s Law Equation – gives magnitude of force Opposite charges – force directed toward each other Like charges – force directed away from each other Charge is measured in Coulombs 16.5

S-114 A mad scientist charges two objects that are 2.45 cm apart. If the measured force between them is 120 N, toward each other, what is the magnitude of the charges (assume they are both the same charge)

Objective I can determine the force that acts between specified point charges.

Homework Problem

Practice Problem A 10C charge is placed so that x=0m and a second charge of -20C is placed so that it x=5m. Where must a third charge of 5C be placed so that the net force on the charge is zero.

16.5 Coulomb’s Law 1 Coulomb is the amount of charge, that if placed 1 m apart would result in a force of 9x10 9 N Charges are quantized – that is they come in discrete values The constant k relates to the constant called the permittivity of free space 16.5

16.5 Coulomb’s Law AP tends to write k as We will assume that charges are point charges - size is negligible 16.5

Electric Charge and Electric Field 16.6 Solving Problems Involving Coulomb’s Laws and Vectors

16.6 Coulomb’s Law and Vectors Principle of Superposition – Electrostatic forces are added just like any other force As vectors 16.6

Practice Problem Three charges are placed in space. The first charge has a value of 5nC. The second is placed 3 m above the first and has a value of - 8nC. The third has a value of 2nC and is placed 4 m to the right of the first. What is the net force on the 5nC charge? 16.6

Practice Problem Using the same charges as the previous problem, what is the net force on the -8nC charge? 16.6

Practice Problem 4 charges, each with a value of 5  C are placed at the corners of a square 2 m on a side. What is the net force on the charge in the upper right corner? 16.6

Practice Problem 4 charges, each with a value of 5  C are placed at the corners of a square 2 m on a side. What is the net force on the charge in the upper right corner? 16.6

Electric Charge and Electric Field 16.7 The Electric Field

Electrical forces act over distances Field forces, like gravity Michael Faraday electric field – extends outward from every charge and permeates all of space The field is defined by the force it applies to a test charge placed in the field 16.7

16.7 The Electric Field The Electric field would then be Or q is the test charge We can also say that Remember that E is independent of the test charge. The electric field is also a vector (free body diagrams are probably a good idea) 16.7

S-112 A 3 mc charge is on the right, and a -2 mc charge on the left. They are 0.8 m apart. What is the field strength.2 m from the right?

Electric Charge and Electric Field 16.8 Field Lines

To visualize electric fields Draw electric field lines Direction of the lines is the direction of force on a positive test charge The density of the lines indicates relative strength of the field Note: the field density increase as you get closer 16.8

16.8 Field Lines For multiple charges, keep in mind 1.Field lines indicate the direction of the field The actual field is tangent to the field lines 2.The magnitude of the field is relative to the field line density 3.Fields start at positive and end at negatives 16.8 Field Lines

16.8 Field Lines If the field is produced by two closely spaced parallel plates The field density is constant So the electric field is constant 16.8

Electric Charge and Electric Field 16.9 Electric Fields and Conductors

For a conductor 1.The electric field inside a conductor is zero (static) 2.Any net charge is distributed on the surface of a conductor 3.Electric field is always perpendicular to the surface of a conductor 2.Charges concentrate at the area of greatest curvature 16.9

S-113 Four charges, each of 1.2 nC are placed on the corners of a square 2.5 cm per side. What is the magnitude and direction of the electric field on the upper right corner? What is the force on the charge in that corner?