Complete the tutorial on electrostatics (both sides of handout) Tape activity http://www.sos.siena.edu/%7Erfinn/phys140s09/tape.pdf

Whiteboard For the tape activity, with your group (3 or less), answer these questions from Tuesday’s class. What is the minimum # of different types of charge needed to account for your observations? Explain. How do two positively charged objects interact? How can you tell? Which tape was positively charged? How did you know?

The total charge of an isolated system is conserved. Charge Conservation The total charge of an isolated system is conserved. Rubbing a balloon on hair Triboelectric series Decreasing tendency to lose electrons

The Mystery of Newton The Rabbit You just won a rabbit in a contest at the state fair. You’re so excited! You’ve always wanted a rabbit. You decide to name your rabbit Newton because you love physics and your plan is to name all of your pets and future children after famous physicists. Along with the rabbit comes a metal cage, wood shavings, a metal water dish, and a plastic food bowl. After convincing your family that the rabbit is a good addition to the family, you start noticing that your rabbit is exhibiting some weird behaviors, but only when he’s inside the cage. The rabbit startles often in the cage, especially near it’s food and water bowls. He won’t come near the side of the cage to let you pet it. After seeing these weird behaviors your dad wonders out loud why the people at the fair were giving the rabbit away and mentions that there’s a local animal shelter that probably deals with weird rabbits all of the time. You’re determined to keep your rabbit and want to find out what’s wrong. Assignment: Using what you know about electrostatics, collect your evidence and determine what’s happening to Newton and how you’re going to remedy the problem.

Nature of charge Amber is a natural substance that can be naturally charged Few naturally occurring objects can be electrically charged Static electricity is easily shown with plastic, rubber, and synthetic fibers Electrical charges behave like positive and negative numbers

Atoms and Electricity An atom consists of a very small and dense nucleus    surrounded by much less massive orbiting electrons. The nucleus is a composite structure consisting of    protons, positively charged particles, and neutral neutrons. The atom is held together by the attractive electric force    between the positive nucleus and the negative electrons. Electrons and protons have charges of opposite sign but    exactly equal magnitude. This atomic-level unit of charge, called the fundamental    unit of charge, is represented by the symbol e.

Charge quantization A macroscopic object has net charge Where Np and Ne are the number of protons and electrons    contained in the object. The process of removing an electron from the electron    cloud of an atom is called ionization. An atom that is missing an electron is called a positive    ion. Its net charge is q = +e.

Insulators and Conductors In metals, the outer atomic electrons are only weakly bound to the nuclei. These outer electrons become detached from their parent nuclei and are free to wander    about through the entire metal. The metal as a whole remains electrically neutral, but the electrons are now like a    negatively charged liquid. The electrons in the insulator are all tightly bound to the positive nuclei and not free to    move around. Charging an insulator by friction leaves patches of molecular ions on the surface, but these patches are immobile.    

Illustrations of Coulomb’s Law Electroscope induction conduction

Polarization Force

The Electric Dipole

Coulomb’s Law - magnitude K = 9.0 x 109 N m2/C2 Coulomb’s contant q1, q2 = charges measured in Coulombs (C) 1 C is a lot of charge! r = distance between q1 and q2 ε0 =permittivity of free space (more relevant to future chapters)

Electric Force Similar form as gravitational force (weird, huh?), except positive and negative charges Like charges repel; opposite charge attract

Net Force from Multiple Charges An electrostatic force exists between each pair of charges according to Coulomb’s law Add components of forces to get net force when adding multiple charges

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.

The Electric Field

Group Problems

What is the net force on the 48 C charge? Group Problems What is the net force on the 48 C charge? Q1 Q4 Q3 Q2 2. Find net force on Q1 +y +x

EXAMPLE 26.6 Lifting a glass bead F(1 on 2) exceeds force of gravity on bead by a factor of 60, so the glass bead will have a net force upward. F(1 on 2) 9 x 10^-3 N F(G) = 1.5 x 10^-4 N

EXAMPLE 26.8 The electric field of a proton