Electrostatics

•  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?

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

•  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.

•  A macroscopic object has net charge

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

€

Fe =K q1q2r2

€

Fe =14πε 0

q1q2r2

(more relevant to future chapters)

ε0 =permittivity of free space

Electric Force

•  Similar form as gravitational force (weird, huh?), except – positive and negative charges

•  Like charges repel; opposite charge attract €

Fe =K q1q2r2

€

Fg =Gm1m2

r2

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

€

Fe = Fe,12 + Fe,13 + Fe,23 =k q1q2r2

+k q1q3r2

+k q2q3r2

The Electric Field We begin our investigation of electric fields by postulating a field model that describes how charges interact: 1. Some charges, which we will call the source charges, alter the space around them by creating an electric field. 2. 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.

€

Fon q =K q1q2

r2 + ...

The Electric Field

Group Problems

Group Problems

Q1

Q4 Q3

Q2 2. Find net force on Q1

+y

+x

1.  What is the net force on the 48 µC charge?

EXAMPLE 26.6 Lifting a glass bead

EXAMPLE 26.8 The electric field of a proton