Electrostatics Complete the tutorial on electrostatics...
Transcript of Electrostatics Complete the tutorial on electrostatics...
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