Physics 121. Instructor: Dr. Chris McCarthy. If you are enrolled, please mark the signup sheet

Transcription

1 Physics 121 Instructor: Dr. Chris McCarthy If you are enrolled, please mark the signup sheet If you wish to add this class, add your name and ID# to the sheet. Pre-requisite: Physics 111. If you are not yet enrolled and took it at another university, please hand me an (unofficial) transcript this week

2 Physics 122 Lab You must take Physics 122 lab this semester *. Attend the first lab section meeting in Trailer N *Unless you have already passed the lab.

3 Handouts Today: -Course Syllabus -Reading Schedule -Units Sheet Physics 121 Required Textbook: Walker, Physics (5th or 4th edition) Required Homework submission service: MasteringPhysics See syllabus for instructions.

4 Getting Help Dr. McCarthy s Office Hours: & Help Session: Tuesday PM in TH 520 Wednesday PM in TH 520 Your Lab Instructor also holds Help Sessions ASK your lab instructor when.

5 Getting Help CARP (Campus Academic Research Program) provides tutoring to SFSU students in HSS

6 Physics 121 In this class, you ll learn about the most powerful forces in nature & aspects of physics that are crucial to modern life: -electricity -magnetism -electromagnetic waves: light, x-rays, radio and microwaves -relativity theory & quantum physics. -nuclear energy & reactions Many of the important aspects of physics are unseen, and must be deduced from observations. Some aspects of modern physics challenge our notion of common sense

7 What you ll need in Physics 121 Attention to quantities & UNITS See big-picture concepts, like... Understand FORCE & Newton s 2nd Law How would you know if a force was present? Understand ENERGY & how it can transform. Be able to work with vectors & components Be a problem solver, not an answer-getter

8 Amber (fossilized tree resin) will pick up little objects after being rubbed. (known since antiquity) Ελεκτρον = elektron in Greek

9 Ben Franklin ( ) investigated electricity and made several discoveries. -Electricity is composed of two types of charge, which he called positive and negative -Franklin (bravely) proved that lightning is a form of electricity. -He also invented a defense against it: the lightning rod.

10 Experiments show that these substances become charged upon rubbing: Rabbit Fur Glass Human Hair ++++ Silk ++ Cotton - Amber --- Rubber ---- PVC Pipes PVC becomes negatively charged because it gains negative particles (electrons) Rabbit fur becomes positively charged because it loses negative particles (electrons) demo

11 Positively and negatively charged objects exert forces on each other...either a force of attraction or repulsion. Amber (-) repels amber (-) Amber (-) attracts silk (+) Fur (+) attracts rubber (-) Rubber (-) repels rubber (-) All of these observations can be summarized by two easy rules: Like repels like Opposites attract

12 An uncharged ( neutral ) object has equal numbers of positive and negative charges. (Its net charge is zero) However, if it comes near a charged object (say a positive rod), its own charges will move... The positive charges will move away from the rod & negative charges will move toward the (+) rod. The neutral object will become polarized with its negative charges near the positively charged rod. Since Opposites attract, the charged rod and neutral object will attract each other!

13 If they touch charge will transfer.

14 Conductors & Insulators Polarization of a neutral object depends on how easily its charges move. But what if they can t move? Such an object is called an insulator. Insulators (eg. wood, rubber) do not conduct electricity. They also don t conduct heat well. A conductor is an object whose charges move easily. (eg metals like copper, silver and salt solutions, etc.) Conductors easily conduct both heat and electricity. A semi-conductor partially conducts electricity. They are used extensively in modern electronics

15 Measuring Electric Charge Charge is a quantity that can be measured. The SI unit used to measure charge is the: Coulomb (C). An unknown quantity of charge? Use a variable: q Comparison: Mass (m) is a quantity, measured with units of Kilograms (kg) Typical charges are much smaller than 1 C. Eg. rubbing a balloon might give it a charge of 5 microcoulombs (5 μc = 5 x 10-6 C ) handout

16 Electrons Most electrical phenomena are caused by movement of electrons (e - ) Every electron has the same charge: x Coulombs We use the symbol e to mean the amount of charge found in one electron, whether positive or negative. (eg. +e, -2e, +4e) If you wanted one Coulomb of negative charge, how many electrons would you need to collect? 1/(1.60 x )= 6.24 x electrons or 6,240,000,000,000,000,000 e - (6 quintillion!)