22-1 (SJP, Phys 1120)1120 Fa '04.Steve Pollock. x2-2495 F419 Mike Dubson, x2-4938, 10th floorWeb page: www.colorado.edu/physics/phys1120 Lots there!Office hours: MWF after class for quick questions (down front)And many more - see online syllabusProf Dubson: See syllabus!We can see you other times, just call or email [email protected],[email protected]: 1) Read the handout (syllabus),2) Check out www.colorado.edu/physics/phys11203) Read Halliday, Resnick, Walker ("HRW") 22.1-4 for the 2nd class (we'llcover all of Ch. 22 in the next few lectures.)1st class: Intro, Syllabus, introduction to "electrostatics"Welcome! 1120 is the 2nd semester of calculus-based Physics. We'll bestudying electricity, magnetism, and light: the foundations of our technologicalsociety.Let's begin by reviewing some highlights from last semester. Physics 1110 canseem a little overwhelming, there were a lot of facts and ideas in that class! Butthey are all tied together with a few basic underlying principles, that reflect ouruniverse and how it works: Phys 1110 was primarily about mechanics, how andwhy things move. The key elements of Phys 1110 were the following:1) kinematics, the description of motion, velocity, and acceleration.2) dynamics, the explanations of why things move as they do.The essential ideas of dynamics are stated in Newton's laws:Newton 1: Bodies will continue in a state of uniform (unchanging) velocity,unless acted on by a net outside force.Newton 2: F=ma, forces cause acceleration, another word for change in velocity(because a = Δv/ Δt, or more precisely, dv/dt = d2x/dt2)This is a vector equation: forces (and acceleration) have direction andmagnitude. To be a bit more accurate, Newton's second law really says r F i= mv a i∑, or if you like, F_net = ma.22-2 (SJP, Phys 1120)Newton 3: The force on A by B is equal and opposite to the (reaction) force onB by A, or: F_AB = F_BA. (For every force applied on a body, there is anequal and opposite force applied to whatever caused that force)Other important and useful concepts, extensions of Newton's laws:1) Total energy is conserved.Energy is a defined, mathematical quantity, which can take various differentforms: kinetic energy, potential energy, thermal energy, chemical energy,electrical energy..You really only focused on the first two, last semesterRecall, kinetic energy is given by the formula KE = 1/2 m v^2(m is mass, and v is velocity.) KE is a number, a scalar.2) Momentum is conserved if there are no net external (outside) forces acting.Momentum is also a defined quantity:m*v (mass*velocity) It's a vector, it has a direction.Finally, we learned Newton's universal law of gravity: any two massive objectsattract one another, with a force of magnitudeF = G M1 M2 / r^2.(G is a constant of nature, r is the distance between the objects, M's are themasses of the objects)Then you applied these ideas, and learned about e.g. gravity, friction, andcontact (normal) forces; circular motion, springs, simple harmonic motion(vibrations), waves, and more.Together, Newton's laws and the conservation laws constitute one of the mostamazing and important intellectual breakthroughs in human history. Physics Iwas concerned with, and described (very successfully!) a lot of everyday stuff:baseballs, juggling balls, airplanes, air tracks, Olympic divers, Tai Chi masters,fireworks, the moon, helium balloons, beds of nails, fire extinguishers, pendula,water waves,… Most of this is "17th century" physics, understood by IsaacNewton, and used today by engineers, architects, physical therapists, etc.22-3 (SJP, Phys 1120)Physics 1120 concentrates on (mostly) 19th century discoveries: electricity,magnetism, and light, and the 20th and 21st century applications of these ideas.Many of the phenomena described by this physics are also everyday, but perhapsa little higher tech than what we saw last semester. Now we want to describe andunderstand microwave ovens, light bulbs, stoves, computers, lasers, and so on.Much of what we'll be studying will feel a little more abstract, but it's all veryreal. By the end of the semester, you will develop some good intuitions aboutelectricity, currents, lights and radiation! We begin in Ch. 22 of HRWElectric charge and Electric Fields: "Electrostatics"Last semester, our "favorite" force was gravity. But there are other types offorces that have been studied since Newton. We will spend a good portion of thissemester studying electric forces (and then we'll move on to magnetic forces,different but related). These forces have been known about for 1000's of years,and most likely you have discovered for yourself that e.g. rubbing the carpet andtouching someone can make annoying sparks, and rubbing a balloon on yourshirt can make it stick to the wall (for a while). So, what's going on? If you'd likea more "experimental" or "phenomenological" background, visit my old 2020notes (there's a link from the "lecture notes" page). Here, I'm just going tosummarize the conclusions of lots of (often quite simple!) experiments:It all boils down to a "new" force of nature, arising from a new relevant physicalproperty of matter: electric chargeThere are two (and only two) distinct types of electric charge:Positive charges (+), and Negative charges (-)These are just NAMES, we could have called then "North" and "South" charges,or "Chocolate" and "Vanilla". (Though, our choice of names provide usefulmetaphors) Ben Franklin was one of the first people to experiment with, andbegin to understand, "electrostatic forces" (between static or slowly movingcharges) Ben postulated the following simple rules:1) Electric charges of the same sign repel each other2) Electric charges of opposite sign attract each other.Summary:There are more rules, but this is already the basis ofelectrostatics!22-4 (SJP, Phys 1120)The central idea we hold now about charges is this: The world is made of atoms.Everything!! Atoms themselves contain parts which are electrically charged.The nucleus (made of protons and neutrons) is positively charged.The electrons which "orbit" around the nucleus arenegatively charged.Opposite charges attract, remember, that's what holds theelectron in orbit (Not gravity!)The naming choice (which is which, electrons are -,protons are +) is a human convention, established by Ben Franklin. (As we'll see,it's a little bit of bad luck he didn't do it the other way around, but he didn't, sowe have to stick with this.
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