25 1 SJP Phys 1120 1120 Fa 07 Steve Pollock x2 2495 F419 Victor Gurarie x5 5898 F617 Web page www colorado edu physics phys1120 Lots there Office hours MWF after class for quick questions down front And many more see online syllabus We can see you other times just call or email Steven Pollock Colorado EDU victor gurarie colorado edu Reading 1 Check out www colorado edu physics phys1120 2 Read the syllabus in particular 3 Knight intro and Ch 25 1 4 this is a lot it will take time 1st class Intro Syllabus introduction to electrostatics What follows is a summary of some key points of this Chapter not quite an alternative to the text but perhaps a different perspective Welcome 1120 is the 2nd semester of calculus based Physics We ll be studying electricity magnetism and light the foundations of our technological society Let s begin by reviewing some highlights from last semester Physics 1110 can seem a little overwhelming there were a lot of facts and ideas in that class But they are all tied together with a few basic underlying principles that reflect our universe and how it works Phys 1110 was primarily about mechanics how and why 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 v t or more precisely dv dt d2x dt2 This is a vector equation forces and acceleration have direction and magnitude To be a bit more accurate Newton s second law really says r v Fi m a or if you like F net ma i 25 2 SJP Phys 1120 Newton 3 The force on A by B is equal and opposite to the reaction force on B by A or F AB F BA For every force applied on a body there is an equal 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 different forms kinetic energy potential energy thermal energy chemical energy electrical energy You really only focused on the first two last semester Recall 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 objects attract one another with a force of magnitude F G M1 M2 r 2 G is a constant of nature r is the distance between the objects M s are the masses of the objects Then you applied these ideas and learned about e g gravity friction and contact normal forces circular motion springs simple harmonic motion vibrations waves and more Together Newton s laws and the conservation laws constitute one of the most amazing and important intellectual breakthroughs in human history Physics I was 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 Isaac Newton and used today by engineers architects physical therapists etc 25 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 perhaps a little higher tech than what we saw last semester Now we want to describe and understand 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 very real By the end of the semester you will develop some good intuitions about electricity currents lights and radiation We begin in Ch 25 of Knight Electric charge and Electric Fields Electrostatics Last semester our favorite force was gravity But there are other types of forces that have been studied since Newton We will spend a good portion of this semester 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 and touching someone can make annoying sparks and rubbing a balloon on your shirt can make it stick to the wall for a while So what s going on If you d like a more experimental or phenomenological background visit my old 2020 notes there s a link from the lecture notes page Here I m just going to summarize the conclusions of lots of often quite simple experiments It all boils down to a new force of nature arising from a new relevant physical property of matter electric charge There 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 useful metaphors Ben Franklin was one of the first people to experiment with and begin to understand electrostatic forces between static or slowly moving charges Ben postulated the following simple rules 1 Electric charges of the same sign repel each other 2 Electric charges of opposite sign attract each other Summary There are more rules but this is already the basis of electrostatics 25 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 are electrons negatively charged Opposite charges attract remember that s what holds the protons electron 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 so we have to stick with this Sorry for the pun When you rub things you might separate and charges tearing apart some atoms but you can never create a new isolated electric charge You just move them
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