Unit1WEBLectureNotes2008 - Physics 2B Summer Session 1 Unit...

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Physics 2B Summer Session 1 Unit 1 Lecture Notes Motivation The Electromagnetic Force (EM) is one of the F our F undamental F orces found in nature: 1. Gravitational Force 2. Electromagnetic Force 3. Strong Nuclear Force 4. Weak Nuclear Force The last two don’t affect our daily life on earth (see Physics 2D ) The EM is much, much stronger than Gravity E&M determines the structure of matter, both atomic & molecular So E&M is an integral part of Chemistry, Biology, Engineering, Etc. Prolific use of electronic devices is found in all branches of science Properties of Electric Charge ± Charge is a fundamental property of matter, like mass so charge doesn’t exist apart from some kind of particle ± Charge was first discovered by the Greeks ( ελεκτροη ) ± Charge comes in two flavors – positive & negative ± Like charges repel – Unlike charges attract ± Charge is Quantized – electron ( e ) & proton ( e + ) ± Charge appears to be balanced in nature There are an equal number of plus & minus charges So we get Neutral Atoms and a Neutral Universe ± Conservation – charge is neither created nor destroyed MKS Unit = Coulomb (C) +1 9 18 + 1 e 1.6 10 C 1 C 6.25 10 e
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First a quick word about units , symbols and the like . Take a look at the PDF file “MKS Units” on the course website under the “Handouts” section. It is important to know that this system is closed under all of the formulas and equations in this course. In other words, your solution will come out in the correct fundamental units provided that you input all your data in the other appropriate fundamental units of the system. For example, you wouldn’t expect the distance formula s vt = to yield an answer in miles per hour if you put in 20 meters/second v = and 22 minutes t = . It was fairly easy in mechanics ( Physics 2A ) to cancel physical units – remembering, say, that a Joule is equal to a kilogram-meter per second squared. Here, however, there are many new and colorfully- named units which can be described in terms of other fundamental units in a variety of ways. So be careful with units. Coulomb’s Force Law Let’s begin with two electric point charges, q 1 and q 2 separated by a distance r and calculate the force on q 2 by q 1 . Note that we have drawn a line from q 1 to q 2 and extended that line with a unit vector which points away from the source charge q 1 . The force is found using 12 2 ˆ qq k r = 12 Fr Coulomb Force Equation where the constant 2 9 0 1 9.00 10 4 N m k C πε == × i Note the resemblance to the gravitational force equation 2 ˆ mm G r = 12 which also Is an inverse square law with respect to distance, Depends on the product of a fundamental particle property and has Force Symmetry, i.e., = − 12 21 FF For a numerical exercise, try to follow example 23-1 on page 574.
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This note was uploaded on 08/23/2008 for the course PHYS 2b taught by Professor Schuller during the Summer '08 term at UCSD.

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Unit1WEBLectureNotes2008 - Physics 2B Summer Session 1 Unit...

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