PHY 184 Week 2 Notes- Electrostatics
PHY 184 Week 2 Notes- Electrostatics PHY 184
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This 6 page Class Notes was uploaded by Cameron Blochwitz on Friday September 23, 2016. The Class Notes belongs to PHY 184 at Michigan State University taught by Oscar Naviliat Cuncic in Fall 2016. Since its upload, it has received 16 views. For similar materials see Physics for Scientists and Engineers II in Physics at Michigan State University.
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Date Created: 09/23/16
PHY 184 Lecture Notes Week 2 9/6-9/8 Electrostatics Quark Structure of matter o Quarks are the smallest unit of matter Atomic Structure Atom Nucleus Nucleons Quark There are 6 different kinds of quarks +2 o Up/Charm/Top all have a charge of 3 e −1 o Down/Strange/Bottom all have a charge of 3 e All matter is made up of these three elementary particles Up/Down/Electron Isolated quarks have never been observed The carry a fractional charge Structure of an Atom Atoms are neutrally charged o Composed of a positive nucleus and negative electrons Nucleus itself is made of positive protons and neutral neutrons Describing an Atom Atomic Number (Z): Number of Protons or Electrons Mass Number (A): Number of Nucleons (Protons and Electrons) Number of Electrons: equal to Z, charge -Ze Number of Protons: equal to Z, Charge Ze Number of Neutrons N: A-Z=N Atomic Mass: Z*M +p*M n Electrons and Protons Electron is an elementary particle o It cannot be further subdivided Proton is not an elementary particle o Composed of 2 Up Quarks and 1 Down Quark o Charge of a Proton 2 2 1 qp=3e+3 e−3e=e Up Up Down Neutron is not an elementary particle o Composed of 2 Down Quarks and 1 Down Quark o Charge of a Proton n = e− e− e=0 3 3 3 UP Dow n Down Quarks are held together by the strong force Electric charge is a fundamental property that a particle or object must have in order to experience any electromagnetic interaction o Without charge the object cannot interact with electromagnetic fields. There are 2 main kinds of charges positive and negative Charged Rods Demonstration o Rubbing the rods will charge them Like charges will repel each other Opposite charges will attract each other Unit of charge is the Coulomb, C o Defined in terms of the Ampere 1 C = 1 A*s (Ampere Second) Unit Charge is defined by the electron o qe=−e −19 o e=1.602×10 C o Proton carries a charge of +e 1 C is a very large amount of charge o Often will be using smaller charges −6 1 μC = 1×10 C 1×10 −9 1 nC = C 1×10−12 1 pC = C Number of Electons to produce 1 C 1C N e −19 o 1.602×10 C The electric charge is quantized o There is a minimum amount of charge a particle can have This charge is e This is too small to be noticed in daily life But the Quarks have fractional charges! o This is possible because quarks are never isolated They are unable to exist on their own Charges of macroscopic objects are neutral o If it carries a negative it has extra electrons o If it carries a positive charge it is missing electrons ONLY ELECTRONS MOVE!!! Charges are able to be moved around o They can never be created o In any process the net charge is always a constant The electron structure of materials determines their ability to conduct electricity o Conductors conduct very well o Insulators conduct very poorly Electric Forces by Polarization Demonstration (Rotating Rod/Hanging Insulator) o There is an electric force between a charged rod and an uncharged one Why does this occur o The charged rod induces polarization it attracts the electrons and produces a net force + + + + + + + + + + + + - + + + + + + + + + + - 2 Point Charges Consider a single point charge q 1 Add a second point charge q 2 r q2 We can use Coulomb’s Law to find the force q1q2 F 1→2k 2 ^ r k is known as Coulomb’s constant 9N m2 8.99×10 2 C r^ is a unit vector pointing from q to 1 2 o This direction is fixed; the direction of the force will be determined by the signs on the charges o k and r are always positive Signs of charge are opposite the attract Signs of charge are the same they repel Coulomb’s Law contains the Law of Charges Comparing Electromagnetism and Gravity qe m e F ek 2 F gG 2 r r r will cancel and so 2 2 F ekq e F gG m e F e 42 =4.20×10 F g The electric interaction is vastly stronger than gravity o So much stronger that we are able to ignore gravity System of Charges When working with a system of charges you will need to consider each interaction of all the charges The force from system of charges (q , q , … , q 1 on2Q is n q1 q2 qn r r^1+r r^2+…+ r ^n o 1 2 n F=kQ¿ o This is known as the superposition principle It shows a vector sum of forces Equilibrium Position Two positive charges q and q are1placed a2 shown, where will a third charge q keep the system in equilibrium 3 o We know that the F 1 on 3must be equal to F 2 on 3 o When added the forces net to zero. o We therefore have the equation q q 1 = 2 (x −x ) 2 (x −x ) 2 3 1 2 3 Solve for x 3 Note the charge of q is irrele3ant! This method is the basis for all equilibrium problems
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