Slide 1

Electrostatics Slide 2 10.1 Properties of Electric Charges Slide 3 Static electricity not moving Two types of charge positive (+) when electrons are lost negative (-) when electrons are gained Objects can gain charges by rubbing Slide 4 10.1 Properties of Electric Charges Like charges repel Unlike charges attract Law of Conservation of electric charge the net amount of electric charge produced in a process is zero Slide 5 10.1 Properties of Electric Charges Robert Millikan charge is always a multiple of a fundamental unit Quantized occurs in discrete bundles The discrete bundle is an electron The charge on a single electron is Slide 6 10.2 Insulators and Conductors Slide 7 Conductors outer electrons of atoms are free to move through the material Insulator electrons tightly held, do not move Slide 8 10.2 Insulators and Conductors Semiconductors conduct electricity under some circumstances, dont under other conditions Charges can be transferred by contact Called Charging by Conduction Slide 9 10.2 Insulators and Conductors Induction charging without contact Object is brought near a charged object Electrons move Object is grounded An electroscope measures if an object has a charge on it Slide 10 10.3 Coulombs Law Slide 11 Electric charges apply forces to each other From experiments Force is proportional to charge Inversely proportional to square of distance Slide 12 10.3 Coulombs Law Equation gives magnitude of force Opposite charges force directed toward each other Like charges force directed away from each other Charge is measured in Coulombs Slide 13 10.3 Coulombs Law 1 Coulomb is the amount of charge, that if placed 1 m apart would result in a force of 9x10 9 N Charges are quantized that is they come in discrete values The constant k relates to the constant called the permittivity of free space Slide 14 10.3 Coulombs Law These are forces, so be sure to use vector math, draw free body diagrams For multiple objects, require multiple free body diagram Slide 15 10.4 The Electric Field Slide 16 Electrical forces act over distances Field forces, like gravity Michael Faraday electric field extends outward from every charge and permeates all of space The field is defined by the force it applies to a test charge placed in the field Slide 17 10.4 The Electric Field The Electric field would then be Or q is the test charge We can also say that Remember that E is independent of the test charge. The electric field is also a vector (free body diagrams are probably a good idea) Slide 18 10.5 Electric Field Lines Slide 19 To visualize electric fields Draw electric field lines Direction of the lines is the direction of force on a positive test charge The density of the lines indicates relative strength of the field Note: the field density increase as you get closer Slide 20 10.5 Electric Field Lines For multiple charges, keep in mind 1.Field lines indicate the direction of the field The actual field is tangent to the field lines 2.The magnitude of the field is relative to the field line density 3.Fields start at positive and end at negatives Field Lines Slide 21 10.5 Electric Field Lines If the field is produced by two closely spaced parallel plates The field density is constant So the electric field is constant Electric Diple two point charges of equal magnitude but oppsite sign Slide 22 10.6 Conductors in Electrostatic Equilibrium Slide 23 Electrostatic Equilibrium when no net motion of charge occurs within a conductor 1.The electric field is zero everywhere inside a conductor 2.Any excess charge is on the surface of a conductor Slide 24 10.6 Conductors in Electrostatic Equilibrium 3.The electric field just outside a charged conductor is perpendicular to the conductors surface 4. The charge accumulates on areas of greatest curvature Slide 25 10.7 Potential Difference and Electric Potential Slide 26 Electricity can be viewed in terms of energy The electrostatic force is conservative because it depends on displacement Now We can calculate this value for a uniform electric field Slide 27 10.7 Potential Difference and Electric Potential Positive test charge increases when moved against the field Negative test charge increases when moved with the field Electric Potential (Potential) electric potential energy per unit charge Slide 28 10.7 Potential Difference and Electric Potential Only difference in potential are meaningful Potential Difference (Electric Potential Difference) is measureable Measured in volts (after Alessandro Volta) Slide 29 10.7 Potential Difference and Electric Potential If we want a specific potential value at a point, we must pick a zero point. That point is usually either A. The ground B. At an infinite distance Slide 30 10.8 Electric Potential & Potential Energy Slide 31 Using calculus it can be shown that the electric potential a distance r from a single point charge q is Assuming that potential is zero at infinity Like Potential Difference, this value is a scalar So Slide 32 10.9 Potentials and Charged Conductors Slide 33 1.All points on the surface of a charged conductor in electrostatic equilibrium are at the same potential. 2.The electric potential is a constant everywhere on the surface of a charged conductor in equilibrium. 3.The electric potential is constant everywhere inside a conductor and equal to its value at the surface. Slide 34 10.10 Capacitance Slide 35 Capacitor device that stores electric charge In RAM, Camera Flash, Slide 36 10.10 Capacitance Simple capacitors consist of two plate The symbol for a capacitor is The symbol for a cell is The symbol for a battery is Slide 37 10.10 Capacitance When a potential difference is placed across a capacitor it becomes charged This process takes a short amount of time The charge on each plate is the same, but opposite charge The amount of charge is proportional to the potential difference A constant C (Capacitance) gives Charging a Capacitor Time for RC Circuit Slide 38 10.10 Capacitance Capacitance Unit Farad For a parallel plate capacitor, the capacitance depends on the area of the plates, the distance between the plates Slide 39 10.11 Combinations of Capacitors Slide 40 Parallel more than one pathway For a parallel set of capacitors the total charge is the sum of the individual charges In all parallel circuits the potential across each branch is the same as the total Slide 41 10.11 Combinations of Capacitors The equivalent capacitance is the value of one capacitor that could replace all those in the circuit with no change in charge or potential Since And We combine and get Slide 42 10.11 Combinations of Capacitors Series components of a circuit are in one pathway The magnitude of the charges is the same on each plate Slide 43 10.11 Combinations of Capacitors The total potential is the sum of the potential drops across each capacitor We then use that equation and the equation for capacitance We get Slide 44 10.12 Energy Stored in a Charged Capacitor Slide 45 Capacitors store energy Energy can be defined as change in work Or the are under a plot of Q vs. V Slide 46 10.13 Capacitors with Dielectrics Slide 47 Most capacitors have an insulator between the plates Called a Dielectric Increases the capacitance by a factor K Called the dielectric constant Slide 48 10.13 Capacitors with Dielectrics Some Dielectric Constants MaterialK Paper3.7 Glass5 Rubber6.7 Mica7 Strontium Titanate 300