Physics 104 Study Guide Exam 1 Lecture 1 SEPT 3 Charges - concept to describe behavior of objects (positive & negative) - charged objects have an excess of 1 flavor EX: 1C=huge nC= 10^-9 μC= 10^-6 e= “elementary charge” 1e= 1.6 x 10^-19 How many 1C of charge? Each electron has 1.6 x 10 ^-19 in it # of electrons = 1C # of C in each electron Coulomb’s Law (insert eqution) - k is a number: 9 x 10^9 Q causing the electrical field q feeling the electrical field Electrical fields one that causes the field determines the direction of the field arrows don’t represent movement, just the direction that it points inLecture 2 SEPT 8 Induction/Polarization and Superposition - electrical fields in 2D, superposition - define insulator, conductor, induction and polarization - potential, voltage, energy Superposition - finding x and y components Clicker Q 1: What direction does E field point at A due to Q1? E1, A? *insert diagram B. Points away from the charge CQ2: What does it point at A due to Q2? D. points down CQ3: Which way does EA point? C. - Conductor: any object where outer shell e-‘s are free to move around (metal) - Insulator: any object where outer shell e-‘s are not free to move around but shell can be distorted (plastic, wood) - Induction: (conductors) a charge distribution in a conductor, caused by an external E field- Polarization: (insulators) a charge distribution caused by an insulator, caused by an external E field. Some molecules are naturally polarized. Induction example Electroscope (conductor): or conducting ball When you put a negative charge on the tip, the 2 leaves come together Polarization example Paper (insulator)Lecture 3 SEPT 10 E field lines: - radiate away (start on) from positive charges and radiate toward (and on) negative charges - they never cross, always start and end on charges - # of lines starting on or ending on a charge is proportional to the size of the charge o big charge = draw lots of lines o E field = thick means very strong, lines are closer together - density of field lines in a region indication magnitude of E field - direction of field lines indicate the direction of the E field CQ1: What direction is A? 2. at C? 3. Where is it strongest? At C Sheet of Charge - “charge density”: the number of Coulombs spread over 1 square meter of the sheet If we stay close to the sheet of charge or if its very large, the E field doesn’t change with distance like it does for point charges Superposition - for 2 sheets, we do same method as for point chargesLecture 4 SEPT 15 Charged Sheets, Charged Plates and Voltage - thin insulating sheets of charge - charged plates (thick conducting plates) - voltage: joule/coulomb or the work to move 1C from A to B o ΔV_A, B - Work and energy - Voltage and E field (if there is E field, there is ΔV) Charged sheets - what is the E field, left, between and right? CQ1: What is E field left of σ_2? B. 1130 N/C CQ2: What’s E field between the charged sheet? D. 3390 N/C What if we apply (add) an external E field = 1130 N/C? What should Eext be so that E_B=0? - 2260 cancels Neutral plate in an E field -the external E field = 1130 N/C. How does conducting plate respond? What is the E field inside the plate? -in conductor, E field = 0 always (positive charges get put into direction of external E field)Neutral cylinder “plate” (thick aluminum can) Single charged plate (positive charge) Two charged plates neat each other (equal and opposite charge) Voltage ΔV_A, B - ΔV_A, B is the work by my hand as I move +1C of charge form A to B - Insert pic Voltage and E field If E field, there will be voltage differenceLecture 5 SEPT 17 Charged plates, voltage - Work and energy - ΔV_A,B - Connect E and ΔV Voltage ΔV Batteries Voltage goes nowhere. It exists between 2 points. Kinetic and Potential Energy - energy conservation o ΔKE = -ΔPE o ΔKE = ½mv^2 - Gravity o ΔPE_A,B = mgΔh_A,B for gravity fields o ΔPE_A,B = qEΔx_A,B for E fields E and x can point in any direction Equations and ideas - Newton’s 2nd law and FBD: F_net = ma - E = 0 inside a conductor, ΔV (inside a conductor) = 0. - E = σ single sheet 2E_o - E = σ between 2 plates (equal and opposite charge densities) E_o - ΔPE_A,B = qΔV_A,B - E_o = 8.85 x 10^-12 - ΔPE_A,B = - ΔKE - σ= Q Area - F_E,q = qE - W ^H,q _A,B = N_H.q Δx_A,B = ΔPE - KE = ½ mv^2- ΔV = Ed EX: σ = 45 nC/m^2 EX: move plates closer ΔV_A,C = +750 more pump because the plates got closer When you cross a battery, it doesn’t mean anything how far you go. If I start with neutrals, the battery will suck charges and becomes equal and opposite charges If one was charged and other not, then there will not be equal and opposite charge EX: Disconnect the battery then move the plates apart the voltage changesLecture 6 SEPT 22 E fields and potential voltage - Equipotential lines - Can we use ΔV = Ed? Capacitance - start with the relationship between Q and E between plates connected by a battery. Look for relationship between ΔVbatt and Q. EX: - 2 conduction plates 3cm x 4cm separated by 2mm. What is capacitance, C? - a battery with ΔVbatt = 12V is connected across the plates. What is the total charge stored? Q = CΔV = (531 x 10^-12)(12) = 6.37 nC Energy stored by a capacitor -what does a battery do to a capacitor? -how much work does the battery do to charge the capacity: replace the battery with a charge mover. i.e my hand until fully charged to Q and ΔV.-how much energy is stored in previous capacitor? -know its 12V battery -know Q CQ1: if separation decrease ½ and NOT connected by battery… Energy goes DOWN by a factor of 2 CQ2: if separation decreases ½ and IS connected: Energy goes UP by factor of 2 Inserting plates and dielectrics (insulators) - insert conduction plate, what happens? Insert a dielectric. These are insulators and fill entire space. They also keep the plates apart. What happens? Dielectrics are characterized by a pure number, k. Dielectrics increase capacitance by this factorLecture 7 SEPT 24Lecture 8 SEPT
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