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Electrostatics and Electricity

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ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – + + – + – + – + – + – – negative charge: excess (gain) of electrons positive charge: deficiency (loss) of electrons neutral: electrons equal protons (no net charge )

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ELECTRIC CHARGE law of conservation of charge: total charge stays constant (for every + charge produced, there is a – charge produced) – – – + + – –

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ELECTRIC CHARGE law of conservation of charge: total charge stays constant (for every + charge produced, there is a – charge produced) – – + + – – –

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ELECTRIC CHARGE law of electrostati cs: like charges repel, unlike charges attract

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ELECTRIC CHARGE Charge transfer conductor: readily transfers charge (free electrons) insulator: doesn’t transfer charge (electrons in bonds)

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ELECTRIC CHARGE Charging by Conduction direct contact same sign permanent charge divides evenly between objects

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ELECTRIC CHARGE Charging by Induction no contact opposite sign temporary unless grounded

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Electric Charge Charge by Friction The heat generated by rubbing two objects together energizes electrons causing them to transfer.

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ELECTRIC CHARGE Conductor that has induced charge by neighboring positive wall. Free electrons move towards the wall. Insulator that has induced charge by neighboring positive wall. Molecules are polarized.

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ELECTRIC CHARGE Why does the water bend towards the cup?

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ELECTRIC FORCE electric force is a fundamental force of nature: holds atoms together, holds molecules together, causes friction & most forces (except gravity) Amount of charge, q or Q: measured in coulombs, C 1.00 C = 6.25×10 18 electrons charge of one proton or electron, e = ±1.60×10 – 19 C

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ELECTRIC FORCE Coulomb’s Law: force between charges depends on amounts of charge and distance between them inverse square law like the force of gravity F e = kq 1 q 2 /r 2 F e : electric forceq: charge r: distance between chargesk: 8.99×10 9 Nm 2 /C 2 +F e : repulsion, –F e : attraction

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ELECTRIC CHARGE Grounding: discharging by connecting to a large charge sink (such as earth) Charge Distribution: only on the surface; spreads evenly on spherical conductor; stays put on insulator; concentrates at points Spark Discharge: when charge is large enough, air ionizes and conducts the charge away (lightning)

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ELECTRIC FORCE Electric field: region around a charge where it exerts electric force on other charges field lines: show direction & amount of force (by how close the lines are) on a + test charge

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ELECTRIC FORCE electric fields exert force on charged objects electric field strength, E: force exerted on a charge by an electric field E = F/q unit: N/C (Newtons/Coulomb), or V/m (Volts/meter)

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ELECTRIC FORCE constant electric fields are used to accelerate charged particles field is constant between parallel plates force F = qE change in kinetic energy K-K 0 = Fd d: distance traveled in electric field, K = ½mv 2

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CIRCUIT BOARD INTRO

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ELECTRIC CIRCUITS Basic Circuit: conductor loop for transferring energy load: energy user (bulb, resistor, heater, motor) source: energy provider (battery, generator)

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ELECTRIC CIRCUITS Current, I : rate of “flow” of electric charge. unit: ampere, A I = Q/t 1 A = 1 C/s Charge, Q, is measured in Coulombs. Think of current as the number of electrons that pass by a point each second!

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ELECTRIC CIRCUITS Voltage, V: work done per charge between two points, unit: volt, V The voltage is the “push” on the current! Examples: Batteries, Electrical Outlets, Capacitors.

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ELECTRIC CIRCUITS Resistance, R: opposition to charge flow, unit: ohm, resistance limits the flow of current resistance turns electric energy into heat (& light) resistor: fixed resistance, symbol:

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ELECTRIC CIRCUITS Ohm’s law: current is proportional to voltage and inversely proportional to resistance: V = IR V: voltage, V I: current, A R: resistance, Example: How much current is there if the voltage is 6V and the Resistance is 3 ?

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ANALYZING CIRCUITS Resistances in Series: I T = I 1 = I 2 = I 3 V T = V 1 +V 2 +V 3 R T = R 1 +R 2 +R 3 adding resistors in series increases R T, decreases I T removing one resistor stops current in the whole circuit

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ANALYZING CIRCUITS EXAMPLE CIRCUIT 1 - assume 12 V battery R T =____ V T =____ I T =____ P T =____ R 1 = 8 V 1 =____ I 1 =____ P 1 =____ R 2 = 8 V 2 =____ I 2 =____ P 2 =____

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ANALYZING CIRCUITS EXAMPLE CIRCUIT 2 - assume 4 V per cell R T =____ V T =____ I T =____ P T =____ R 1 = 8 V 1 =____ I 1 =____ P 1 =____ R 2 = 16 V 2 =____ I 2 =____ P 2 =____

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ANALYZING CIRCUITS Resistances in Parallel: I T = I 1 + I 2 + I 3 V T = V 1 = V 2 = V 3 1/R T = 1/R 1 +1/R 2 +1/R 3 adding resistors in parallel decreases R T, increases I removing one resistor stops current only in that branch

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ANALYZING CIRCUITS EXAMPLE CIRCUIT 3 - assume 12 V R T =____ V T =____ I T =____ R 1 = 8 V 1 =____ I 1 =____ R 2 = 8 V 2 =____ I 2 =____

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ANALYZING CIRCUITS EXAMPLE CIRCUIT 4 - assume 12 V R T =____ V T =____ I T =____ R 1 = 1 V 1 =____ I 1 =____ R 2 = 2 V 2 =____ I 2 =____

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UNIT 7 FORMULAS F e = kq 1 q 2 /r 2 k = 8.99×10 9 Nm 2 /C 2 e = ± 1.60×10 –19 C F = qE K-K 0 = Fd I = Q/t V = W/Q R = L/A V = I R P = V I = I 2 R E = Pt R T = R 1 +R 2 +R 3 1/R T = 1/R 1 +1/R 2 +1/R kWh = 3.60×10 6 J

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