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PHYSICS UNIT 7: 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 electrostatics: 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 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 Charging by conduction & induction

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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 on 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 charges k: 8.99×10 9 Nm 2 /C 2 +F e : repulsion, –F e : attraction

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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 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 Field Lines E field lines are constructed by determining what a positive charge would do if placed in the field The denser the lines the stronger the field. Lines always emanate from positive charge and end at negative charges.

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Lines of Equipotential The grey dotted lines represent places where the Net E-field magnitude is equal. Note how on the parrallel plate scenario The E- field is equal for any point within the plates.

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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 = Work K f -K 0 = Fd (Work done by the field) d: distance traveled in electric field K = ½mv 2

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Electric Potential Imagine that a positive charge q is released from contact with Q and is allowed to accelerate to an infinite distance away picking up KE as it goes. The Change in KE is the Work required to bring the test charge from an infinite distance back to Q. Electric Potential, V, is work per unit charge that is needed to bring q toward Q V = W/q Units are Joules/Coulomb Q q

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Examples What is the electric potential at point A of a 2 Coulomb charge that requires 10 J of work to move from B to A? V= W/q = 10/2 = 5 J/C = 5V The Electric Potential Difference is called VOLTAGE! B A

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Potential Energy Is PE increasing or decreasing as q, 5 C, moves towards Q? IF V b =0 and V a =10, Then Voltage is 10 volts at A. Potential Energy is equal to work needed to move q to A. So… W = qV = U = 50 Joules! AB

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Negative Charges? What happens if negative q, -5C, is moved from A to B? Assume V b =0 and V a =10 Then as q moves to A PE decreases. U=qV a =(-5)(10)= -50 J A B

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Conclusion F = qE E = kQ/r 2 W = q V U = q V Positive charges move naturally from high electric potential to low electric potential Negative charges move naturally from low electric potential to high electric potential All charges move from high PE to low PE

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The Electron Volt The Joule is a huge unit of energy when dealing with electrons moving across electric potentials. How much energy would an electron gain if it moved across a potential difference of 1 V? U = qV = (1.6 X 10 -19 C) (1V) = 1.6 X 10 -19 J So.. 1.6 X 10 -19 J is defined as an electron volt. This unit can be used as an energy unit for situations dealing with small charges.

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PHYSICS UNIT 7: ELECTRICITY

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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 conventional current flow: positive to negative (real current is electrons, flowing negative to positive)

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ELECTRIC CIRCUITS Potential Difference or Voltage Drop, V: work done per coulomb of charge between two points, unit: volt, V V = W/q 1 V = 1 J/C 12 V gives 12 J/C to the electrons

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ELECTRIC CIRCUITS Sources of Potential Difference capacitor: stores charge battery: cells connected in series cell: stores chemicals; reactions produce V for cells in series, battery voltage is the sum of cell voltages anode cathode

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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

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resistance of a length of wire, R = L/A : resistivity ( ·cm), L: length (cm), A: cross-section (cm 2 ) silver =1.59×10 –10 copper =1.68×10 –10 carbon =3.00×10 –7 silicon =0.00100 for solids, as T increases, increases and vice versa

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ANALYZING CIRCUITS Ohm’s law: current is proportional to voltage and inversely proportional to resistance: V = IR V: voltage, V I: current, A R: resistance, applies to circuit as a whole: V T = I T R T applies to each part of a circuit: V 1 = I 1 R 1 V 2 = I 2 R 2

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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 R1R1 R2R2 R3R3

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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 R1R1 R2R2 R3R3

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ANALYZING CIRCUITS Kirchoff’s 1 st Rule: total current entering a junction equals total current leaving a junction (conservation of charge) Kirchoff’s 2 nd Rule: total voltage change around any closed loop of a circuit is zero (conservation of energy) I 1 = I 2 + I 3

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ELECTRIC ENERGY & POWER Electric Power: rate of electric energy supply or use, in Watts, W power supplied or used, P = V I, 1 W =1 J/s power used, P = I 2 R (appliance and light bulb ratings)

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ELECTRIC ENERGY & POWER Electric Energy: work done (energy transferred) by electric current, in Joules, J (electric companies bill for energy, not power) energy, E = Pt electric bill in kilowatt-hours, 1.00 kWh = 3.60×10 6 J

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ANALYZING CIRCUITS EXAMPLE CIRCUIT 1 - assume 4 V per cell 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 EXAMPLE CIRCUIT 3 - assume 4 V per cell 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 4 - 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 EXAMPLE CIRCUIT 5 - assume 5 V per cell R T =____ V T =____ I T =____ P T =____ R 1 = 1 V 1 =____ I 1 =____ P 1 =____ R 2 = 6 V 2 =____ I 2 =____ P 2 =____ R 3 = 12 V 3 =____ I 3 =____ P 3 =____

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CIRCUIT BOARD INTRO

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Springs are connectors for wires and components. Some springs are connected to devices on the board (like the D-cells). If a spring is too loose, squeeze the coils.

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CIRCUIT BOARD INTRO When you connect a circuit to a D-cell note the polarity (+ or –). Only connect things long enough to make your observations & measurements, then disconnect one wire so the D-cells don’t run down and resistors don’t overheat

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ELECTRIC ENERGY & POWER Electric Hazards effect of shock depends on location skin: burns, muscles: spasms, nerves: pain, heart: disruption effect of shock depends on current <10 mA: pain, no damage >10 mA: severe muscle contraction, paralysis 70 mA chest: heart fibrillation 1 A chest: heart stops completely, but may restart

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ELECTRIC ENERGY & POWER Electric Hazards body resistance 10 4 to 10 6 dry, 10 3 wet short circuit: low resistance path low resistance = large current shock, fire fuses & circuit breakers: disconnect circuit above a specific current level

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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 3 1.00 kWh = 3.60×10 6 J

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