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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) + – neutral: electrons equal protons (no net charge) + – positive charge: deficiency (loss) of electrons + – negative charge: excess (gain) of electrons

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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 = 6.25×1018 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 Fe = kq1q2/r2 Fe: electric force q: charge r: distance between charges k: 8.99×109 Nm2/C2 +Fe: repulsion, –Fe: 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 Kf-K0 = Fd (Work done by the field) d: distance traveled in electric field K = ½mv2

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Electric Potential Q q 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

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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 Vb=0 and Va=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! A B

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

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**Conclusion F = qE E = kQ/r2 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 C) (1V) = 1.6 X J So X 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 load: energy user (bulb, resistor, heater, motor)**

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 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 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 anode cathode cell: stores chemicals; reactions produce V battery: cells connected in series for cells in series, battery voltage is the sum of cell voltages

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ELECTRIC CIRCUITS Resistance, R: opposition to charge flow, unit: ohm, W 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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**ELECTRIC CIRCUITS resistance of a length of wire, R = rL/A**

r: resistivity (W·cm), L: length (cm), A: cross-section (cm2) rsilver=1.59×10–10 rcopper=1.68×10–10 rcarbon=3.00×10–7 rsilicon= for solids, as T increases, r 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, W applies to circuit as a whole: VT = ITRT applies to each part of a circuit: V1 = I1R V2 = I2R2

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ANALYZING CIRCUITS Resistances in Series: IT = I 1 = I2 = I3 VT = V1+V2+V3 RT = R1+R2+R3 adding resistors in series increases RT, decreases IT removing one resistor stops current in the whole circuit R1 R2 R3

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**ANALYZING CIRCUITS Resistances in Parallel: IT = I1=I2+I3**

VT = V1 = V2 = V3 1/RT = 1/R1+1/R2+1/R3 adding resistors in parallel decreases RT, increases I removing one resistor stops current only in that branch R1 R2 R3

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ANALYZING CIRCUITS Kirchoff’s 1st Rule: total current entering a junction equals total current leaving a junction (conservation of charge) Kirchoff’s 2nd Rule: total voltage change around any closed loop of a circuit is zero (conservation of energy) I1 = I2 + I3

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**ELECTRIC ENERGY & POWER**

Electric Power: rate of electric energy supply or use, in Watts, W power supplied or used, P = VI, 1 W =1 J/s power used, P = I2R (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 = 3.60×106 J

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**ANALYZING CIRCUITS EXAMPLE CIRCUIT 1 - assume 4 V per cell**

RT=____ VT=____ IT=____ PT=____ R1= 8 W V1=____ I1=____ P1=____ R2= 8 W V2=____ I2=____ P2=____

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**ANALYZING CIRCUITS EXAMPLE CIRCUIT 2 - assume 4 V per cell**

RT=____ VT=____ IT=____ PT=____ R1= 8 W V1=____ I1=____ P1=____ R2= 16 W V2=____ I2=____ P2=____

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**ANALYZING CIRCUITS EXAMPLE CIRCUIT 3 - assume 4 V per cell**

RT=____ VT=____ IT=____ PT=____ R1= 8 W V1=____ I1=____ P1=____ R2= 8 W V2=____ I2=____ P2=____

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**ANALYZING CIRCUITS EXAMPLE CIRCUIT 4 - assume 4 V per cell**

RT=____ VT=____ IT=____ PT=____ R1= 8 W V1=____ I1=____ P1=____ R2= 16 W V2=____ I2=____ P2=____

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**ANALYZING CIRCUITS EXAMPLE CIRCUIT 5 - assume 5 V per cell**

RT=____ VT=____ IT=____ PT=____ R1= 1 W V1=____ I1=____ P1=____ R2= 6 W V2=____ I2=____ P2=____ R3= 12 W V3=____ I3=____ P3=____

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CIRCUIT BOARD INTRO

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CIRCUIT BOARD INTRO 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 104 to 106 W dry, 103 W 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 Fe = kq1q2/r2 k = 8.99×109 Nm2/C2 e = ± 1.60×10–19 C**

F = qE K-K0 = Fd I = Q/t V = W/Q R = rL/A V = IR P = VI = I2R E = Pt RT = R1+R2+R3 1/RT = 1/R1+1/R2+1/R3 1.00 kWh = 3.60×106 J

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