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Homework Test Corrections p. 526: 9, 10, 13

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Quiz What is an electric circuit? What are the fundamental units of an Ampere? How do I find the voltage required to push a current through a given resistance? What is the relationship? How do I find the power generated when a current is moved through a potential difference? What is a potentiometer? What is a kilowatt-hour? What is a kilowatt-hour a measure of? (_ _ _ _ _ _ )

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A flow of charged particles in a closed system. (Which can be very large…) Electricity

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Electron Energy Kinetic Energy Schematic Current (I): Definition Potential Difference Battery Internal Resistance Circuit kWH and Usage Ohms / Mhos V: Definition Franklin RT = R1+ R2 + … RT = 1/R1+1/R2 +1/R3 + … R = (T)L/A P = IV Parallel V = IR P = I 2 R Graphs: VI(Linear) IV(Linear) V=IR Light bulb R(t) Work Resistance (R): Definition Series Positive and Negative Current SuperConductor Ohm’s Law Tools: Voltmeter Ammeter VOM DMM Rheostat Potentiometer Battery Material Resistivity / Reference Tables R=R(T)

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Voltage, Current & Circuits Voltage is the potential difference that drives the movement of charged particles. (Named after Volta, who rediscovered the battery) Current is the flow of charged particles. An electric circuit is composed of three elements: 1) conductor 2) potential difference (V), supplied by battery, solar cell, thunderstorm, … 3) closed loop

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Voltage, Current & Circuits Voltage is the potential difference that drives the movement of charged particles. Voltage is analogous to height in a gravitational field: gh Multiply by m mgh – and you now have gravitational potential energy (Joules). Electrical potential energy is W = qV = Joules. The unit of V is Joules / coulomb

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Current Electric current is the rate of flow of charge I = Δq/ Δt Ampere = Coulomb per second (passing by a point in a circuit) Ampere made discoveries regarding the interrelationships between electricity and magnetism in the early 1800’s

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Current – which way does it go? Two types of current: Conventional current: Current flows from the positive terminal to the negative terminal Electron flow: Follow the electrons! Which are going the wrong way (by definition) Why are these not the same? –What famous American guessed wrong? When was the electron discovered?

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Current – which way does it go? Two types of current: Conventional current: Current flows from the positive terminal to the negative terminal Electron flow: Follow the electrons! Which are going the wrong way (by definition) Why are these not the same? –What famous American guessed wrong? When was the electron discovered? –1898 (JJ Thompson) with his cathode ray tube. Long after current was defined.

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How do I visualize what is going on in an electric circuit?

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What is driving the current?What is driving the flow?

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Ancient Video http://www.stmary.ws/highschool/physics/home/ notes/electricity/circuits/default.htm http://www.stmary.ws/highschool/physics/home/notes/electricity/circuits/ CurrentBasics.htm http://www.stmary.ws/highschool/physics/home/notes/electricity/circuits/ OhmsLawEquation.htm 30 seconds with Mr. G.

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Ohm’s Law: Ohm's Law …says that, for many materials under a wide range of conditions, the voltage, V, and current, I, are linearly related, which implies resistance, R, is independent of V and I.

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Linear equation with zero intercept: y = mx V = IR The slope is the resistance, R

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Resistance –the ratio of V/I Units: Ohms –Its inverse is conductance, unit is mhos Georg Ohm promulgated his famous law in 1827 –He was a high school Physics teacher!

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Which is the dependent and which the independent variable? The slope is the inverse of the resistance, 1/R

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Ohm’s Law V = IR Why do I care? Fix wiring problems in my home* Fix wiring problems in my car Fix my kid’s toys Figure out why my circuit breaker / fuse blows Do my job at work (the one I have now, not the one I had for the last 27 years) *Disclaimer: Consult your parents or a qualified electrician before trying this yourself.

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Ohm’s Law Mnemonic

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V = IR

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V ----- = I R

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V ----- = R I

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Ohm’s Law

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Ohm’s Law: Ohm's Law …says that, for many materials under a wide range of conditions, the voltage, V, and current, I, are linearly related, which implies resistance, R, is independent of V and I. When does it not apply? Circuit elements that change temperature Examples? Circuit elements with large capacitance or strong magnetic fields (that are changing) Semiconductors: materials that are natural insulators that are made to be somewhat conductive Diode – like a one-way path

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Homework p. 526: 9, 10, 13

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Example Units: Volt Current Resistance

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Example A 10V battery is placed across a 5 Ohm resistor. What is the current? A 10A current flows through a 10 Ohm resistor. What is the voltage? A 10V battery drives a 1A current through a resistor. What is the resistance?

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Power Power is the rate of doing work (Watts) Power = Work / time Power = Volt * q / t but q/t = I Electrical Power = Voltage * Current = VI Electrical Energy = Power * Time = VIt

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Homework: Page: 465 –Problems 1-7

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Resistance and Ohm’s Law Resistance is the impedance to the flow of charge Resistance is defined as the ratio of potential difference to current R = V/I Unit is the Ohm, Ω A device is said to obey Ohm’s Law of its resistance is independent of the Voltage

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Find Resistance of a material Depends on four factors –R ∞ Temp –R ∞ RHO (ρ) resitivity –R ∞ Length –R ∞ A -1 (cross-sectional Area)

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Low resistance Short Fat cold

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High Resistance Long Thin Hot

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Problem Find the resistance of a 20 meters length of Aluminum with a diameter of 12 mm

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Givens Length 20 m RHO =2.82 * 10 -8 Ω * m Area = π r 2 D= 12 mm =.012 m r =.006 m A = π (.006m) 2 = m 2

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Chap 22 Problems: 8-13, 15-20 Ohm’s Law Worksheet

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Diagramming circuits

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Series Connection: –Within the circuit, –ammeters are always in the circuit, they measure flow of charge Ammeters should always have low resistance

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Parallel Connection –Provides multiple paths for flow –Voltmeter is always placed parallel to the device you wish to measure the potential difference and have very high resistance

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More Electric Energy Equations P = W/t Watt P = VI = V 2 /R =I 2 R Electric Energy (W)= Pt = VIt = V 2 /Rt =I 2 Rt Joules for all types of Energy

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Chapter 22 Problems: 21-25

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Kirchoff’s Junction Rule Conservation of charge

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Kirchoff’s Loop Rule Conservation of Energy The sum of the changes in potential around any closed path (loop) of a circuit must be zero.

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Series Circuit Current : One path for the flow of charge I t =I 1 =I 2 =I 3 Resistance must increase because length increases R t -=R 1 +R 2 +R 3 Voltage must add to Zero V t -V 1 -V 2 -V 3 =0 V t =V 1 +V 2 +V 3

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Series Diagram

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VIRP Table V t = V 1 = V 2 = I t = I 1 = I 2 = R t =R 1 =R 2 = P t = P 1 = P 2 =

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Practice Series Circuits /www.stmary.ws/physics/home/animations3/ electricity/ElectricPowerChallenge.html.net /ed1_files/circuits1.html http://www.stmary.ws/highschool/physics/ho me/notes/electricity/circuits/default.htm

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Homework: Chapter 23: –Problems:page 488 –1-6

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Parallel Circuit More than one path for the flow of charge More room for the flow of charge so Resistance goes down Voltage must stay the same since all charges have the same drop

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Parallel Equations

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Parallel Diagram

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Practice Parallel Homework: Problems 8-14, ****16

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