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DC Tutorial

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PW is over!!!

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DC Circuits Website: http://dccircuits.wordpress.com/ Pilot Test

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Which self-attempt questions do you want to go through? Comments and areas for improvement on the website?

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Today’s Plan Discussion Questions Today’s lecture Slides uploaded Will continue it online if we cannot finish today Self-attempt Questions DC Circuits Website http://dccircuits.wordpress.com/ Simplifying circuits takes time to master

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Four Stages of Life: 1.) You believe in Santa 2.) You don’t believe in Santa 3.) You are Santa 4.) You look like Santa

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Only a kid would think that these are true World is more complex than you previously thought

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DC Circuits: Full of traps You think that you got it right, but actually, you were tricked

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Current flow as water flow Current takes the path of least resistance All the current will flow here “short circuit” No current flows here Visit website to find out why

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D1 Key LessonsWorking and Calculations Assumption Voltmeter : Infinite Resistance Equation Solve for equivalent resistance: Voltmeter doesn’t affect resistance of circuit much 8

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D1 Key Lessons Working and Calculations Assumption Light Bulb : Finite Resistance Equation Solve for equivalent resistance: Equivalent Resistance is lower than expected Larger Resistor gets a larger share of emf

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D2 Key Lessons Working and Calculations Diode allows current to flow in one direction only For Diode, V = RI does not work. Instead, we read the graph. For ammeter, V = RI = 100 x 0.008 = 0.8V Current through diode = 6A (read from graph) Total Current = 6mA + 8mA=14mA I V

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D2 Key Lessons Working and Calculations Why does current not flow like this?Energy will not be conserved. Show Excel Simulation

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D3 Parallel circuit?

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Is this series? Visit website to find out why

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Follow the wire Split into Three Three combine into one

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D3 Split into Two Combine back

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5 Ω 0.80 Ω Component C D3 Key LessonsWorking and Calculations Parallel Circuit: 5 Ω resistor and component C have same potential difference across them For 5 Ω resistor, V = RI = 0.85 x 5.0 = 4.25V For Component C, p.d = 4.25V (parallel) Current = 1.3 A (read graph)

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5 Ω 0.80 Ω C D3 Key LessonsWorking and Calculations Around a closed loop, sum of p.d. is equal to the sum of the emf For 0.80 Ω resistor, V = RI = 0.8 x 2.15 = 1.72V Sum of emf = sum of p.d. = 1.72 + 4.25 = 5.97V 1.3A 0.85 A 2.15A 4.25V 1.72V

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5 Ω 0.80 Ω C D3 Key LessonsWorking and Calculations Around a closed loop, sum of p.d. is equal to the sum of the emf For 0.80 Ω resistor, V = RI = 0.8 x 2.15 = 1.72V Sum of emf = sum of p.d. = 1.72 + 4.25 = 5.97V 4.25V 1.72V

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D4 Key LessonsWorking and Calculations Misconception Practice Question Resistance = NOT V I Resistance Is NOT Gradient NIL

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D1 Key LessonsWorking and Calculations Larger resistor gets a proportionally larger share of battery emf (closed loop) Potentiometer O E X S R Battery emf (2V) 0.005V 1.995V wire R 3Ω3Ω??

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Revision Time!

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A B E PD “No Deflection” Concept

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A B E PD

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A B E

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A B E

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A B E

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A B E No deflection No current PD = E P P is called balance point AP is called balance length

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D1 Key LessonsWorking and Calculations “No Deflection” Concept Potentiometer O E X S R 0.005V 60% of length p.d. is _____ E = 0.003V

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D1 Key LessonsWorking and Calculations “No Deflection” Concept Potentiometer O E X S R 0.005V 60% of length p.d. is _____ E = 0.003V

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D2 Key LessonsWorking and Calculations 1.Know how to simplify circuits 2.Larger Resistor Proportionally larger share of emf 3.Potentiometer Potentiometer 0.005V 60% of length p.d. is _____ E = 0.003V 80 1.50 V 20 2.00 V AC J B S1S1 S2S2 a ii)

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E S Galvanometer No deflection No Current Flow in lower circuit Pg from lecture notes Resistance does not matter

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Does the resistance matter? E X S Galvanometer No deflection No Current Flow in lower circuit Refer to Page 16 of lecture notes

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Any Current Flow? Does internal resistance matter? Why? No No current means no power dissipated due to internal resistance. I 2 R = 0 Yes There is power loss! I 2 R > 0. Part of the energy provided by the battery will be lost due to internal resistance. V = RI If I is zero, V = 0

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Does the resistance matter? E X S 1Ω1Ω2Ω2Ω 3Ω3Ω5Ω5Ω 10Ω 1337Ω No current no energy dissipated p.d is zero resistor is useless Answer to b(ii) : No change in answer. Lower circuit resistance does not matter

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D2 Key LessonsWorking and Calculations Know how to simplify circuits Potentiometer 2V Need to balance 1.5V 80 1.50 V 20 2.00 V AC J B S1S1 S2S2 Length is 0.75m (75% of 1m)

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Bottom Circuit Top Circuit 1.5V Opposes Net emf is zero No current flow

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Bottom Circuit Top Circuit 1.5V 1.2V Opposes Net emf is not zero current still flows

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D2 Key LessonsWorking and Calculations Use DC Circuit Construction Kit to understand this problem fully! Potentiometer 80 1.50 V 20 2.00 V AC J B S1S1 S2S2 80%20% Blue Circuit(Top) is opposing 80% of red circuit 80% of 1.5V = 1.2V 2V Need to balance 1.2V There is still current flow. Resistance cannot be neglected Balance length: 60cm If battery has internal resistance, BL drops

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Summary of part i and ii Top Circuit Bottom Circuit Current flow in lower circuitResistance ii1.5V no Doesn’t matter i1.2V1.5Vyesmatters No current no power no heat loss resistor does nothing

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