# DC Tutorial. PW is over!!! DC Circuits Website: Pilot Test.

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DC Tutorial

PW is over!!!

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

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

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

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

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

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

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

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

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

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

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

D3 Parallel circuit?

Is this series? Visit website to find out why

Follow the wire Split into Three Three combine into one

D3 Split into Two Combine back

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)

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

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

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

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??

Revision Time!

A B E PD “No Deflection” Concept

A B E PD

A B E

A B E

A B E

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

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

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

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)

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

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

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

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

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)

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

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

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

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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