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ENGR 111 Lecture 3 Reading: Chapters 19, Class notes

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Lecture 3: DC Fundamentals Electrical Charge (q): more or fewer electrons In an atom, # of electrons = # of protons When they differ, electrical charge is present Each electron/proton carries a unit charge Electron negative, Proton positive More electrons than protons, negatively charged More protons than electrons, positively charged

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Electrical Charge Unit of Charge: 1 coulomb (1C) Equal to charge of 6.24x10^18 elementary charges An electrical (or electrostatic) field surrounds a charge The field strength proportional to charge The field strength inversely proportional to square of distance from the charge

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Electrical Charge Charges of opposite polarity attract Charges of similar polarity repel Electrical charge can be created through chemical processes Batteries

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Electrical fundamentals Voltage is the potential difference of charge at two points in an electrical field Voltage symbol V, unit Volts Voltage results in the flow of charge between two points

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Current Flow of charge = Current Current symbol I, unit Amperes 1 Ampere current = Flow of 1 coulomb of charge past a point per second Charge flows through movement of electrons Current is said (by convention) said to flow in the opposite direction

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Current Current can be DC (Direct) or AC (Alternating) DC current always flows in the same direction Batteries, cells AC current changes direction periodically Wall power outlets (120V, 60 Hz)

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Resistance Materials offer different resistance to current Conductors (Aluminum, copper, gold) –low Insulators (Glass, rubber, plastic) – high Semiconductors (Silicon, gallium) – in between Resistance, symbol R, unit Ohms (Ω)

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Water Analogy Charge flow through a wire similar to water flow in a pipe Water flow measured in gallons/sec, not molecules/sec Current measured in coulombs (6.24x10^18 elementary charges)/sec

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Water Analogy Harder to push water through a thinner pipe (smaller current, higher resistance)

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Water Analogy For water to flow, there has to be pressure difference at the two ends of the pipe Voltage has to exist across a wire for current

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Water Analogy Another model for voltage

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Some basic laws (Kirchoff) Kirchoff’s Current Law (KCL): Current flowing into and out of a node should be equal Conservation principle

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KCL I I1 I2 I = I1 + I2 I2 I

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Kirchoff’s voltage Law Voltages around a closed circuit should sum to zero When you come to the same point, voltage difference should be zero Start End V1 V2 V3 V4 V5 V1 + V2 + V3 +V4 + V5 = 0

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KVL

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Summary Rate of flow of charge = current Differences in charge potential = voltage Different materials offer different resistance to charge flow KCL = current at a node sums to zero KVL = Voltage around a loop sums to zero Resistors are color codedcolor coded

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Example 1: KCL

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Example 2: KCL

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