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ENGR 111 Lecture 3. Reading: Chapters 19, Class notes. 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

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ENGR 111 Lecture 3

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Engr 111 lecture 3 l.jpg

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

I2

I

I2

I1

I

I = I1 + I2


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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

V2

V1

V3

Start

End

V5

V4

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 coded


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Example 1: KCL


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Example 2: KCL


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