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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

ENGR 111 Lecture 3

Reading: Chapters 19, Class notes

lecture 3 dc fundamentals
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
electrical charge
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
electrical charge4
Electrical Charge
  • Charges of opposite polarity attract
  • Charges of similar polarity repel
  • Electrical charge can be created through chemical processes
    • Batteries
electrical fundamentals
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
current
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
current7
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)
resistance
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)
water analogy
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
water analogy10
Water Analogy
  • Harder to push water through a thinner pipe (smaller current, higher resistance)
water analogy11
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
water analogy12
Water Analogy
  • Another model for voltage
some basic laws kirchoff
Some basic laws (Kirchoff)
  • Kirchoff’s Current Law (KCL):
    • Current flowing into and out of a node should be equal
    • Conservation principle
slide14
KCL

I2

I

I2

I1

I

I = I1 + I2

kirchoff s voltage law
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

summary
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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