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ENGR 111 Lecture 3

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

  2. 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

  3. 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

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

  5. 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

  6. 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

  7. 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)

  8. 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)

  9. 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

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

  11. 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

  12. Water Analogy • Another model for voltage

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

  14. KCL I2 I I2 I1 I I = I1 + I2

  15. 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

  16. KVL

  17. 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

  18. Example 1: KCL

  19. Example 2: KCL

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