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 Electron negative, Proton positive  More electrons than protons, negatively charged  More protons than electrons, positively charged

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

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 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

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

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 Analogy Another model for voltage

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

KCL I I1 I2 I = I1 + I2 I2 I

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

KVL

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

Example 1: KCL

Example 2: KCL