engr 111 lecture 3 reading: chapters 19, class notes
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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
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 attractCharges of similar polarity repelElectrical 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 = CurrentCurrent symbol I, unit Amperes1 Ampere current = Flow of 1 coulomb of
charge past a point per secondCharge 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
V1V2
V3
V4
V5
V1 + V2 + V3 +V4 + V5 = 0
KVL
Summary
Rate of flow of charge = currentDifferences in charge potential = voltageDifferent materials offer different
resistance to charge flowKCL = current at a node sums to zeroKVL = Voltage around a loop sums to zeroResistors are color coded
Example 1: KCL
50 30 0 I
Example 2: KCL
1 2 3 4
2 1 3 4
0 I I I I
I I I I