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