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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 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

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

- 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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