# ENGR 111 Lecture 3 - PowerPoint PPT Presentation

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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

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ENGR 111 Lecture 3

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## ENGR 111 Lecture 3

### 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

I2

I

I2

I1

I

I = I1 + I2

### 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