Transcript ENGR 111 Teaching plan

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
I2
I
I1
I
I = I1 + I2
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
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 coded
Example 1: KCL
50  I  30  0
Example 2: KCL
I1  I 2  I 3  I 4  0
 I 2  I1  I 3  I 4