1. ENGS2613 Intro Electrical Science Week 2. 23 & 25 January 2017 Dr
ENGS2613 Intro Electrical Science Week & 25 January 2017 Dr. George Scheets
Read 2.1 – 2.5
Problems 1.15, 16, & 29; 2.1, 12, & 16
Quiz #1, 27 January
Solve I's, V's, P's given a circuit
See Fall 2016 Quiz #1
Yours will be ≈ 20% easier

2. Quiz #1 Closed Book & Neighbor
Up to 3 unattached sheets of notes OK Keep on your desk
Calculator OK- No Sharing
Smart Phones NOT OK
Reaching down for something in backpack?
Get permission first

4. Wires Generally Have Resistance
12 gauge copper wire (diameter = inches) R ≈ Ω/foot
Exception: Superconducting wires have R ≈ 0 Can carry very large currents
77 K = -321◦ F
Sources:
&

5. Circuit Elements as of 23 January
Active Devices (Ideal. Can generate power) Current flow is Hi → Lo V outside device
Independent Voltage Source
Independent Current Source
Dependent Voltage Source
Dependent Current Source
Passive Device (can't generate power) Current flow is Hi → Lo V inside device
Resistor (Resists current flow)
Wire

6. Standard Assumptions Sustained current requires a loop
Path from High to Low voltage side of source
Connecting wires have R = 0 Ω
No voltage drop regardless of current
Treat ground as = 0 volts
Ohm's Law: V = IR Voltage drop across device = (Current thru it)(Resistance)
Power: P = VI

7. Simple Circuits No current loop exists. Hence no current flows.V+
40 Ω
50 Ω
9 V V2
V- = 0 Volts
= 0 Volts
9 V V+
40 Ω
50 Ω V2
= 9 Volts + - +
= 0 Volts
= 9 Volts -
V- = 9 Volts
No current loop exists.
Hence no current flows.
Using V = IR…

8. Resistors in Series I + - + - R1 R2 I + - R = R1 + R2
Exact same current flows thru R1 & R2? Yes? R = R1 + R2 I + - + - R1 R2 I + -
R = R1 + R2

9. Electrocution & You Current is the killer 120 VAC
Dangerous if you're wet
Image Source:

10. Current Amounts Source: hypertextbook.com/facts/2000/JackHsu.shtml
Source:

11. Hand-to-Hand Resistance, Dry
1.228 MΩ
I = V/R = 120 VAC 1,228,000 = μ amps AC = m amps AC = amps AC ≈ amps AC
You'll Feel This

12. Hand-to-Hand Resistance, Wet
522 KΩ
I = V/R = 120 VAC ,000 = μ amps AC = m amps AC ≈ amps AC
You'll Feel This

13. Hand-to-Hand Resistance, Wet & Salty
213 KΩ
I = V/R = 120 VAC ,000 = μ amps AC = m amps AC
You'll Feel This

14. 120 V AC Respect It! Full Body Immersion in Water Can Be Deadly
For Example: Today.Com

15. Current Amounts Those in the water were here.
Source: hypertextbook.com/facts/2000/JackHsu.shtml
Source:

16. Residential Areas 240 V AC to House Insulated Lines
120 V AC Neutral 120 V AC
Top Wire Not Insulated
Sine Wave
≈ 13,200 V AC

17. Hand-to-Hand Resistance, Wet
522 KΩ
I = V/R = 13,200 VAC ,000 = 25.3 m amps AC
TROUBLE!!

18. Current Amounts Source: hypertextbook.com/facts/2000/JackHsu.shtml
Source:

19. High Voltage Situations
Someone Can't Let Go?
Do NOT grab them to pull away
Ranked Options
Kill Power
Pry Them Off
Non-metallic Tool
Knock Them Off
More Dangerous

20. High Voltage Situations
In a Car under fallen Power Lines?
Stay There! Car sits on 4 rubber insulators
Have to get out?
Do NOT step out Do Not simultaneously touch Car & Ground or (Power Line & Ground)
Leap Out Still May Kill You if Ground is Wet

21. Lightning
30,000 amps & 15 Coulombs → 500 μsec 120,000 amps & 350 Coulombs → 2.9 msec
Human Struck? 10% - 25% Death Rate
Source: Wikipedia

22. Kirchhoff's Laws Named after Gustav Kirchhoff 1824 - 1887
Prussian Physicist University Professor
1845: Developed His Laws While a Student
Became his PhD dissertation
Source: Wikipedia

23. Node generally = Wire Current going in = Current going out
Kirchoff's Current Law
Plumbing System
Electrical System
α Gps
α A
3 Gps
3 A
β Gps
β A
α + β Must = 3
α + β Must = 3
Node generally = Wire Current going in = Current going out

24. Kirchoff's Voltage Law Electrical System CW KVL Loop Plumbing System+
6 PSI +
50 Ω
9 V -
4 V + -
40 Ω
3 PSI
Pump -
0 Volts
1 PSI
Electrical System
CW KVL Loop
= 0
CCW KVL Loop
– 9 = 0
Plumbing System
+5 – 3 – 2 = 0 psi

25. DC Circuit with Resistors
To Solve, Need… Ohm's Law Kirchhoff's Current Law Kirchhoff's Voltage Law
… Properly Applied Algebra

26. Using Kirchhoff's Voltage Law
Draw current loops
Arbitrary direction (clockwise or counter-clockwise)
Hit each circuit element at least once (preferably only once)
Label voltage drops for each element
MUST follow current directions
Passive device (Resistor):
Current into + side, Current out on – side
Active device (Voltage or Current Source):
Current exits + side, Current enters – side
+ side is at a higher voltage than – side (if current direction is correct)
Write equation for voltage drops or gains around loop
Arbitrary direction (with or against current flow)
Going from – side to + side? Positive value in equation
Going from + side to – side? Negative value in equation
Solve Equations