intro to electricity - Copy.ppt
- 2. Electricity ~ water simili water pressure water out second constrict hose water Wire (electrical component) Electrical pressure (voltage) electronic charge charge out second resistance Official circuit symbol for fixed value resistor
- 3. Effect on R of increasing hose or wire length on flow Effect on R of increasing hose or wire cross section on flow Effect on R of increasing pressure or voltage Resistance analogies none R~ Ro *L/A
- 4. Temperature effects on wire resistance R T R~RoeE/T T increase causes resistance to increase in normal conductors…conversely, lower temperatures decrease resistance. As temperature increases, more random motion in wire disrupts flow of current (~scatters e-) Some materials’ R 0 below T threshold= superconductors
- 6. Japan’sMag Lev `Bullet Train’ 200 mph !
- 7. Basic Electrical units unit Symbol unit Qualitative description charge Q or q Coulomb (coul=C) or Faradays (F)* Basic material pushed around in electronic components potential V or E Volts (joules/coul) `pressure’ that pushes charge across the components current I or i Amperes=amp (coul/second=A) Charge flow rate Resistance (impedance) R or Z Ohms (volts/amp=) `friction’ slowing down charge flow * 96,485 C=1 F
- 8. Ohm’s law Simple : V= I x R General: V = I Z Dot product => I*Z*cos along I vector direction Vector versions of V, I Z=complex impedance including capacitance, inductance Pertinent manipulations: R R= V = volts = ohms I coul/s Scalar versions of V, I R=simple resistance = joules *s = ohms coul2 DC AC
- 9. Power Power = P=energy/second = joules/second = watts P=V*I = joules *coul = joules coul sec sec Applying Ohm’s law: substitute V=IR or I= V/R P= (IR)*I = I2R P = V* (V) = V2 ( R) R
- 10. Kirchoff’s Voltage and Current Laws (KVL and KCL) KVL: In any closed loop the voltage sum =0 Vapplied = IjRj KCL: At any point in a circuit, the sum of current in and out of that point = 0 Iin = Iout
- 11. G Basic Instrumentation Device Using Resistors: Wheatstone Bridge (Pierpont 1843 !) G= galvanometer Or…current meter I (usually adjustable) Often the test R At `null’ on G*: R4 = R2 R3 R1 R1 and R2 are usually fixed *no current across bridge where G is V
- 12. Modern meters that measure across `G’: Digital volt-ohm meters (DVM)
- 13. Red probe here for mA current (I) Red probe here for high (I) current (20 mA max) Com= common=`ground’ Red probe here for V (volts) and (ohms)
- 14. Circuit configurations for V, I and R in DVMs (your text)
- 15. AC vs DC V DC is simple & extension of a battery Time Time AC is time varying & not an extension of a battery V In the beginning… there was only DC Then came AC on off + -
- 16. Some history…..in the time before there were wall sockets Thomas Edison sez…DC is the all-American way, baby !” Nickolai Tesla sprach1…”Nein, nein..diese AC ist ganz besser, dumpkopf !!” (Use AC, dummy !) 1.. Actually, he was Serbian, not German, but the point is, he was not American Die, Euro trash ! Eat dirt, American scum2 2 Ralph Fiennes…winner of the Nicolai Tesla look-a-like contest Do we use DC or AC ???
- 17. History (continued): the big fight `Tesla coil demonstrates that AC can be `transformed’ up and down in V without (much) loss…. (and still used safely…) I have 1093 US patents and you don’t, you greasy foreign maggot… Tesla showboating the safety features Serbia rules, American pigdog ! (and I got 700 patents, turkey) The Winner SUPERIOR PHYSICS The challenger The Champ Left standing
- 18. Historic aside: Who is credited with inventing the radio? The usual answer: Marconi Actually, after a long fight, US Supreme Court invalidated all of Marconi’s patents in 1937 and declared this guy the real inventor: Tesla
- 20. AC generators: How to make AC power Coils Coils parallel with field ~reverse of Lorentz force Coil anti-perpendicular to field Coil perpendicular to field
- 21. Advantages of AC generation 1) V and I increase with: •Speed of turning : mechanical electrical conversion) • strength of magnet (H) • Number (N) and area (A) of wire turns 2)Can `transform’ or `step’ voltage through winding pairs with low loss. Vin Vout Vout = # windings out Vin # windings in ) I~ N*A*sin *H/t •Angle it makes with field () A
- 22. One more advantage… Sine wave input into capacitor Sine wave out of capacitor Output shape = input shape DC into capacitor DC out of capacitor Output shape input shape volts
- 23. Niagara Falls AC generator (Tesla’s big chance) First commercial AC generator: Buffalo NY (Tesla design) US side of Niagara Falls Schematic of hydroelectric power plant
- 24. Niagara (continued) Total generation ~ 4.4 GW
- 25. One Problem with AC Generation: Coal-fired power stations Six 100 MW stations
- 26. Global warming consequences Part of why we’re in such a mess… No action or initiative on alternative energy in 8 years of Bush rule “Hey, I’m not analytical. I don’t spend time thinking about why I do things.” Our ex -Prez Aboard Air Force One 6/4/03
- 27. And this guy is making it still worse