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Current Electricity

Current Electricity

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Current Electricity

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  1. Current Electricity

  2. Current Electricity –what have we discussed static electricity • Properties of Conductors & Insulators. • Transfer of charge through various methods. • Calculation of Forces between charged objects. • Calculation of Forces exerted by electric fields. • Energy of charges at positions electric fields (voltage)

  3. We’ll apply what we know to current electricity. What does current mean? Flow or motion. What does electric current mean? Flow or motion of electric charge.

  4. What will happen if two oppositely charged metal plates touch? Hint: Remember The conducting spheres. How long? Until the plates have equal charge.

  5. We can use E of charges in motion to do work. If we transfer charges from high PE to low PE, the charge can do some work converting E– but not for long.

  6. With static I can charge an object, & transfer the electric charge to do work. Static electricity is quick transfer of charge but charge stops when equilibrium reached. Demo with bulb We often want continuous flow of charge because we want continuous work.

  7. Current Electricity provides continuous motion or flow of charges.

  8. Current still needs a potential difference/voltage to push charges to move. Each volt does work on each coulomb of charge & each charge changing q’s Energy: V = W/q. V = DE/q. The van der Graff & sphere set up p.d. The vdg is not continuous enough for constant work.

  9. Current Electricity requires constant source of potential difference or voltage. • Battery • Wall outlet • Generator • Solar Cell

  10. Constant Potential Dif/Voltage induces charges to move continuously.

  11. Voltage/p.d. provides the “push” or “pressure” to charges. Think of 1 side as more + and 1 side more neg + - The higher the voltage/p.d., the more push or pressure each charge gets, the more E each q has, the more W each q can do.

  12. An electric circuitallows current to flow an to do continuous work. Circuits need the following components: • A source of continuous voltage. • A pathway made of conducting materials. • Appliance(s) to convert electric E to some other form (heat, light, sound etc.)

  13. Which type of charges can move? Solids only e- flow. Liquids any charged particles or ions can flow.

  14. How many ways can you set up a circuit of 1 battery, 2 bulbs, 3 wires to li • Use all the components for each circuit, but no extra components allowed. • Sketch all the arrangements that light the bulbs. • Sketch all the arrangements that did not light the bulbs. • Look at your sketches. Explain what you think is happening physically. Why do the bulbs light with certain arrangements? Why won’t they light with others? Write your hypothesis under your sketches. • For a circuit that worked, list the E transformations. • Write down any questions you have about the circuit. • If time allows, find the circuit symbols in your reference table. Re-sketch your circuits using proper symbols.

  15. Discussion Review Sheet

  16. Measuring Current • Rate of flow of charge.

  17. Amperes (A) measures rate of coulombs passing a point in a wire. # charges per sec or # Coulombs per sec. 1 A = 1 C/s passing a point or cross section of wire.

  18. I = q/t I = current C/s or A Q = charge in Coulombs t = time in seconds

  19. Ex 1: 100 C pass a section of wire every 5 sec. How much current flows in the wire? I = q100C 20 C/s or 20 A. t 5 s

  20. Ex 2: The current in a light bulb is 0.835 A. How long does it take for a total charge of 1.67 C to pass a point in the wire? I = Q/t t = Q/I t = 1.67C = 2.00 s 0.835 C/s

  21. Ex 3: If, in 1 second 6.25 billion billion (6.25 x 10 18) electrons pass through a point in a wire, what is the current? 1 A

  22. Hwk read text 19-1 • Do page 695 # 3 – 5 & pg 717 #1 – 6 neatly on lined in full sentences! Show work.

  23. Electric Current Clip 9 minutes • http://www.youtube.com/watch?v=5laTkjINHrg Battery clip old but nice. 10 min. http://www.youtube.com/watch?v=IpaEGhjpZgc

  24. To get continuous flow of charge we need: 1. voltage (p.d.) to push charges. 2. Charge Pump = do work on charge which gains PE. Pumps are batteries, generators. 3. Closed Circuit – continuous pathway for charges to flow –metal wire.

  25. Outlet has p.d. = 120V.Don’t stick a fork in it!!

  26. How does p.d. make charge flow??

  27. Pot. Dif. Causes electricfield to spread through wire at near light speed. e- in wire respond by moving in field & colliding with neighboring e- starting to flow.

  28. Electric field in wire caused by voltage source induces all e- to move. Each e- moves slowly but all begin at once.

  29. Circuit – Closed pathway for charges. All components need p.d. to move. Conducting path (wire). Easy to move current through very little p.d. needed. Potential Dif/ Voltage Gives the push Resistors – (bulbs) Difficult for current to flow. Larger p.d. needed.

  30. Work & Energy on Charges (q) in circuit. PEelc used up, PEchm, starts cycle again. Q pushed into Conducting path (wire). Q in motion. PEelc - KE Battery does work on q. PEchm – PEelc Resistors – (bulbs) Work on q, PEelc &KE – heat, light.

  31. Voltage E transformations as skiiers. 9 min. • http://www.youtube.com/watch?v=F1p3fgbDnkY

  32. Resistors Bulbs, toasters, etc. convert KE of e- to other forms of E – heat, light etc.The e- loses KE. Devices are called resistors or loads. They slow down the e- so they resist current flow. (Like paddle wheel in river).

  33. As e- flow through filament, KE lost to heat & light. What must be around the bulb to push e- through it? p. d.

  34. Resistors convert Eelc or KE to other types by resisting/slowing the flow of charge. Resistance (R) unit = ohm’s W.

  35. Ohm’s Law The resistance of a conductor V = volts J/C I = current A, C/s R = resistance ohm’s W or V/A. R = V/I

  36. Ex A: A 120-V potential difference is applied to a toaster which draws a current of 4 A. What is the toaster’s resistance? • R – V/I • = 120-V/4 A • =30 W.

  37. Resistance in Wires Occurs in wires as well as appliances. Certain factors affect how much resistance a wire will offer to current flow.

  38. Factors affecting wire resistance.1. Length2. Area3. Temperature4. Type of material

  39. Length – longer wire offers more resistance. More chances for friction in wire. Less resistance More resistance

  40. Cross Sectional AreaThick wires offer less resistance.

  41. TemperatureHot offers more RCold offers less R

  42. At a given temperature, R = resistance (W)r = constant of resistivity (Wm)l = length (m) A = cross sectional area (m2) See table

  43. Ex 1: A 9.5 cm length of copper wire has a cross sectional area of 2.5 x 10-3 m2. What is the resistance of the wire at 20oC?

  44. From table r = 1.72 x 10-8Wm.So R = (1.72 x 10-8Wm)(.095 m )2.5 x 10-3 m2= 6.5 x 10-7W.

  45. Ex 2: Find the resistance of a copper wire 10 m long and 1.2 x 10-9 m2 in area. 143 W.

  46. Resistance For wire with known p and length use: R = rL/Afor R with known V and I useR = V/I.

  47. Film clip with questions. 9:30 minutes. Resistance

  48. 1. _____V Name _______________________________After Film: Given the circuit connected with copper wires to a 6V battery, what should each voltage (p.d.) read at position 1 and 2? Write the voltages in the space. • If the potential difference around the bulb is actually measured as 5.5V, what might you conclude happened ? • _____________________________ • _______________________________ 6 V 2. _____V

  49. Ex: A spool of gold wire with area 1.2 x 10-7 m2, has a resistance of 1.5 W. What is its length? • 7.4 m

  50. Hwk Read tx. 19 – 1 & 19 - 2Do text p 717 #15, 20ANDdo review bookpg 225 #2 - 7