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Electric Current

Electric Current. Chapter 20. Static Electricity… not real useful. Lot of work for a lousy spark or to attract bits of stuff to a rubber stick. To make electricity more useful, we need. We need a CONTINUOUS flow of electrons through a conductor VOLTAGE SOURCE

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Electric Current

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  1. Electric Current Chapter 20

  2. Static Electricity… not real useful.. Lot of work for a lousy spark or to attract bits of stuff to a rubber stick

  3. To make electricity more useful, we need • We need a CONTINUOUSflow of electrons through a conductor • VOLTAGE SOURCE • 1st voltage source was a battery Voltage Source: Escalator

  4. Luigi Galvani (1790s) • “Yikes! I can make this dead frog’s leg twitch!” • It was caused by a flow of charge generated by two dissimilar metals (scalpel and metal lab tabletop) • “galvanized”: when the muscles suddenly contract

  5. Alessandro Volta (1800s) • Manufactured the first wet cells • Led to our modern battery • Used bits of dissimilar metals and bowls of electrolytes (like saltwater) • “Crown of Cups” CONTINUOUS ELECTRICITY!

  6. Crown of Cups was often arranged in a circle

  7. Volta’s pile • An even better idea • Stack of discs that alternate between two different metals with saltwater soaked cardboard between them • Top and bottom were connected by a conductor • Continuous flow of electrons VOLT: unit for potential difference - named after Volta

  8. Volta’s Pile Unusual b/c it has a spark gap It is on display at Dartmouth

  9. Volta’s Pile opened the door for many new experimentation with electrodes Electrochemical Cell Two dissimilar metals and electrolyte Zn  Zn 2+ + 2e- Cu2+ + 2e-  Cu Oxidized (loses e-) Reduced (gains e-) REDOX Reaction

  10. Modern Day Batteries • Dry Cell (Alkaline) • Rechargeable Batteries (NiCd and Lead acid batteries)

  11. Properly Speaking… • Battery is a collection of cells • A single AA Duracell thingee is a CELL, not a battery

  12. NaOH or KOH George Leclanche (1800s) Reduced: gains e- Carbon rod Cathode • Invented dry cell • Modern dry cell is same except uses a basic paste instead of acidic • Zn can last longer with basic paste • Both produce 1.5V • Amount of charge depends on size of cell (large D can produce more charge than AAA cell) • Primary cells: produce electricity as long as chemical rxn takes place. Zinc Can Anode Oxidized: loses e-

  13. Secondary Cells (Storage Cells) • Can be recharged • Examples • Old automobile lead acid battery • NiCd batteries in portable electric devices • Work because of Redox (Oxidation/Reduction Rxns) • Oxidized: loses e- • Reduced: gains e-

  14. Electric Current • Flow of electrons through a conductor • Must have • Voltage source (cell, battery, generators, solar cells, etc) • Path for e- to move • Carrier of charge in a solid conductor is the electron (protons are tied up in nucleus) • Two types of current • DC: direct current – e- flow one way • AC: alternating current – direction of current changes • To maintain the flow of e- (electric current) a constant potential difference must be maintained. • Produced by voltage source

  15. Potential Difference Work will be done Particle will have a higher potential energy thus, higher potential difference (Voltage) Remember: Voltage is Energy per charge Electric potential difference is the difference in the electric potentials at each point

  16. If the electric potential difference between two locations is 12 volts, then one coulomb of charge will gain 12 joules of potential energy when moved between those two locations. INSIDE BATTERY

  17. Moving Internally in battery • Work must be done on a positive test charge to move it through the cells from the negative terminal to the positive terminal • Potential energy would increase • Potential difference would increase Again, we are talking about INSIDE the battery

  18. Moving externally through a conductor • It decreases its electric potential energy and thus is at low potential by the time it returns to the negative terminal • Loss of this electric potential energy in the external circuit results in a gain in light energy, thermal energy and other forms of non-electrical energy

  19. WITHIN BATTERY: Every coulomb of charge is gaining 12 joules of potential energy as it moves through the battery • WITHIN WIRE: Every coulomb of charge loses 12 joules of electric potential energy as it passes through the external circuit.

  20. Direction of Flow • Area of controversy • The direction that a positive charge carrier would travel in the circuit • … the US Navy doesn’t even use this convention… • … but the AP Test does! • This is from positive side of battery to negative side

  21. Water has been pumped to the top HIGH Electric Potential Charge and water BOTH Spontaneously and naturally Move to low pressure

  22. Must be a Potential Difference to flow

  23. Voltage Droploss of electric potential while passing through a circuit

  24. Difference is “AA” and “D” size cells • Each have a rating of 1.5 V • So each Coulomb of charge that moves from the neg. to pos. terminals inside the cell, can do 1.5J of work • D cell has more Coulombs of charge, so it will last longer • D will do more work, but will do it at the SAME RATE at AA.

  25. Check Your Understanding http://www.physicsclassroom.com/Class/circuits/u9l1c.cfm

  26. Current (I)

  27. 3.25 x 1017 e- flow past a point in 0.235 s. What is the current?

  28. Current Flow Basics • Circuit: path of e- flow • Resistance: opposition to its flow • Units: Ohm • Symbol: Ω • Load: thing that uses up electricity and provides resistance • Lightbulb, electric motor, solenoid, heating coil, etc • Conductor have small resistance • Resistors: a components deliberately placed in a circuit to add resistance • Voltage drop: voltage difference across a resistor • this creates current flow through the resistor

  29. W W 29

  30. Eskimo – W Cool - W 30

  31. W of the (opera) 31

  32. W’s - mobile 32

  33. mobile - W 33

  34. Far Fr –W W 34

  35. There’s no place likeW 35

  36. W goodness W gosh 36

  37. Sherlock W - s 37

  38. What did the acorn say when he grew up? G – W – A - tree (Trig – a – n – W – a – tree) 38

  39. W - boy W - girl 39

  40. Free – D -W 40

  41. SchematicsDrawing of a circuit • Electronic devices used to come with a schematic, but today circuitry is shrunk down and put on microchips • So, you can’t do much to repair it

  42. Symbols Resistor Battery Conductor Switch

  43. When a circuit has a battery, switch and resistor. The resistor represents the LOAD. The LOAD uses up the electricity. LOAD

  44. Ohm’s Law Potential difference across a conductor causes current to flow. The flow is opposed by the resistance.

  45. A waffle iron draws 7.1 A when plugged into a 120 V circuit. What is its resistance?

  46. W sweet W 46

  47. W - run stealing W sliding W 47

  48. cond – W – mini - W 48

  49. king d - W super d - W astro d - W 49

  50. Bring W the bacon 50

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