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2. 5 – 19 Do Now Problems 1.Define electric current and give the associated units 2.What is the formula associated with Ohm’s law? 3. Differentiate between the terms voltage source and potential difference Homework Read & outline 34.5 – 34.11

4. Water vs. Electric Current There are many analogous properties between water and electric current Characteristics such as Pressure Volume Flow

5. Pressure In an electrical system pressure is measured in A higher voltage pushes the charges through the wire with greater force In a water system we measure water pressure in foot lbs or PSI (pounds per square in) Volts

6. Pressure How a water tower works: 1. Pump station 2. Reservoir 3. Water user

7. Voltage Voltage is a measure of electric potential energy, just like height is a measure of gravitational potential energy. Voltage is measured in volts (V). Using a voltmeter A voltage difference of 1 volt means 1 amp of current does 1 joule of work in 1 second.

8. Voltage Since 1 joule per second is a watt (power), you can interpret voltage as measuring the available electrical power per amp of current that flows.

9. Voltage Source A pump is a source of fluid pressure difference A battery is a source of electrical potential difference

10. Voltage & Batteries The positive end of a 1.5 volt battery is 1.5 volts higher than the negative end. If you connect batteries positive-to- negative, each battery adds 1.5 volts to the total. Three batteries make 4.5 volts. Each unit of current coming out of the positive end of the three-battery stack has 4.5 joules of energy.

11. Voltage source Charges do NOT flow unless there is potential difference The battery or source is represented by an escalator which raises charges to a higher level of energy. A voltage source is needed to provide a sustained potential difference i.e. batteries or generators VOLTAGE causes current

12. Current and voltage A battery uses chemical energy to create a voltage difference between its two terminals. In a battery, chemical reactions provide the energy to pump the current from low voltage to high voltage. A fully charged battery adds energy proportional to its voltage.

13. Volume In our water system volume is in The Electrical equivalent is a Coulomb is a unit of charge, consists of 6.3 x 10 18 electrons (a big number indeed) cubic feet (ft3) or liters Coulomb

14. Electric Charge A building up at the negative terminal of a cell/battery which then flows from this terminal as negative charges repel one another.

15. Flow Fluid flowing through a system of pipes can behave in many respects like electricity flowing in a circuit The main difference is that water will fill any space whereas electricity will 'fill' only a conductor

16. Flow Charge can only flow when there is a potential difference or a difference in voltage across the ends of a conductor

17. Current is a flow of charge

19. Current Electric current is measured in units called amperes, or amps (A) for short. One amp is a flow of a certain quantity of electricity in one second. The amount of electric current entering a circuit always equals the amount exiting the circuit.

20. Electric Current

21. What is need for current to flow? Electrons have a charge of –1.6  10 -19 Coulombs – one amp = 6  10 18 electrons per second By convention, electrical current flows from high voltage to low voltage In other words, a positive current flows from higher to lower electrical potential NO POTENTIAL DIFFERENCE = NO FLOW OF CURRENT

22. Recap Relate water pressure and electric pressure. Units for electric pressure? Relate water volume to electric volume. Units for electric volume? Relate water flow and electric flow. Units for electric flow? Condition needed for electric flow.

23. 5 – Do Now Problems 1.What condition is necessary for the sustained flow of charge in a conductor? 2.Differentiate between AC and DC. 3. What is voltage? HW. Pg. 545 - 546 Review questions

24. How does current move through a circuit?

25. Conventional current Conventional current was proposed by Ben Franklin in the 1700’s. Scientists later discovered that the particles that carry electricity in a wire actually travel from negative to positive. Today, we still use Franklin’s definition.

26. Current Getting it Started A potential difference is established between two points and some charges are released The positive terminal of a battery will attract electrons charges They will be acted on by the electrical force and start to move

27. Voltage source Charges do NOT flow unless there is potential difference The battery or source is represented by an escalator which raises charges to a higher level of energy. A voltage source is needed to provide a sustained potential difference i.e. batteries or generators VOLTAGE causes current

28. Current & flow of charges Electrical circuits, consist of current in the wire moving from higher to lower electrical potential

29. Current Electricity Example The continuous flow of charge in a complete circuit.

30. What does a battery do? A battery uses chemical energy to move charges. If you connect a circuit with a battery the charges flow out of the battery carrying energy.

31. Effects of Electric Current on the Body

32. Shock examples

34. Electrical Resistance and Ohm’s Law Key Question: How are voltage, current, and resistance related?

35. Electrical resistance Resistance measures how difficult it is for current to flow.

36. Electrical Resistance The total amount of electrical resistance in a circuit determines the amount of current that in the circuit for a given voltage. The more resistance the circuit has, the less current that flows.

37. The ohm Resistance is measured in (  ). One ohm is the resistance when a voltage of 1 volt is applied with a current of 1 amp.

38. Ohm’s Law  The formulae

39. The resistance of electrical devices The resistance of electrical devices ranges from very small (0.001 Ω) to very large (10×10 6 Ω). Each device is designed with a resistance that allows the right amount of current to flow when connected to the voltage the device was designed for.

40. Ohm Quiz Ohm plate Ohm on the range Broken Ohm Ice cream Ohm Ohm sweet Ohm Ohm Alone

41. Practice Quiz 1. If the resistance of your body were 100,000 ohms, what would be the current in your body when you touched the terminals of a 12 volt battery? 12 V / 100000== 0.00012 A 2. If your skin were very moist so that your resistance was only 1000 ohms, and you touched the terminals of a 24 volt battery, how much current would you draw? =24 V / 1000 =0.024 A See slide 18

42. More Practice Problems I = V/R I = 110 V / 2200 Ώ I =.05 A R = V/I R = 110 V/.5A R = 220 Ώ V = IR V = 1.2 A (100 Ώ) V = 120 V

43. Electric Resistance 1. Length as the electrons will have more distance to move and will therefore be bumping around more. resistance varies with the length of a conductor Ex: The length of a conductor is similar to the length of a hallway. A shorter hallway would allow people to move through at a higher rate than a longer one.

44. 2. Cross-sectional area resistance varies inversely with the cross sectional area If the area doubles then the resistance is halved. The wider the pipe, the easier it is for water to flow through …………………………………………………

45. Type of material 3. Type of material good conductors have low resistance and many free electrons electrical conductance is measured in siemens (S).siemens the inverse of the resistance

46. Temperature 4. Temperature most materials experience an increase in resistance as their temperature increases experience a decrease in resistance as their temperature decreases due to the kinetic theory of matter ……………………………………………… …

47. Electric Power Power = Volts x Amps The work done by an electric current moving through a circuit is given by 1 watt = 1 volt x 1 amp W = V I t

48. Power Equations Power (P) Rate at which work is performed Measured in watts (W) Power P = V I P = (I R) I = I 2 R

49. Decided to power homes with DC power –DC means direct current: just like what batteries deliver Getting Power to Our Homes Thomas Edison's GE company held the patents on the DC generators being used in the early days

50. Do we want power plants close to our home? Probably NOT…so we need to have –an ability to “ship” electricity across states So power lines are long –Therefore resistance no longer negligible Getting Power to Our Homes

51. Power Dissipated in an Electricity Distribution System 120 Watt Light bulb 12 Volt Connection Box Power Plant on Colorado River 150 miles Estimate resistance of power lines: say 0.001 Ohms per meter, times 200 km = 0.001 W/m  2  10 5 m = 20 Ohms We can figure out the current required by a single bulb using P = VI so I = P/V =120 Watts/12 Volts = 10 Amps

52. Power Dissipated in an Electricity Distribution System 120 Watt Light bulb 12 Volt Connection Box Power Plant on Colorado River 150 miles Power in transmission line is P = I 2 R = 10 2  20 = 2,000 Watts!! “Efficiency” is έ = 120 Watts/4120 Watts = 0.3%

53. The Tradeoff The big problem is the high current through the (fixed resistance) transmission lines Need less current The square in I 2 R that has the most effect Our appliances needs a certain amount of power I = P /V less current means higher voltage

54. The Solution The Solution is high voltage transmission I = 120 Watts/12 kV = 0.01 Amps for one bulb, giving P = I 2 R = (0.01) 2 20 = 20  10 -4 Watts, so P = 0.002 Watts dissipated in transmission line Efficiency in this case is έ = 120 Watts/120.004 = 99.996%

55. The Tradeoff But having high voltage in each household is a recipe for disaster –sparks every time you plug something in –risk of fire –not cat-friendly We need a way to step-up/step-down voltage at will –We can’t do this with DC, so go to AC

58. Basic Terminology Direct Current (DC) – charge flows in one direction. –Batteries –Solar Panels

59. Basic Terminology Alternating Current (AC) –Magnitude & direction of current flow periodically change –Each sequence called a cycle –Frequency is cycles per second (Hz)

60. AC Circuits An AC voltage source reverses the positive and negative terminals many times per second. The current flows one direction and then the other then the other….. This changing of polarity or cycling is called frequency, measured in cycles per second (hertz).

61. Why AC? AC is like a battery where the terminals exchange sign periodically! approx. 60 cycles per sec in household current AC sloshes back and forth in the wires Although net electron flow over one cycle is zero, useful work can still be done! Imagine sawing (back & forth), or rubbing hands together to generate heat

62. DC (direct current) and AC (alternating current) I Batteries produce DCHousehold/power plant: AC

63. DC vs AC DC Electrons flow constantly Electrons flow in only one direction Batteries AC Electrons flow in short burst Electrons switch directions (60 times a second) House current

64. AC Nikola Tesla invented & George Westinghouse introduced his system based on high-voltage alternating current (AC), which could carry electricity hundreds of miles with little loss of power

65. RESISTOR COLOR CODES The resistor code is a color code. Around each resistor are colored stripes. Each color stands for a number. You can tell how much resistance by looking at the numbers. Gold or silver tells you how accurate the resistor is gold is (+) Or (–) 5% silver is (+) Or (–) 10% Black = 0 Yellow = 4 Gray = 8 Brown = 1 Green = 5 White = 9 Red = 2 Blue = 6 Orange = 3 Violet = 7

66. RESISTOR COLOR CODES This is how to crack the code. Write the numbers for the first two colors. The third color tells you how many zeros to add. Resistance is measured in ohms. This resistor has a resistance of….. 1000 ohms

67. Resistors Electrical components called resistors can be used to control current. Resistors have striped color codes to record their "values" (writing on them is difficult).

68. Resistors Used to control current or potential difference in a circuit.

69. Pop Quiz 2,500,000,000  740  880,000,000  6,200,000  7,400,000,000 