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What you need: NotebookNotebook CalculatorCalculator Writing utensilWriting utensil

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Ohm’s Law, Power, & Energy

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Currents and Circuits Current - the flow of charged particlesCurrent - the flow of charged particles Circuit - a closed continuous path through which current flowsCircuit - a closed continuous path through which current flows - requires a voltage source to maintain the potential difference - requires a voltage source to maintain the potential difference

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Ohm’s Law Resistance = Voltage/ CurrentResistance = Voltage/ Current R = V / IR = V / I UnitsUnits –Resistance (R) = ohms (Ω) –Voltage (V) = volt (V) –Current (I) = ampere (A)

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Ohm’s Law Increasing voltage increases resistanceIncreasing voltage increases resistance Increasing current decreases the resistanceIncreasing current decreases the resistance

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Resistors Any device that changes electrical energy into another formAny device that changes electrical energy into another form

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Example #1: A 30.0 V battery is connected to a 10.0 Ω resistor. What is the current in the circuit?A 30.0 V battery is connected to a 10.0 Ω resistor. What is the current in the circuit? I = V/RI = V/R I = 30.0 V / 10.0 ΩI = 30.0 V / 10.0 Ω = 3.00 A= 3.00 A

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Power Measures the rate at which energy is transferredMeasures the rate at which energy is transferred Power = Current x VoltagePower = Current x Voltage P = IVP = IV The unit of power is the WattThe unit of power is the Watt

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Example #2: A 6.0 V battery delivers a 0.50 A current to an electrical motor. What power is consumed by the motor?A 6.0 V battery delivers a 0.50 A current to an electrical motor. What power is consumed by the motor? P = IVP = IV P = (0.50 A)(6.0 V)P = (0.50 A)(6.0 V) P = 3.0 WP = 3.0 W

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Electrical Energy Energy = Power x timeEnergy = Power x time E = PtE = Pt If you are using Watts, use seconds for time. In this case, energy is measured in Joules.If you are using Watts, use seconds for time. In this case, energy is measured in Joules.

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Continue Example #2: Using the information from the Example #2, how much electric energy is delivered if the motor runs for 5.0 minutes?Using the information from the Example #2, how much electric energy is delivered if the motor runs for 5.0 minutes? E = PtE = Pt E = (3.0 W)(300 s)E = (3.0 W)(300 s) E = 9.0 x 10 2 JE = 9.0 x 10 2 J

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The Kilowatt-Hour The joule (a “watt-second”) is a relatively small amount of energy; too small for commercial sales use.The joule (a “watt-second”) is a relatively small amount of energy; too small for commercial sales use. For that reason, electric companies measure their energy sales in “kilowatt-hours”.For that reason, electric companies measure their energy sales in “kilowatt-hours”.

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The Kilowatt-Hour A kilowatt-hour is equal to 1000 watts delivered continuously for 3600 seconds (1 hour).A kilowatt-hour is equal to 1000 watts delivered continuously for 3600 seconds (1 hour). 1 kWh = (1000 J/s)(3600 s) = 3.6x10 6 J

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Example #3: A plasma tv draws 2 A when operated on 120 V.A plasma tv draws 2 A when operated on 120 V. –How much power does the tv use? –If the tv is operated for an average of 7 hr/day, what energy in kWh does it consume per month (30 days)? –At 11¢ per kWh, what is the cost of operating the tv per month?

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Electric Current. Charge in motion (“flow” of charges). Charge in motion (“flow” of charges). Current: The number of electrons that pass a specific point.

Electric Current. Charge in motion (“flow” of charges). Charge in motion (“flow” of charges). Current: The number of electrons that pass a specific point.

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