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What you need: • Notebook • Calculator • Writing utensil Ohm’s Law, Power, & Energy Currents and Circuits • Current - the flow of charged particles • Circuit - a closed continuous path through which current flows - requires a voltage source to maintain the potential difference Ohm’s Law • Resistance = Voltage/ Current •R=V/I • Units – Resistance (R) = ohms (Ω) – Voltage (V) = volt (V) – Current (I) = ampere (A) Ohm’s Law • Increasing voltage increases resistance • Increasing current decreases the resistance Resistors • Any device that changes electrical energy into another form Example #1: • A 30.0 V battery is connected to a 10.0 Ω resistor. What is the current in the circuit? • I = V/R • I = 30.0 V / 10.0 Ω • = 3.00 A Power • Measures the rate at which energy is transferred • Power = Current x Voltage • P = IV • The unit of power is the Watt Example #2: • A 6.0 V battery delivers a 0.50 A current to an electrical motor. What power is consumed by the motor? • P = IV • P = (0.50 A)(6.0 V) • P = 3.0 W Electrical Energy • Energy = Power x time • E = Pt • If you are using Watts, use seconds for time. In this case, energy is measured in Joules. Continue Example #2: • Using the information from the Example #2, how much electric energy is delivered if the motor runs for 5.0 minutes? • E = Pt • E = (3.0 W)(300 s) • E = 9.0 x 102 J The Kilowatt-Hour • 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”. The Kilowatt-Hour • A kilowatt-hour is equal to 1000 watts delivered continuously for 3600 seconds (1 hour). 1 kWh = (1000 J/s)(3600 s) = 6 3.6x10 J Example #3: • 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?