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9.2 THE POWER OF ELECTRICITY IN THIS SECTION Objectives: • Electric Power (P=VI) is the rate at which electrical potential energy is transformed • Power Consumption multiplied by time of use equals the amount of electrical energy used by a device UP A HILL • Both of these vehicles will gain the same potential energy • It depends how the energy is converted (load) • Because the race car could transform its energy faster, it has more power POWER • Power – the rate of change in energy • Power is also the rate at which work is done or energy is transformed • The unit for measuring energy is joule (J) • One joule (J) of energy transformed in one second(s) is called one watt (W) of power ELECTRICAL POWER • Electrical Power – the rate of change in electrical energy • The unit for measuring energy is joule (J) • Electric loads convert electrical energy into other forms of energy (i.e. heat/light). P = VI P = power (watt) V = voltage (volts) I = current (amperes) Pg. 323 #1-3 P V I SAMPLE POWER PROBLEMS 1. A flashlight bulb operates on 3.0 V and draws a current of 4.0 A. What is the power of this bulb? P = VI P = (3.0 V)(4.0 A) P = 12 W The power of this bulb is 12 W. SAMPLE POWER PROBLEMS 2. A 60 W light bulb is connected to 120 V. What current passes through the light bulb? I = P/V I = 60 W/120 V I = 0.5 A The current passing through the light bulb is 0.5 A. POWER RATING • Power rating – the measurement of how much electrical energy a device consumes for every second it is in use. Example: 100 W light bulb uses 100 J of electrical energy every second and produces 100 J of heat and light energy. POWER IN RELATION TO ENERGY AND TIME • Power – is the rate of change in energy (or work). P = E/t P = power (watt) E = energy (joules) t = time (seconds) E P t Power (P) is defined as energy transferred (E) per time interval(t) EXAMPLE QUESTION • P = E/t • How much electrical energy is consumed by a 1200W hair dryer if it used for 5.0 min? • Use the formula • 5.0 min = 5 x 60s = 300s • E = Pt • = (1200W)(300s) = 360 000Ws = 3.6x105J • A: A 1200W hair dryer consumes 3.6x105J of electrical energy if it used for 5.0 min. CALCULATE • Pg. 324 #1-3 SAMPLE ENERGY CONSUMPTION PROBLEM 1. How much electrical energy is consumed by a 60 W light bulb left on for 25 minutes? NOTE: Must convert minutes seconds 1 minute = 60 seconds E = P*t E = (60 W)(25 min. x 60 s) E = (60 W)(1500 s) E = 9.0 x 104 J The electrical energy consumed was 9.0 x 104 J. UNITS FOR ENERGY • 1.0 joule = 1.0 W x 1.0 s (Energy Units) • 1.0 kilowatt-hour = 1.0 kilowatt x 1.0 h • 1.0 kW*h = 1.0 kW x 1.0 h (Larger Energy Units) CALCULATING ELECTRICAL ENERGY AND COST • Using large amounts of electrical energy; therefore measure in kW*h (not W*s). • Remember to convert W kW by dividing by 1000. • How much kW*h of electricity is consumed by a 500 W light bulb that burns for 5 hours. • First: 500 W 0.5 kW (divided by 1000) • E = Pxt = 0.5 kW x 5 h = 2.5 kW*h COST OF ENERGY USED BILL PAYMENT • Cost of energy used = amount charged/1 kW*h x kW*h • Cost: put in $ units/kW*h. Example - $0.05/kW*h Example: If the power company charges 5 cents for every kW*h of energy, how much is the electric bill for the month if a family uses 1300 kW*h of energy? Cost of energy used = $0.05/1kW*h x 1300 kW*h Cost of energy used = $65.00 PRACTICE: 1. A 300 mA current runs through a resistor connected to 100 V. a) b) c) What is the power? It runs for 100 hrs, how much electricity is used? Electricity costs 7 cents per kW·hr, how much did it cost to run this resistor? PRACTICE: 2. Mom is yelling at her teenager for leaving the lights on in the basement for one week (7 days!!). There are 3 lights each drawing 75 W. What did it cost to run the lights if electricity costs 7 cents a kW·hr? PRACTICE: 3a. An arc welder used 100 A of current on 220 V, what is the resistance? b. If the arc welder is used for 6 hrs, how much electricity did it use? c. If electricity costs 7 cents per kW·hr, how much did it cost to run that arc welder? THE COST OF ELECTRICITY • Page 326 – Complete the table and answer questions 1 & 2 ELECTRICAL SURGES • Electrical surges are brief increases in voltage to tens of thousands of volts • Can occur through all of your wiring • Can be caused by lightning, turning large appliances on and off • Surge protectors