2. Electric Energy

3. Using Electric Energy Energy can be neither created nor destroyed,…but it can be transformed from one kind to another

4. Objectives Explain how electric energy is converted into thermal energy Determine why high-voltage transmission lines are used to carry electric energy over long distances Define kilowatt-hour Perform energy and power calculations

5. Power and Energy Power is the time rate of doing work or expending energy Unit of work or energy is the joule Unit of power is the watt (1 joule per second)

6. Energy Work is equal to the product of the power consumed and the time of charge flow

9. Equivalent Expressions

10. Thermal Energy Current through a resistor generates heat Light bulbs Space heaters Stoves, etc. Voltage “drops” across a resistor Energy converted to heat is lost “Joule heating” or I 2 R loss

11. Example A heater has a resistance of 10 Ω It operates on 120.0 V What is the current through the resistor? What thermal energy is supplied by the heater in 10.0 seconds?

12. Example Sketch the problem List the knowns and unknowns Strategy Use I = V/R to find the current Use W=I 2 Rt to find the energy* *The textbook uses “E” for energy See solution on Page 521 of textbook

13. Where Does Electric Energy Come From? Chemical energy (battery) Mechanical energy (generator) Gravitational energy (water falls) Steam turbines & wind turbines (heat) Solar panels (light) Nuclear energy Energy can be neither created nor destroyed,…but it can be transformed from one kind to another

14. Where Does Electric Energy Go? Chemical energy (making batteries) Mechanical energy (motor) Heat (intentional or byproduct) LEDs (light) Nuclear energy (particle accelerators) Energy can be neither created nor destroyed,…but it can be transformed from one kind to another

16. Transmission of Electricity Power is lost as heat in transmission line Reduce the loss Low resistance wire High conductivity Large diameter Lower current Why high-voltage? Increase voltage and reduce current

17. Units of Electrical Energy Your electric bill is for energy, not power (even though we call the utility, “the power company”) Electric meters measure the power consumption in kilowatts and the time in hours 1 kWh = (1000 J/s)(3600 s) = 3.6 x 10 6 J

18. Example The resistance of a stove element is 11Ω at operating temperature with a voltage of 220V. What is the current through the stove element? How much energy is used in 30 minutes? What would the cost be at 12 cents per kilowatt-hour?

19. Extra Credit Find the name of each person for whom the following units are named Watt Joule Coulomb Ampere Volt

20. Practice Problems Textbook, Page 522 #14 – 17 Textbook, Page 525 #18 – 20 Two people do #20