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Chapter 6, Section 8

Chapter 6, Section 8. Energy Consumption: Cold Shower. HW: PTG 6.8 p 686 #1-8. Learning Objective Calculate the energy efficiency of electrical devices. Success Criteria Calculate the heat gained by a sample of water.

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Chapter 6, Section 8

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  1. Chapter 6, Section 8 Energy Consumption: Cold Shower

  2. HW: PTG 6.8 p 686 #1-8 • Learning Objective • Calculate the energy efficiency of electrical devices. • Success Criteria • Calculate the heat gained by a sample of water. • Calculate the electrical energy converted into heat by a resistor. • Calculate the efficiency of a transformation of electrical energy to heat. • Explore the power ratings and energy consumption levels of a variety of electrical appliances. • Do Now • WDYS/WDYT p 678 • Agenda • Do Now • Investigation • Summary

  3. What do you see?

  4. What do you think? • The entire daily energy output of a Homes For Everyone (HFE) generator would not be enough to heat water for an average American family for a day. • If an electrical heating coil (a type of resistor) were submerged in a container of water, and if a current were to flow through the coil to make it hot, what factors would affect the temperature increase of the water?

  5. Investigation 6.8 • #1 – setup and measurements of waterand coil • #2 Heat water for 10 minutes • While water heats set up notebook

  6. Investigation 6.8 • Recall that the heat gained by an object can be found using the equation: • ΔQ = mcΔT • ΔQ is a measure of heat in joules, • m is the mass of the substance in grams • c is the specific heat of the substance, and • ΔT is the change in temperature in degrees Celsius. • #3 – Calculate the heat gained using your data table and the specific heat value of4.18 J/g°C.

  7. Investigation 6.8 • P=E/t so E=Pt • where E is energy in joules (watt-seconds), • P is power in watts (joules/second), • t is time in seconds. • #4 - Calculate the energy from your experiment using this equation • Answer 4 a and 5b

  8. Investigation 6.8 • #5 – Calculate the efficiency using the equation above and answer 5a

  9. Summary • Do you think the heated coil is very efficient? Why or why not? Provide evidence to support your claim.

  10. HW: PTG 6.8 p 686 #1-8 • Learning Objective • Calculate the energy efficiency of electrical devices. • Success Criteria • Calculate the heat gained by a sample of water. • Calculate the electrical energy converted into heat by a resistor. • Calculate the efficiency of a transformation of electrical energy to heat. • Explore the power ratings and energy consumption levels of a variety of electrical appliances. • Do Now • List some ways to make the foam cup setup more energy efficient. • Agenda • Do Now • Investigation • Summary

  11. Physics Talk 6.8 • What is a kilowatt-hour? • 1 kilowatt-hour of energy power time • 1000W x 1 h = • 1000W x 3600 s • = 3,600,000 J • That’s a lot of joules!

  12. Physics Talk 6.8 • What is heat? • Heat is the transfer of thermal energy. • In order for an object to heat up, there must be a source of thermal energy • When objects cool down, the thermal energy is being transferred elsewhere

  13. Physics Talk 6.8 • How does a thermos work? • A thermos creates a barrier to isolate the contents. This means that the amount of energy in the thermos must stay relatively the same. • Cold things stay cold (blocks thermal energy from leaving) • Hot things stay hot (traps thermal energy in)

  14. Physics Talk 6.8 • How do we measure a change in heat? • ΔQ = mcΔT • ΔQ is a measure of heat in joules, • m is the mass of the substance in grams • c is the specific heat of the substance, and • ΔT is the change in temperature in degrees Celsius.

  15. Physics Talk 6.8 • Another method for calculating a change in heat • We know energy = power x time • We also know power = voltage x current • This means, energy = voltage x current x time • E = VIt • where E is the energy in joules (J) • V is the voltage in volts (V) • I is the current in amperes (A) • t is the time in seconds (s)

  16. Sample Problem • A 12-V starter battery in a car supplies 48 A of current to the starter. If the starter draws energy for 15 s, how much energy does the starter use? • Given: • V = 12V • I = 48A • t = 15s • Unknown: • Energy! (E) • Tool: • E = VIt • Solution: • E= 12V·48A·15s • E = 8640 J

  17. Canceling Units

  18. Physics Talk 6.8 • What happened to the missing energy in the experiment? • Some of the heat could have been lost out of the cup, into the thermometer, or somewhere else. The important thing is that it went somewhere because ENERGY IS NEVER DESTROYED!

  19. Physics Talk 6.8 • What is the equation for efficiency?

  20. Essential Questions • P 684 WDYTN • Questions p 685 • What does it mean? • How do you know? • Why do you believe? • Why should you care?

  21. Physics To Go #5-8 as a class

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