Chapter 6 Section 9 Comparing Energy Consumption: More for Your Money.

Chapter 6 Section 9

Comparing Energy Consumption More for Your

Money

HW PTG 69 p 698 1-9

bull Learning Objectivendash Calculate the energy efficiency of

electrical devices and distinguish between heat transferred by conduction convection and radiation

bull Success Criteriandash Measure and compare the energy

consumed by appliancesndash Compare the costs of operating a

variety of electrical appliances in terms of power ratings amount of time each appliance is used and billing rate

ndash Distinguish among the three ways of heat transfer

bull Do Nowndash WDYSWDYT

p 691

bull Agendandash Do Nowndash Investigation

69ndash Summary

Investigation 69

bull 1 and 2 ndash teacher demo

bull 3 and 4 ndash using a hot plate

bull 5 Repeat with heating coil

bull Record measurements and observations in log

bull Heat water to same temperature as above

bull Heat water to same temperature as above

Investigation 69

bull 6 - Calculate

bull 7 ndash in notebook

bull 8 and 9

bull Energy (J)=power (W)timestime (s)

bull E = Pt

bull Calculate costs for energy use

bull Cost = energy times price per unit of energy

Summary

bull Are energy efficient methods worth the added cost of the appliance

bull Think of some scenarios where you think it would be cheaper and some scenarios where it would not be cheaper

How does this impact the decisions of large global

corporations

HW PTG 69 p 698 1-9

bull Learning Objectivendash Calculate the energy efficiency of

electrical devices and distinguish between heat transferred by conduction convection and radiation

bull Success Criteriandash Measure and compare the energy

consumed by appliancesndash Compare the costs of operating a

variety of electrical appliances in terms of power ratings amount of time each appliance is used and billing rate

ndash Distinguish among the three ways of heat transfer

bull Do Nowndash Which

method of heating is best and why

bull Agendandash Do Nowndash Physics Talkndash Essential

Questions

Physics Talk 69

bull How do you calculate the electrical energy consumed by a device

bull Assuming it uses power at a constant rate you multiply the power of the device in kilowatts by the number of hours used

bull E=Pt

Physics Talk 69

bull What if the appliance does not use power at a constant rate

bull The device uses a kilowatt meter that takes into account the amount of power used and the amount of time

Physics Talk 69

bull What are examples of devices that use a meter

bull Your refrigerator turns on and off at different rates so it uses a meter

bull You entire house is hooked up to a meter This is how the power company charges you for electricity

Physics Talk 69

bull Do all power companies use these meters

bull No Some companies use statistical data about the appliances you have in your home and just automatically charge you the average amount

Physics Talk 69

bull How can you calculate the cost of running an electronic device

bull Cost = energy times price per unit of energy

bull Energy is measured in kilowatt-hours

bull (1000 Wh = 1 kWh)

Sample Problembull An electric coffee maker uses an average of 225 kW

bull h of energy each month If the family is charged $012kW bull h for electricity what is the average monthly cost of operating the coffee maker

bull Given ndash E = 225 kWhndash Rate = $012kWh

bull Unknownndash Cost

bull Toolndash Cost = energy times price per unit of energy

bull Solutionndash Cost = 225 kWh times $012kWhndash Cost = $270

Physics Talk 69

bull What is heat transfer

bull What are the three ways heat is transferred

bull The transmission of heat energy from a warmer substance to a cooler substance

bull Conductionbull Convectionbull Radiation

Physics Talk 69

bull What is conduction

bull The transfer of heat energy from particle to particle between substances through contact or within a substance

bull Always requires direct contact

bull Example When the heating coil touched the water molecules to heat them

Physics Talk 69

bull What is convection

bull The transfer of heat energy through the movement of air or liquid currents

bull Example When the warmer water molecules moved away from the heat and transferred their energy to colder water molecules

Physics Talk 69

bull What is radiation bull The transfer of heat energy by emission of electromagnetic radiation in all directions

bull Examples ndash When the microwaves

heat the waterndash When an object is

warmed by being in the sun

Physics Talk 69

bull Conduction convection and radiation

Essential Questions

bull p 696 WDYTN bull P 697 ndash What does it meanndash How do you knowndash Why do you

believendash Why should you

care