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Understanding EfficiencyHuman Circuit1Exam- Read Your Textbook!!!!Friday, May 15, 2015Tuesday, May 19 2015(9A will have social on Thursday morning instead of science science on Friday)Exam bank exambank.com (AB K-12, grade 9)Top 5 scores Login: saintmark Password: lionsRewordify.com

How does it work?Remember how lie detectors work? People are conductors.Electricity is nothing more than free electrons moving from atom to atom through a material. This flow is called a current.Since your body is mostly water and there are water and minerals on your skin, your body can be a conductor, but its not a great one.Direct and Alternating Current

Some motors run on direct current:Electricity runs in one directions.Mp3 players, computers, cell phones & calculators also use DC.Plug-in devices that run on DC come with their own power supplies that convert the power companys 120-V AC to DC.The Electricity in household circuits is ACAlternating Current flows back and forth, 60 times per second.

Why do power companies generate AC?With AC they can use transformers.Transformers change the amount of voltage with hardly any energy loss.Changing voltage is necessary because the most efficient way to transmit current over long distances is at a high voltage.ButWhen transmission lines carry current at 500 000 V, the voltage must be reduced before it can be used in your home.

Generating ElectricityRemember our good friend Michael Faraday?

Electromagnetic InductionFaraday demonstrated that electrical current could be generated (by moving a conducting wire through a magnetic field).This changed the world because it introduced a way to generate a steady supply of large amounts of electricity.Generators coils of wire that are moved through a magnetic field.DC Generator is structurally the same as a DC motor:

AC generator is slightly different:

Power: Rate at which a device converts energy.

Tokyo's Toshima Incineration PlantBurns 300 tons of garbage a day, turning it into electricity, hot water and a kind of recyclable sand.

The unit of power is:The Watt (W) Named for James WattA watt is equal to one joule per second.The faster a device converts energy, the greater its power rating.

Power Ratings:Most small appliances: 1500 W or lessStove: 7000 WCalculator: 0.4 mWReview

Ohm's LawOne the most important and fundamental laws of electronics is that of ohms law.This law defines the relationships between current, voltage and resistance.Ohm's LawStates that the current through a conductor between two points is directly proportional to the potential difference across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship- its a powerful predictorMathematically:Power is equal to the current multiplied by the voltage.Power (P); current (I); voltage (V)

Energy: Measured in Joules (watts x seconds)You can use the power rating of a device to figure out the amount of energy the device uses.Remember that power is the rate at which a device converts energy. You can find the amount of energy by multiplying this rate by the length of time the device operates.

Kilowatt HoursPeople use A LOT of joules of energy in their homes/businesses, so kilowatt hours are often used as a unit for energy.Calculation is the same, except hours are substituted for seconds, and kilowatts (kW) are substituted for watts.Electricity meters measure the energy used in kilowatt hours.

Remember what Mr. Edwards taught us?

Law of Conservation of EnergyEnergy cannot be created or destroyed.It does not just appear or disappear it must be transformed from one form to another.Energy DissipationHowever A lot of the time we find that the energy output of a device or system is smaller than the input energy.

Sometimes, a lot smaller???HeatMost often the missing energy is lost or dissipated (scattered in all directions) as heat.Hot plate/beaker with water.The hot plate radiates some heat to into the surrounding air instead of into the water, or into nearby objects, including you.Heat is lost.Mechanical DevicesAll mechanical systems dissipate some energy, so their useable energy is always less than their input energy.

Irrigation System/Pumping SystemYou can hear it running, so some energy is dissipated as sound.If the motor has been running for a while, if you touch it, it will be hot.Current flowing through wires always produces some heat, and the friction between moving parts generates some heat as well.All of the missing input energy has been transformed into energy you cannot use.

Understanding EfficiencyThe efficiency of a device is the ration of useful energy that comes out of a device compared to the total energy that went in.Efficiency is usually calculated as a percent:

SoThe more input energy that a device converts to useable output energy, the more efficient the device is.

Easy Bake Oven?

Comparing EfficienciesBy comparing efficiencies of devices we can judge both their energy cost and their environmental impact.

Reducing the Energy Wasted by Devices

Limits to EfficiencyAn electric heater can be 100% efficient in converting electricity to heat, but devices that convert electricity to other forms of energy can never be 100% efficient.Moving partsGenerate a certain amount of thermal energy that is not useful output.

Friction is one cause of energy lost in engines.Increasing Efficiency?Decrease friction as much as possible- use improved bearings and lubricants.Heat escapes- add more insulation (for example, around a stove), or refrigerator- reduces the amount of heat that transfers into the fridge.You need less energy to keep the fridge cold.

Homework: Due Friday, May 22Check & Reflect, page 338, 1-9Check & Reflect, page 342, #1-7