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ENERGY

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TYPES OF ENERGY Kinetic - energy in motion Potential - stored energy Chemical - energy stored in the bonds of molecules and atoms Nuclear energy- energy stored in the nucleus of atom Electrical energy- electrons moving through a wire Mechanical, sound, motion, thermal and radiant are other examples

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Energy Efficiency percentage of energy input that does useful work in an energy conversion system

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When we’re energy efficient we use less energy to do something as good as before or better. We have to remember there is a limited amount of non-renewable fuel sources such as coal and gas. Even if we don’t run out of fuel, we can damage our environment by using too much and wasting energy.

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Energy transformations are great, but very inefficient! Our bodies are about 20% efficient, and burning the gasoline in your car is only 10% efficient! Most of the energy is lost in the form of heat!

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Efficiency of Some Common Devices Device Efficiency (%) Dry-cell flashlight battery90 Home gas furnace85 Storage battery 70 Home oil furnace 65 Small electric motor62 Steam power plant38 Diesel engine 38 High-intensity lamp32 Automobile engine25 Fluorescent lamp 22 Incandescent lamp 4

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UNITS OF ENERGY JOULE (J) or kilojoule (kJ) Calorie (cal) amount of heat required to raise the temp of 1 g of water 1⁰C BTU (British thermal unit) amount of heat required to raise the temperature of 1 pound of water 1⁰ F Used in US to rate water heaters, furnaces and air conditioners. (coal) Therm (thermal units) often used by gas companies (natural gas)

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CONVERSIONS 1 cal = joules 1 BTU = 1.05 kJ = 252 cal 1 therm = 100,000 BTU 1 kWh= 3413 BTU 1 cubic foot of gas contains 1031 BTUs 1 ton of coal contains 2.5 x 10 7 BTUs

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POWER- THE RATE AT WHICH ENERGY IS USED P = E/t E= energy t= time THE UNIT FOR POWER IS THE WATT 1 W = 1 J/s A 100 watt light uses 100 J/s of electrical energy Power (watts)= current (amps) x voltage(volts)

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FIRST AND SECOND LAWS OF THERMODYNAMICS If a bulb is 20% efficient, then the bulb converts 20% of the electrical energy into light and 80% is lost as waste heat. First Law of Thermodynamics Energy can be converted from one form to another ( electrical → heat) Second Law of Thermodynamics In any energy conversion, some energy is converted into lower quality energy

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Knowing the relationship between energy and power allows you to find the energy used when an appliance of known power (watts) operates for a known amount of time. EXAMPLE: how much energy (kJ) does a 75 watt light bulb use when it is turned on for 25 minutes? ANSWER: E = Pxt E = 75 J/s x 60s/min x 25 min = 110,000 J or 110 kJ

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The kWh (kilowatt hour) Kilowatt hour is a unit of energy. This is the term used by the electric company. E= P x t 1 kWh is equal to 1 kW (1000 W) delivered continuously for 1 hour (3600 seconds) 1 kWh = 1000 J/s x 3600s = 3,600,000 J or 3.6 e 6 J or 3600 kJ

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1 kWh is the energy required to power watt light bulbs for 1 hour. The average US home uses about 10,000 kWh electric energy/year.

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Remember your metric conversions! GMKHD – DCMMNP 5000 w = ____ kW 4800 J = ____ MJ 88 kc = ____ cal

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Energy Calculations Worksheet Take a look and highlight terms. See if you can do the first problem tonight.

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Energy Calculation Answers 1. A.) 100 kg x 1kcal x 20 `C = 2,000kcal kg/`C B.) 2,000 kcal x 4.186j x 1,000 cal= 8.37 MJ cal 1kcal C.) 1Btu x 8.37 x 10 6 J = 7934Btus 1,055J D.) 7,934Btu x 1h x 60m = 11.9 min 40,000Btu hr

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2. A.) 1KWH x 3.6 x 10J x 1BTU = 3,412Btu KWH 1,055J B.) The second law of thermodynamics prevents 100% conversion of heat to mechanical or electrical energy. A typical coal fired plant operates at 33% efficiency, meaning that only 1/3 of the energy in coal is converted to electricity.

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3.A. 120MBtu x 1,000,000 = 1.2x10 Btu x 1 ft 3 x 1/ccf = 1,165ccf (gas) 1 MBtu 1030Btu 100ft 3 B. 1165ccf = 1,942ccf x $0.9 = $1,748 to heat 60% eff.6 ccf C = ccf x $0.9 = $1,310.6 to heat 80% eff.8 ccf Money saved per year in heating costs = $1748-$1,310.6 = $ Years to payback cost of furnace = $4000/$ = 9.1 years

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4.A.) 8760hrs/yr x 160 W/m 2 x 10m 2 x.20 =2,803.2 KWH 2,803.2 KWH x 3.6x10 6 J x 1BTU = 9,565,421 BTU = 9.57 MBT KWH 1055 J B.) 2,803.2 KWH =.2803 =28% 10,000 KWH 9,565,421 Btu = 1,200 BATHS 7,934 Btu/BATH

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5.A.) 250 W/m 2 x 10m 2 x.10 x 1KW x 8,760HR = 2,190 KWH 1,000W B.) 2,190 KWH =.219 x 100 = 21.9% 10,000KWH C.) 10,000KWH x 10m 2 = m 2 2,190KWH

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6. 8,760 x 200W/m x 10 6 m x.001 x Btu = 498 Tons 12 X 10 6 Btu/ ton W

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7A.) 1,000MW x 1,000KW MW = 4,000 Wind Turbines 250 KW B.) Solar energy can help to decrease the total amount of energy provided by the power company by 21.9 %. Solar energy used to heat water could be feasible for small families. It would not be economical or practical to use solar energy as your only energy source at this time. Wood is very inefficient as a heat source. The forest in example 7 could provide enough energy for almost 50 homes. The trees would require continuous replenishment; the soil would require fertilization to maintain……

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8.) Battery: 4.5 WH =.0045KWH 1,000W/KW 1 = $ / KWH KWH

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9. Natural gas cost is $1.14 x 1cf x 1 ccf = $11.07/MBTU ccf 1030 BTU 100cf 120/.8 x $11.07 = $ Propane gas cost: $1.69 x 1 gal x 1x 10 6 BTU = $18.37/MBTU Gal BTU MBTU 120/.8 x $18.37= $2775.

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Fuel Oil cost: $1.93 x 1 Gal x 1x10 6 BTU = $14.51/MBTU Gal 133,000 BTU MBTU 120/.8 x $14.51= $2177 Electricity: 0.10 x 1KWH x 1 x 10 6 BTU = $29.51/MBTU KWH 3412 BTU MBTU 120/.8 x $29.51 = $3,517

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SAMPLE PROBLEMS! 1. Dr. Jones’ Nov-Dec power bill shows that his home used 1355 kwh over a 30 day period. a.How much energy was used (kJ) for the 30 day period? b. Find the energy used in J/day. c.At the rate of $.0749/kwh, what is the doctor’s power bill (w/o tax)?

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A kWh x 3600kJ/1 kWh = kJ ( 4.9 e 6 kJ) B kWh/30 days x J/1kWh = 1.6e 10 J/day C kWh x $0.0749/kWh = $101.49

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2. The current through a toaster (110 V) is 8 amps A. What is the power (in watts) of the toaster? B. How much energy (J) will the toaster use in 5 minutes of operation?

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A. W = 110 V x 8 A = 880 W = 880 J/sec B. E = P x t so, 880 J/sec x 60 sec/1 min x 5 min = 264,000 J

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3. A 100 watt light bulb is 20% efficient. A. how much energy does it use in 12 hours of operation? B. How much energy does the bulb convert into light over the 12- hour period? C. How much energy does the bulb convert into heat over the 12-hour period? D. Convert the total energy use into kwh.

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A. 20 J/sec x 60sec/1min x 60 min/1 hr x 12 hrs = 864,000 J B. 864,000 J C. 80 J/sec x 60 sec /1min x 60min/1hr x 12 hrs= 3,456,000 J D. 4,320,000 x 1kwh/3.6e 6 J = 1.2 kwh

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4. An electric clothes dryer has a power rating of 4000W. Assume that a family does 5 loads of laundry each week for 4 weeks. Also assume that each load takes 1 hour. A.Find the energy used in both J and kJ B. If the cost of electricity is $0.758/kwh, find the cost of operating the dryer for 3 months.

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A. E = P x T = 4000J/sec x 60 sec/1min x 60 min/1hr x 20 hrs x 1kw/3.6e 6 B. 80 kwh x $0.0758/kwh = $6.06

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5. Dr. Nick’s natural gas bill states that his household used 110 therms of energy for a 30- day period. A.Convert 110 therms to kwh B.His charge for the energy was $ Find the cost of this natural gas in $/kwh.

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A.110 therms x 100,000 BTUs/1 therm x 1.05kJ/1BTU x 1 kw/3500kJ= kwh B. $88.78/ kwh = $0.03 kwh

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