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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**EnergyEfficiency**percentage of energy input that does useful work in an energy conversion system www.bio.miami.edu/beck/esc101/Chapter14&15.ppt**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.**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!**Efficiency of Some Common Devices**Device Efficiency (%) • Dry-cell flashlight battery 90 • Home gas furnace 85 • Storage battery 70 • Home oil furnace 65 • Small electric motor 62 • Steam power plant 38 • Diesel engine 38 • High-intensity lamp 32 • Automobile engine 25 • Fluorescent lamp 22 • Incandescent lamp 4**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)**CONVERSIONS**• 1 cal = 4.186 joules • 1 BTU = 1.05 kJ • 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 107 BTUs**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)**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 Lawof 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**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 • = 112.5 kJ**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 3600 kJ**1 kWh is the energy required to power 10 100-watt light**bulbs for 1 hour. • The average US home uses about 10,000 kWh electric energy/year.**SAMPLE PROBLEMS!**1. Dr. Jones’ Nov-Dec power bill shows that his home used 1355 kwh over a 30 day period. • How much energy was used (kJ) for the 30 day period? • Find the energy used in J/day. • At the rate of $ .0749/kwh, what is the doctor’s power bill (w/o tax)?**A. 1355 kWh x 3600kJ/1 kWh = 4878000kJ ( 4.9 e 6 kJ)**B. 1355 kWh/30 days x 3600000 J/1kWh = 1.6e10 J/day C. 1355 kWh x $0.0749/kWh = $101.49**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?**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**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.**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/3.6e6J = 1.2 kwh**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. • 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.**A. E = P x T = 4000J/sec x 60 sec/1min x 60 min/1hr x 20 hrs**x 1kw/3.6e6 B. 80 kwh x $0.0758/kwh = $6.06**5. Dr. Nick’s natural gas bill states that his household**used 110 therms of energy for a 30-day period. • Convert 110 therms to kwh • His charge for the energy was $88.78. Find the cost of this natural gas in $/kwh.**110 therms x 100,000 BTUs/1 therm x 1.05kJ/1BTU x 1**kw/3500kJ= 3208.3 kwh • $88.78/3208.3 kwh = $0.03 kwh