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