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
Energy Efficiency
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
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Device Efficiency (%)
Dry-cell flashlight battery
Home gas furnace
Storage battery
Home oil furnace
Small electric motor
Steam power plant
Diesel engine
High-intensity lamp
Automobile engine
Fluorescent lamp
Incandescent lamp
90
85
70
65
62
38
38
32
25
22
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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)
CONVERSIONS
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1 cal = 4.186 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 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 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
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
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 e6 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.
• Remember your metric conversions!
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GMKHD – DCMMNP
5000 w = ____ kW
4800 J = ____ MJ
88 kc = ____ cal
Energy Calculations Worksheet
• Take a look and highlight terms.
• See if you can do the first problem tonight.
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 106J = 7934Btus
1,055J
D.) 7,934Btu x
1h
x 60m = 11.9 min
40,000Btu hr
2. A.) 1KWH x 3.6 x 10J x
KWH
1BTU = 3,412Btu
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.
3.A.
120MBtu x 1,000,000 = 1.2x10 Btu x 1 ft3 x 1/ccf = 1,165ccf (gas)
1030Btu 100ft3
1 MBtu
B. 1165ccf = 1,942ccf x $0.9 = $1,748 to heat house@ 60% eff
.6
ccf
C. 1165 = 1456.25 ccf x $0.9 = $1,310.6 to heat house@ 80% eff
.8
ccf
Money saved per year in heating costs = $1748-$1,310.6 = $437.40
Years to payback cost of furnace = $4000/$437.40 = 9.1 years
4.A.)
8760hrs/yr x 160 W/m2 x 10m2 x .20 =2,803.2 KWH
2,803.2 KWH x 3.6x106 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
5.A.) 250 W/m2 x 10m2 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 10m2 = 45.6621m2
2,190KWH
6. 8,760 x 200W/m x 106m x .001 x 3.4123 Btu = 498 Tons
12 X 106 Btu/ ton
W
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……
8.) Battery: 4.5 WH
= .0045KWH
1,000W/KW
1
= $222.22 / KWH
0.0045KWH
9.
Natural gas cost is $1.14 x 1cf
ccf
x
1 ccf = $11.07/MBTU
1030 BTU 100cf
120/.8 x $11.07 = $1660.50
Propane gas cost:
$1.69 x 1 gal
x 1x 106 BTU = $18.37/MBTU
Gal
92000 BTU MBTU
120/.8 x $18.37= $2775.
Fuel Oil cost:
$1.93
Gal
x
1 Gal
x 1x106 BTU = $14.51/MBTU
133,000 BTU
MBTU
120/.8 x $14.51= $2177
Electricity:
0.10 x 1KWH x
KWH
3412 BTU
1 x 106 BTU = $29.51/MBTU
MBTU
120/.8 x $29.51 = $3,517
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)?
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.
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.
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 30day period.
A. Convert 110 therms to kwh
B. His charge for the energy
was $88.78. Find the cost
of this natural gas in $/kwh.
A. 110 therms x 100,000 BTUs/1 therm x
1.05kJ/1BTU x 1 kw/3500kJ= 3208.3 kwh
B. $88.78/3208.3 kwh = $0.03 kwh