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ENERGY

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ENERGY

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 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

- 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)

- 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

- 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)

- 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

- 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!
- GMKHD – DCMMNP
- 5000 w = ____ kW
- 4800 J = ____ MJ
- 88 kc = ____ cal

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

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 1BTU = 3,412BtuKWH 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)

1 MBtu 1030Btu 100ft3

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/yrx 160 W/m2 x 10m2x .20 =2,803.2 KWH

2,803.2 KWH x 3.6x106 J x 1BTU = 9,565,421 BTU = 9.57 MBT KWH1055 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 $1.14x 1cf x 1 ccf = $11.07/MBTU

ccf 1030 BTU 100cf

120/.8 x $11.07 = $1660.50

Propane gas cost:

$1.69 x 1 gal x 1x 106 BTU = $18.37/MBTUGal 92000 BTU MBTU

120/.8 x $18.37= $2775.

Fuel Oil cost:

$1.93 x 1 Gal x 1x106 BTU = $14.51/MBTU

Gal 133,000 BTU MBTU

120/.8 x $14.51= $2177

Electricity:

0.10 x 1KWH x 1 x 106 BTU = $29.51/MBTU

KWH 3412 BTU MBTU

120/.8 x $29.51 = $3,517

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