1. 22-2 Using Electric Energy

2. Energy Transfer
A lot of devices are used to convert electrical energy into some other useable form
Light bulb, hair dryer, stove
A lot of energy lost in heat
Light bulb & engines get warm

3. Heating a Resistor
What are some appliances used to convert almost all energy into thermal energy
Hair dryer, space heater, hot plate
To determine how much total energy is converted to thermal energy….
P = E/t  E = Pt
P = IV  V = IR
So…
E = I2Rt

4. A heater has a resistance of 10.0 Ω. It operates on 120.0 V
What is the current through the resistance
Known Unknown
R = 10.0 Ω I = ???
V = V
I = V / R
= V / 10.0 Ω
= 12.0 A

5. A heater has a resistance of 10.0 Ω. It operates on 120.0 V
What thermal Energy is supplied by the heater in 10.0 s
Known Unknown
I = 12.0 A E = ???
R = 10.0 Ω
t = 10.0 s
E = I2Rt
(12.0 A)2(10.0 Ω)(10.0 s)
=14.4 x 103 J
=14.4 kJ

6. Transmission of Electric Energy
Niagara Falls & Hoover dam produce a lot of electricity w/o much pollution
Has to transmitted long distances
Thermal energy lost is P=I2R
Engineers want to reduce I or R
**1km of wire = 0.20 Ω

7. Transmission of Electric Energy
Supposed a farm house is 3.5 km away and has a 41 A electric stove.
The power dissipated (loss) in the wires =
2(3.5 km)(0.20 Ω/km)
= 1.4 Ω
P = I2R
= (41 A)2 x 1.4Ω = 2400 W

8. Kilowatt-Hour Electric Companies charge you for energy
Energy used by any device is its rate of energy consumption
Joules/second (J/s)(sec)
Too small for commercial use so companies use the Kilowatt-Hour or kWh
= 1000 watts delivered continuously for 3600 sec
= 3.6 x 106 J

9. Appliances
Other than heating appliances, most don’t need more than 1000-W of power
Evident in your lab yesterday

10. Cost of operating A television set draws 2.0 A when operated on 120 V.
A) How much power does the TV use?
B) If the set operated for an average of 7.0 h/day, what energy in kWh does it consume/month(30 days)
C) At 11 cents per kWh, what is the cost of operating the set per month?
Known Unkown
I = 2.0 A E = ???
V = V Total Cost = ???
t = (7.0 h/day)(30 days)
Cost = 11 cents / kWh

11. A) How much power does the TV use?
P = IV
= (2.0 A)(120.0 V)
= 240 W

12. E = Pt (240 W)(7.0 h/day)(30 d) = 5.0 x 104 Wh = 5.0 x 101 kWh
B) If the set operated for an average of 7.0 h/day, what energy in kWh does it consume/month(30 days)
E = Pt
(240 W)(7.0 h/day)(30 d)
= 5.0 x 104 Wh
= 5.0 x 101 kWh

13. Cost = (5.0 x 101 kWh)($0.11/kWh) = $5.50
C) At 11 cents per kWh, what is the cost of operating the set per month?
Cost = (5.0 x 101 kWh)($0.11/kWh)
= $5.50