Transcript energy transformed

Electric Energy
Electric energy is made available by the flow of electric charge
through a conductor. It is useful because it can easily be
transformed into other types of energy. For example, motors
transform electric energy into mechanical energy, and electric
heaters, stoves and hair dryers transform electric energy into
thermal energy.
In a light bulb
the filament gets so hot that it glows
and produces visible light. 10% of the
electric energy is transformed into
light and the rest into thermal energy.
What’s really going on when you turn on the lights is that
there are many collisions occurring between the moving
electrons and the atoms of the wire. The electrons transfer
some of their kinetic energy to the atoms when they
collide and thus, the temperature of the atoms increases.
This thermal energy is then radiated as light.
Electric Power
Electric power is the rate at which energy is transformed. The
energy transformed when a charge Q moves through a voltage V
is QV. Thus,
P = power (W or J/s)
Q = charge (C)
P = energy transformed = QV
time
t
V = voltage (V)
t = time (s)
However, the charge per second (Q/t) is simply the electric
current (I). We can therefore change the formula to read:
P = IV
P = power (W or J/s)
I = current (C/s or A)
V = voltage (V)
Electric Power
Ex: What is the power transformed by a device where 15.0 A
current passes through 12.0 V?
P = IV
P = (15.0 A)(12.0 V)
P = 180 W = 1.80 x 102 W
Electric Power
The rate of energy transformation in a resistance R can be
writing using Ohm’s Law (V=IR) in two other ways:
P = I(IR) = I2R
P = (V/R)V = V2/R
P = power (W)
I = current (A)
R = resistance (Ω)
However, these two formulas only apply to resistors, whereas
P = IV can apply to any device.
Electric Power
Ex: Determine the electric power of 150 A current
in 3.5 Ω of resistance.
P = I 2R
P = (150 A)2(3.5 Ω)
P = 78750 W = 7.9 x 104 W
Electric Power
Ex: Calculate the resistance of a 40. W headlight designed for 12 V.
P = V2/R
R = V2/P
R = (12 V)2/(40.W)
R = 3.6 Ω
Ex: A 34 A current runs through a 25 V hair dryer. What is the
power transformed by the hair dryer?
P = IV
P = (34 A)(25 V)
P = 850 W
850 W
Ex: Calculate the resistance of a device with a 15.0 A current
and a power of 635 W.
R = P/I2
R = (635 W)/(15.0 A)2
R = 2.82 Ω
2.82 Ω
Ex: A 65 C current moves through a potential difference of
250 V in 4.0 seconds. Determine the power transformed.
P = QV/t
P = (65 C)(250 V)/(4.0 s)
P = 65000 W = 6.5 x 104 W
6.5 x 10 4 W
Ex: An electric heater draws 15.0 A on a 120 V line. If it
operates 3.0 hours per day and the electric company charges
10.5 cents per kWh, how much does it cost per month (30
days)?
P = IV
P = (15.0 A)(120 V)
P = 1800 W = 1.80 kW
(3.0 h/d)(30 d) = 90 h
(1.80 kW)(90 h)($ .105) = $17
$17
Ex: A bolt of lighting transfers 109 J of energy across a
potential difference of 5x107 V during a time interval of .2 s.
What is the power delivered by the lightning?
E = QV  Q = V/E
Q = (5x107 V)/(109 J) = 20 C
I = Q/t
I = (20 C)/(.02 s) = 100 A
P = IV
P = (100 A)(5x107 V) = 5000000000 W = 5 GW
5 GW