9.2
THE POWER OF ELECTRICITY
IN THIS SECTION
Objectives:
• Electric Power (P=VI) is the rate at which electrical
potential energy is transformed
• Power Consumption multiplied by time of use
equals the amount of electrical energy used by a
device
UP A HILL
• Both of these vehicles will gain the same potential
energy
• It depends how the energy is converted (load)
• Because the race car could transform its energy
faster, it has more power
POWER
• Power – the rate of change in energy
• Power is also the rate at which work is done or
energy is transformed
• The unit for measuring energy is joule (J)
• One joule (J) of energy transformed in one
second(s) is called one watt (W) of power
ELECTRICAL POWER
• Electrical Power – the rate of change in
electrical energy
• The unit for measuring energy is joule (J)
• Electric loads convert electrical energy into
other forms of energy (i.e. heat/light).
P = VI
P = power (watt)
V = voltage (volts)
I = current (amperes)
Pg. 323 #1-3
P
V
I
SAMPLE POWER PROBLEMS
1.
A flashlight bulb operates on 3.0 V and draws a
current of 4.0 A. What is the power of this bulb?
P = VI
P = (3.0 V)(4.0 A)
P = 12 W
The power of this bulb is 12 W.
SAMPLE POWER PROBLEMS
2. A 60 W light bulb is connected to 120 V. What
current passes through the light bulb?
I = P/V
I = 60 W/120 V
I = 0.5 A
The current passing through the light bulb is 0.5 A.
POWER RATING
• Power rating – the measurement of how much
electrical energy a device consumes for every
second it is in use.
Example: 100 W light bulb uses 100 J of electrical
energy every second and produces 100 J of heat
and light energy.
POWER IN RELATION TO ENERGY
AND TIME
• Power – is the rate of change in energy (or work).
P = E/t
P = power (watt)
E = energy (joules)
t = time (seconds)
E
P
t
Power (P) is defined as energy transferred (E) per time
interval(t)
EXAMPLE QUESTION
• P = E/t
• How much electrical energy is consumed by a
1200W hair dryer if it used for 5.0 min?
• Use the formula
• 5.0 min = 5 x 60s = 300s
• E = Pt
• = (1200W)(300s) = 360 000Ws = 3.6x105J
• A: A 1200W hair dryer consumes 3.6x105J of
electrical energy if it used for 5.0 min.
CALCULATE
• Pg. 324 #1-3
SAMPLE ENERGY CONSUMPTION
PROBLEM
1. How much electrical energy is consumed by
a 60 W light bulb left on for 25 minutes?
NOTE: Must convert minutes  seconds
1 minute = 60 seconds
E = P*t
E = (60 W)(25 min. x 60 s)
E = (60 W)(1500 s)
E = 9.0 x 104 J
The electrical energy consumed was 9.0 x 104 J.
UNITS FOR ENERGY
• 1.0 joule = 1.0 W x 1.0 s (Energy Units)
• 1.0 kilowatt-hour = 1.0 kilowatt x 1.0 h
• 1.0 kW*h = 1.0 kW x 1.0 h (Larger Energy Units)
CALCULATING ELECTRICAL
ENERGY AND COST
• Using large amounts of electrical energy;
therefore measure in kW*h (not W*s).
• Remember to convert W  kW by dividing by
1000.
• How much kW*h of electricity is consumed by
a 500 W light bulb that burns for 5 hours.
• First: 500 W  0.5 kW (divided by 1000)
• E = Pxt = 0.5 kW x 5 h = 2.5 kW*h
COST OF ENERGY USED
BILL PAYMENT
• Cost of energy used = amount charged/1 kW*h x kW*h
• Cost: put in $ units/kW*h. Example - $0.05/kW*h
Example:
If the power company charges 5 cents for every
kW*h of energy, how much is the electric bill for the
month if a family uses 1300 kW*h of energy?
Cost of energy used = $0.05/1kW*h x 1300 kW*h
Cost of energy used = $65.00
PRACTICE:
1. A 300 mA current runs through a resistor connected
to 100 V.
a)
b)
c)
What is the power?
It runs for 100 hrs, how much electricity is used?
Electricity costs 7 cents per kW·hr, how much did it cost
to run this resistor?
PRACTICE:
2. Mom is yelling at her teenager for leaving the lights
on in the basement for one week (7 days!!). There
are 3 lights each drawing 75 W. What did it cost to
run the lights if electricity costs 7 cents a kW·hr?
PRACTICE:
3a. An arc welder used 100 A of current on 220 V, what is
the resistance?
b. If the arc welder is used for 6 hrs, how much electricity
did it use?
c. If electricity costs 7 cents per kW·hr, how much did it
cost to run that arc welder?
THE COST OF ELECTRICITY
• Page 326 – Complete the table and answer
questions 1 & 2
ELECTRICAL SURGES
• Electrical surges are brief increases in voltage to tens of
thousands of volts
• Can occur through all of your wiring
• Can be caused by lightning, turning large appliances on
and off
• Surge protectors