Download Section 21.3 - CPO Science

ELECTRICAL SYSTEMS 21.3
Chapter Twenty One: Electrical
Systems
21.1 Series Circuits
21.2 Parallel Circuits
21.3 Electrical Power
Chapter 21.3 Learning Goals
Define electric power and apply a
formula to perform power calculations.
Distinguish direct current and
alternating current.
Discuss applications of electricity in
daily living.
Investigation 21B
Electrical Energy and Power
Key Question:
How much energy is carried by electricity?
21.3 Electrical Power
Electrical power is measured in watts,
just like mechanical power.
Power is the rate at which energy is
changed into other forms of energy
such as heat, sound, or light.
Anything that “uses” electricity is
actually converting electrical energy
into some other type of energy.
21.3 Important review
21.3 Electrical Power
The watt is an
abbreviation for
one joule per
second.
A 100-watt light
bulb uses 100
joules of energy
every second.
21.3 Power
Power is a “rate” and is measured
using current and voltage.
21.3 Different forms of the Power
Equation
21.3 Kilowatt
Most electrical
appliances have a
label that lists the
power in watts (W) or
kilowatts (kW).
The kilowatt is used
for large amounts of
power.
Solving Problems
 A 12-volt battery is
connected in series to
two identical light
bulbs.
 The current in the
circuit is 3 amps.
 Calculate the power
output of the battery.
Solving Problems
1. Looking for:
 …power of battery
2. Given:
 …voltage = 12 V; current = 3 amps
3. Relationships:
 Power:
P=IxV
4. Solution:
 P = 3 A x 12 V = 36 watts
21.3 Buying
Electricity
Utility companies charge customers for the
number of kilowatt-hours (kWh) used each
month.
A kilowatt-hour is a unit of energy.
The number of kilowatt-hours used equals
the number of kilowatts multiplied by the
number of hours the appliance was turned
on.
21.3 Buying Electricity
There are many
simple things you
can do to use less
electricity.
When added up,
these simple
things can mean
many dollars of
savings each
month.
Solving Problems
 How much does it cost to run a
3,000 kW electric stove for 2 hours?
 Use an electricity cost of $0.15 per
kilowatt-hour.
1. Looking for:
 …cost to run stove for 2h
2. Given:
 … P = 3,000W; T = 2h; price $0.15/kW
Solving Problems
3. Relationships:
 1000 watts = 1 kW
 Charge in kWh
4. Solution:
 3000 W x 1 kW = 3 kW
1000 W
 Charge = 3 kW x 2 h = 6 kWh
 Cost = 6 kWh x $ 0.15
1 kWh
= $ 0.90
21.3 AC and DC
Although the letters
“DC” stand for “direct
current” the
abbreviation “DC” is
used to describe both
voltage and current.
DC current flows in one
direction as in a battery.
21.3 AC and DC
The electrical
system in your
house uses
alternating current
or AC.
Alternating current
constantly switches
direction.
21.3 Electricity in homes
Electricity comes into most homes or
buildings through a control panel
which protect against wires
overheating and causing fires.
21.3 Electricity in homes
Electrical outlets in
bathrooms, kitchens, or
outdoors are now
required to have ground
fault interrupt (GFI)
outlets.
GFI outlets are excellent
protection against
electric shocks, especially
in wet locations.
21.3 Distributing electricity
Electricity is a valuable
form of energy because
electrical power can be
moved easily over
large distances.
Alternating current is
easier to generate and
transmit over long
distances.
21.3 Distributing electricity
Many electronic
devices, like cell
phones or laptop
computers, use DC
electricity.
An “AC adapter” is a
device that changes
the AC voltage from
the wall outlet into DC
voltage for the device.
Bright Ideas
What makes one bulb
more efficient than
another? How much more
efficient are the LEDs?
What kind of savings
does this mean in terms
of electricity?