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ELECTRICAL SYSTEMS
Chapter Twenty One: Electrical
Systems
21.1 Series Circuits
21.2 Parallel Circuits
21.3 Electrical Power
Chapter 21.1 Learning Goals
Build and analyze series circuits.
Apply Ohm’s law to calculate the
current in a series circuit.
Explain how energy conservation
applies to electric circuits.
Investigation 21A
Electric Circuits
Key Question:
What are the different types of circuits?
21.1 Electrical Systems
In a series circuit,
current can only
take one path, so
the current is the
same at all points
in the circuit.
21.1 Electrical Systems
Inexpensive strings
of holiday lights are
wired with the bulbs
in series.
If you remove one of
the bulbs from its
socket, the whole
string of mini bulbs
will go out.
21.1 Current and resistance in
series circuits
If you know the resistance of each device,
you can find the total resistance of the
circuit by adding up the resistance of each
device.
21.1 Current and resistance in
series circuits
Think of adding
resistances like
adding pinches to
a hose.
 Each pinch adds
some resistance.
21.1 Current and resistance in
series circuits
Everything has some resistance, even
wires.
Solving Problems
A series circuit contains
a 12-V battery and three
bulbs with resistances
of1Ω, 2 Ω, and 3 Ω.
What is the current in
the circuit?
Solving Problems
1. Looking for:
 …current (amps)
2. Given
 …Voltage = 12V; resistances = 1Ω, 2 Ω, 3
Ω.
3. Relationships:
 Rtot = R1+R2+R3
 Ohm’s Law I = V ÷ R
4. Solution
 Rtot = 6 Ω
 I = 12 V ÷ 6 Ω = 2 amps
21.1 Voltage drop
As each device in
series uses power,
the power carried by
the current is
reduced.
As a result, the
voltage is lower after
each device that uses
power.
This is known as the
voltage drop.
21.1 Voltage drop
The law of conservation
of energy also applies to
a circuit.
In this circuit, each bulb
has a resistance of 1
ohm, so each has a
voltage drop of 1 volt
when 1 amp flows
through the circuit.
21.1 Kirchhoff’s Voltage Law
Kirchhoff’s voltage law states that the
total of all the voltage drops must add up
to the battery’s voltage.
Solving Problems
The circuit shown
contains a 9-volt battery,
a 1-ohm bulb, and a 2ohm bulb.
Calculate the circuit’s
total resistance and
current.
Then find each bulb’s
voltage drop.
Solving Problems
1. Looking for:
 …total resistance; voltage drop each bulb
2. Given
 …Voltage = 9V; resistances = 1Ω, 2 Ω.
3. Relationships:
 Rtot = R1+R2+R3
 Ohm’s Law I = V ÷ R
4. Solution- part 1
 Rtot = 3 Ω
 I = 9 V ÷ 3 Ω = 3 amps
Solving Problems
4. Solution- part 2
 Use resistance to find current
I = 9 V ÷ 3 Ω = 3 amps
 Solution- part 3
 Rearrange Ohm’s law to solve for voltage
 Use current to find each voltage drop
V=IxR
V1 = (3 A) x (1 Ω) = 3 volts
V2 = (3 A) x (2 Ω) = 6 volts (3 + 6 ) = 9 V
Chapter Twenty One: Electrical
Systems
21.1 Series Circuits
21.2 Parallel Circuits
21.3 Electrical Power
Chapter 21.2 Learning Goals
Build and analyze parallel circuits.
Compare and contrast series and
parallel circuits.
Discuss advantages for using
parallel circuits in homes.
21.2 Parallel Circuits
In parallel circuits the current can
take more than one path.
21.2 Kirchhoff’s Current Law
All of the current
entering a branch
point must exit
again.
This is known as
Kirchhoff’s
current law.
21.2 Voltage and parallel circuits
If the voltage is
the same along
a wire, then the
same voltage
appears across
each branch of a
parallel circuit.
21.2 Voltage and parallel circuits
 Parallel circuits have two
advantages over series circuits.
1. Each device in the circuit has a voltage
drop equal to the full battery voltage.
2. Each device in the circuit may be turned
off independently without stopping the
current in the other devices in the circuit.
21.2 Current and parallel circuits
Each branch
works
independently
so the total
current in a
parallel circuit
is the sum of
the currents in
each branch.
21.2 Calculating in circuits
In a series circuit,
adding an extra
resistor increases
the total resistance
of the circuit.
In a parallel circuit,
more current flows
so the total
resistance decreases.
21.2 Parallel vs. Series
 Remember: series/same/current;
parallel/same/voltage.
 Use Ohm’s law for both.
Solving Problems
 All of the electrical outlets
in Jonah’s living room are
on one parallel circuit.
 The circuit breaker cuts off
the current if it exceeds 15
amps.
 Will the breaker trip if he
uses a light (240 Ω), stereo
(150 Ω), and an air
conditioner (10 Ω)?
Solving Problems
1. Looking for:
 whether current exceeds 15 amps
2. Given:
 ……resistances = 240 Ω; 150 Ω; 10 Ω
3. Relationships:
 Assume voltage for each branch = 120 V
 Ohm’s Law I = V ÷ R
 Kirchhoff’s Current Law Itotal = I1 +I2 +I3
4. Solution:
 Ilight = 120 V ÷ 240 Ω = 0.5 amps
 Istereo = 120 V ÷ 150 Ω = 0.8 amps
 Ia/c = 120 V ÷ 10 Ω = 12 amps
0.5
0.8
+12.0
13.3
Breaker will
not trip
21.2 Short circuits
A short circuit is a parallel path in a circuit
with very low resistance.
A short circuit can be created accidentally
by making a parallel branch with a wire.
21.2 Short circuits
Each circuit has its own fuse or circuit
breaker that stops the current if it exceeds
the safe amount, usually 15 or 20 amps
If you turn on too many appliances in one
circuit at the same time, the circuit breaker
or fuse cuts off the current.
To restore the current, you must FIRST
disconnect some or all of the appliances.
21.2 Fuses
In newer homes, flip the
tripped circuit breaker.
In older homes you must
replace the blown fuse (in
older homes).
Fuses are also used in car
electrical systems and in
electrical devices such as
televisions or in electrical
meters used to test circuits.
Investigation 21C
Analyzing Circuits
Key Question:
How do you build and analyze a network
circuit?
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?