Electricity
Unit 4: Electricity
Chapter 13: Electrical Systems
 13.1
Series Circuits
 13.2
Parallel Circuits
 13.3
Electrical Power, AC and DC
Electricity
13.1 Investigation: Series Circuits
Key Question:
How can devices be
connected in circuits?
Objectives:

Build and analyze series circuits.

Apply an understanding of Ohm’s law to explain their
observations.

Describe the effects of short circuits.
Electrical Systems

In in the late 1800s, a major
disagreement over the use of
AC and DC electricity erupted
between two famous
inventors.

Thomas Edison favored the
direct current (DC) method of
moving electrical energy from
electrical generation stations
to homes and buildings.

George Westinghouse argued
that the alternating current
(AC) method worked better.
 Which system do we use
today?
Series Circuits
 In
a series circuit,
current can only take
one path, so the current
is the same at all points
in the circuit.
Series circuits
 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.
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.
Current and resistance in series circuits
 Adding
resistances is
like adding pinches to
a water hose.

Each pinch adds
some resistance.
 Everything
has some
resistance, even thin
wire.
Calculating current
A series circuit contains a 12-V battery and
three bulbs with resistances of 1 W, 2 W, and 3
W. What is the current in amps?
1.
Looking for: …current (amps)
2.
Given: …voltage (12V); resistances = 1Ω, 2 Ω, 3 Ω.
3.
Relationships: Use: Rtot = R1+R2+R3 and Ohm’s Law
I=V÷R
4.
Solution: Rtot = 6 Ω and I = 12 V ÷ 6 Ω = 2 amps
Energy and series circuits
 The
devices in a circuit convert electrical energy into
other forms of energy.
 Remember
that the rate of energy transfer is called
power, and is measured in watts (W).
Voltage drop
 As
devices in series use
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.
Voltage drop and Ohm’s law
 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.
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.
Calculating voltage drops
A circuit contains a 9-volt battery, a 1-ohm bulb, and
a 2-ohm bulb. Calculate the circuit’s total resistance
and current, then find each bulb’s voltage drop.
1.
Looking for: …total resistance; and voltage drop
for each bulb
2.
Given: …voltage = 9V; resistances = 1Ω, 2 Ω.
3.
Relationships: Rtot = R1+R2+R3 and Ohm’s Law
I=V÷R
4.
Solution: - part 1
Rtot = 3 Ω
I = 9 V ÷ 3 Ω = 3 amps
Calculating voltage drop
Solution: - part 2
4.
—
Use resistance to find current:
I = 9 V ÷ 3 Ω = 3 amps
Solution: - part 3
5.
—
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 , so Rtot = (3 + 6 ) = 9 V
Unit 4: Electricity
Chapter 13: Electrical Systems
 13.1
Series Circuits
 13.2
Parallel Circuits
 13.3
Electrical Power, AC and DC
Electricity
13.2 Investigation: Parallel Circuits
Key Question:
How do parallel circuits work?
Objectives:

Build parallel circuits.

Compare and contrast series and parallel circuits.

Discuss applications of parallel circuits.
Parallel Circuits
 In
parallel circuits the current can take more
than one path.
Kirchhoff’s Current Law
 All
of the current
entering a branch point
must exit again.
 This
is known as
Kirchhoff’s current
law.
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.
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.
Resistance in parallel circuits
 Adding
resistance in parallel provides another path
for current, and more current flows.
 When
more current flows at the same voltage, the
total resistance of the circuit must decrease.
Calculating resistance in parallel circuits
A circuit contains a 2-ohm resistor and a 4-ohm resistor in
parallel. Calculate the total resistance of the circuit.
1.
Looking for: …the resistance
2.
Given: …the type of circuit (parallel) and branch
resistances (2  and 4 )
3.
Relationships: Use: the rule for parallel
resistances.
4.
Solution:
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.
Calculating in current and resistance in
a parallel circuit
 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.
Calculating current and resistance
Calculate the total resistance, total current, and current in
each branch for the circuit shown.
1.
Looking for: …total resistance, total
current, and each branch current.
2.
Given: …resistance of each branch
(5,1) and the total voltage (3 V)
3.
Relationships: Use the formula for parallel
resistance and Ohm’s law.
4.
Solution: part 1
Rtot = 1/5  + 1/6  = 5/6  = 0.83 
Calculating current and resistance
Calculate the total resistance, total current, and current in
each branch for the circuit shown.
4.
Solution: part 2
Itot = (3 V) ÷ (0.83 ) = 3.6 A
I1 = (3 V) ÷ (1 ) = 3.0 A
I5 = (3 V) ÷ (5 ) = 0.6 A
Parallel vs. Series

Remember: series/same/current;
parallel/same/voltage.

Use Ohm’s law for both.
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.
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.
Fuses
 In
newer homes, flip the tripped
circuit breaker.
 In
older homes you must replace
the blown fuse.
 Fuses
are also used in car
electrical systems and in
electrical devices such as
televisions or in electrical meters
used to test circuits.
Unit 4: Electricity
Chapter 13: Electrical Systems
 13.1
Series Circuits
 13.2
Parallel Circuits
 13.3
Electrical Power, AC and DC
Electricity
13.3 Investigation: Electrical Energy and
Power
 Key
Question:
How much energy is carried
by electricity?
Objectives:

Build parallel circuits.

Compare and contrast series and parallel circuits.

Discuss applications of parallel circuits.
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.
Important review
Electrical Power
 The
watt is an
abbreviation for one joule
per second.
 A 100-watt
light bulb uses
100 joules of energy
every second.
Power
 Power
is a “rate” and is measured using current and
voltage.
Different forms of the Power Equation
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.
Calculating power
A 12 V battery is connected in series to
two identical light bulbs. The current in the
circuit is 3 A. Calculate the power output of
the battery in watts.
1.
Looking for:... power of the battery
2.
Given: …voltage (12 V); current (3 A)
3.
Relationships: Use Power: P = V x I
4.
Solution: P = (3 A)(12 V) = 36 W
Paying for Electricity
 Utility
companies charge customers for the number
of kilowatt-hours (kWh) used each month.
 A kilowatt-hour
 The
is a unit of energy.
number of kilowatt-hours used equals the
number of kilowatts multiplied by the number of
hours the appliance was turned on.
Paying for 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.
Calculating cost of power
How much does it cost to run a television and a
video game console for 2 hours? Use the reference
table and a price of $0.15 per kWh.
1.
Looking for: …cost of T.V. + video game for 2 hours
2.
Given: … price = $0.15/kWh, P = 250 W and 170 W; t = 2 h
3.
Relationships: Use: no. of kWh= (price) x (t) and 1 kW = 1,000 W
4.
Solution: 250W + 170W = 420W
Convert watts to kW: 420 W x 1 kW = .42 kW
1000 W
No. of kWh = .42 kW x 2 h = .84 kWh
Cost = .84 kWh x $ 0.15 = $0.126 = about 13¢ for 2 hours
1 kWh
Alternating and direct current
 Although
the letters “DC”
stand for “direct current”
the abbreviation “DC” is
used to describe both
voltage and current.
 DC
flows in one direction as
in a battery.
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.
AC and DC
 The
electrical system in
your house uses
alternating current or AC.
 Alternating
current
constantly switches
direction.
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.
120 VAC
 The
120-volt AC (VAC)
electricity used in homes
and businesses
alternates between peak
values of +170 V and –
170 V.
 This
kind of electricity is
called 120 VAC because
+120V is a type of
average positive voltage.
Electricity, power
and heat
 Wires
are made in different
sizes to carry different
amounts of current.
 A large
diameter wire has
less resistance and can
safely carry more current
than a smaller, thinner wire.
Electricity in homes
 Electricity
comes into most homes or buildings
through a control panel which protect against wires
overheating and causing fires.
Electricity in homes

Each wall socket is connected to three wires.
 One wire, called the “hot” wire, carries 120 volts AC.
 The neutral wire stays at 0 volts.
 A third wire is connected to the ground (0 V) near your house.
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.
Plugged In

Most of the vehicles on US roads
today are powered by gasoline.

Concerns about global climate
change, rising oil prices, and
dependence on imported oil
have spurred automotive
engineers to look for
alternatives.

Some of the most promising new
technologies involve “plugging
in”—connecting our vehicles to
the electricity grid.