Teacher Guide:
Circuits
Learning Objectives
Students will…
Define circuit.
Observe the effect
s
of voltage and resistance on current.
Use Ohm’s law to calculate the current in a circuit.
Create series and parallel circuits, and compare their properties.
C
alculate
the total resistance of a series circuit and a parallel circuit.
Vocabulary
ammeter, circuit, current,
electron,
ohmmeter, Ohm’s law, parallel circuit, resistance, resistor,
series circuit, voltage
Lesson Overview
The
Circuits
Gizmo™ allows students to build a circuit on a
circuit board. Available materials include batteries, wires, light
bulbs,
fuses,
and a variety of
resistors. The voltage of the
battery can be varied. Current can be observed qualitatively
and measured using a
n ammeter. A voltmeter and ohmmeter
are also available.
The
Circuits
lesson materials address
topics
similar to
those in
the
Circuit Builder
lesson, but in a more quantitative manner.
The Student Exploration sheet contains three activities:
Activity A
–
S
tudents determine Oh
m’s law, which relates voltage to
resistance, and
current
.
A
ctivity B
–
Students calculate the total resistance of a series circuit
.
Activity C
–
Students calculate the total resistance of a parallel circuit
.
Suggested Lesson Sequenc
e
1.
Pre

Gizmo activity:
Building
circuits
(
30
–
45
minutes)
Gather materials to make circuits. These materials could include wires,
wire
cutters/strippers (scissors can be used to strip wires),
small
light
bulbs, alligator clips,
batteri
es, and battery

holders.
If you have access to an ammeter, bring that in as well.
Test the light bulbs to make sure they light up when connected to a battery.
Divide your students into groups and give each group a
set
of materials. The first
challenge is
to build a simple circuit to light the bulb. Once each group has completed
this task, define series and parallel circuits, and have each group build one of each.
Finally, try to determine what happens when you add and disconnect bulbs from these
circuits.
As you add more bulbs to a series circuit, all the bulbs should become dimmer.
Disconnecting a bulb will break the circuit. In a parallel circuit, there should be no
change in brightness when you add bulbs to or disconnect bulbs from the circuit.
2.
Prior
to using the Gizmo
(
10
–
15 minutes)
Before
students are at the computers, pass out the Student Exploration sheets and ask
students to complete the Prior Knowledge Questions. Discuss student answers as a
class, but do not provide correct an
swers at this point. Afterwards, if possible, use a
projector to introduce the Gizmo and demonstrate its basic operations. Demonstrate how
to take a screenshot and paste the image into a blank document.
3.
Gizmo activities
(
15
–
20 minut
es per activity)
Assign students to computers. Students can work individually or in small groups. Ask
students to work through the activities in the Student Exploration using the Gizmo.
Alternatively, you can use a projector and do the Exploration as a tea
cher

led activity.
4.
Discussion questions
(
15
–
30 minutes)
As students are working or just after they are done, discuss the following questions:
What is the relationship between voltage, resistance, and current in a circuit?
How do you
calculate the total resistance in a series circuit
?
How do you calculate the total resistance in a parallel circuit?
Suppose you have
three
light bulbs connected to a battery. Will the battery last
longer if the bulbs
are
connected in series or in paralle
l?
If too many devices are connected in a parallel circuit, the wire could overheat
and cause a fire. What kinds of safety devices prevent this from happening?
Teaching tip
: The second
page
of Activity C
in the Student Exploration sheet goes
through some
fairly heavy

duty mathematics. Spend some time going over these
equations with your students so they understand how to solve parallel circuits.
5.
Follow

up activities
(
variable
)
There are several ways to follow up the
Circuits
Gizmo.
When stud
ents are comfortable
with series and parallel circuits, they are ready for compound circuits, which combine
elements of both.
Challenge students to find the voltage, resistance, and current in a
compound circuit
. The
Advanced Circuits
Gizmo focuses on this
challenge.
Just for fun, h
ave students design
and build
their
own
“electronic bugs.”
Bug bodies can
be cut from cardboard or foam board. Stiff wire
legs can be cut from metal coat hangers
or other thick wire.
“
Googly eyes
”
and pipe

cleaners can be used fo
r decoration (
figure
A
). The propulsion for the bugs comes from a
1.5

V DC motor (available at
electronics
and hobby
stores). Drill a hole in a
one

inch (
2.5

cm) segment of
dowel, and glue the
dowel
segment
to the motor.
Hot

glue a small nut and bolt to th
e dowel, as shown in
figure
B
.
When the motor is connected to a battery, the bugs will vibrate rapidly and
skitter randomly around. Students can
stage
races and “battles” with their bugs.
Scientific Background
A
circuit
is a closed path along which e
lectronic charges can travel.
A simple circuit consists of a
power source such as a battery, wire, and a
load
that provides resistance. Examples of loads
include light bulbs, motors, buzzers, heating coils,
and
resistors. A battery consists of a
positively
charged terminal and a negatively
charged terminal. When the battery is connected to
a circuit, electrons in the wire will drift toward the positive terminal of the battery, creating
current
throu
gh the wire. Conventional
circuit diagrams
depict the flow
of positive charge
through the circuit, which is opposite the actual movement of electrons. The standard was
established by Ben Franklin, long before it was discovered that current in a wire is carried by
mobile negative charge
s rather than positive charge
s. The
Circuits
Gizmo allows to view either
the conventional current (positive to negative) or the flow of electrons (negative to positive).
Ohm’s law
establishes the relationship between voltage, current, and resistance:
V
=
IR
. In the
equation,
V
repres
ents voltage,
I
represents current, and
R
represents resistance.
Current
increases as voltage increases and
decreases as resistance in
creases.
In a series circuit, the total resistance in the circuit is equal to the sum of each resistance. So a
series cir
cuit with
a 5

ohm, a 10

ohm, and a 15

ohm resistor
has a total resistance of 30
ohms.
If the
battery voltage
is 15 volts (15 V), the current
in the circuit
will be 0.5 amperes (0.5 A).
In a parallel circuit, adding more elements to the circuit causes the
total resistance of the circuit
to
decrease
rather than increase. The total current in the circuit (
I
) is equal to the sum of
currents in each branch:
Ohm’s law states that
V
=
IR
, which can be rewritten as
I
=
V
/
R
. Substituting this expression
into the equation above
we have
:
(Note: Since each branch of the circuit might have a different resistance, we write
R
1
,
R
2
, and so
forth. But the voltage is the same across each branch, so
V
is
used for each term.) Divide each
side of the equation by
V
to
get an expression for the t
otal resistance of the circuit
:
For example, suppose a parallel circuit has a 5

ohm,
a
10

ohm, and a 20

ohm resistor. The sum
of the reciprocals is 1/5 + 1/10 + 1/15 = 11/30. So the tot
al resistance is 30/11 = 2.73 ohms.
Selected Web Resources
Build simple
circuits:
http://www.thesolutionsite.com/lesson/10101/lesson4.html
Circuits basics:
http://www.allaboutcircuits.com/vo
l_1/index.html
Series and parallel circuits:
http://www.allaboutcircuits.com/vol_1/chpt_5/1.html
Solving circuits:
http:
//www.jabe.com/docs/Circuit_Teacher_notes.pdf
Circuits activity:
http://www.furryelephant.com/content/electricity/series

circuits/
Related Gizmos:
Circuit Builder
:
http://www.explorelearning.com/gizmo/id?638
Advanced Circuits
:
http://www.explorelearning.com/gizmo/id?509
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