Teacher Guide - ExploreLearning

Electronics - Devices

Oct 7, 2013 (4 years and 9 months ago)

130 views

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

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

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

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:
l_1/index.html

Series and parallel circuits:

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