Performance Benchmark P.12.C.6
Students know electricity is transferred from generating sources for consumption and
practical uses. I/S
The mass use of electricity has transformed the entire world. Scientific and engineering
research about electricity has been active since the late 1700s, but it was the transmission
of electricity to virtually everyone that propelled the last 100 years of technological
revolution. In fact, the National Academy of Engineering has ranked electrification (a
term used to describe the transition of society to rely on electrical power) as the #1
greatest engineering achievement of the 20th Century; greater than space travel, invention
of the automobile and airplane, and even, the advent of the Internet.
To learn more about the top twenty greatest engineering achievements of the 20th
Century, including electrification, go to http://www.greatachievements.org/
The heart of electrification lies with generation and transmission of energy in very large
and complex electrical circuits. However, despite their complexity, the basics of electrical
circuitry are fundamental to reliable operation of the transmission system.
Figure 1. A schematic of electrical transmission. Although not explicitly shown, electricity flow in a
closed loop (circuit) from the power plant to the house and back from the house to the power plant. (From:
http://www.eia.doe.gov/basics/electricity_basics.html)
At its most basic, an electrical circuit is a mechanism for transferring energy. As such, an
electrical circuit must have the following components: (1) an energy source, (2) a closed
path for energy transfer, and (3) an object where the energy is converted for use.
For the electricity used in our homes, offices, factories, and anywhere there is an
electrical socket, the most common energy source is the generator. In an electrical
generator, kinetic energy from a moving turbine rotates conductive wires relative to a
magnetic field. First discovered by Michael Faraday, this rotation induces an electric
potential energy in the circuit, which we commonly call voltage (V). More precisely,
voltage is the electrical potential energy per charge. In smaller, more portable circuits, the
most common energy source is the battery. The unit for voltage is called the Volt (V).
To learn more about electric generators, go to http://www.wvic.com/how-gen-works.htm.
The generator is part of the closed path that allows energy to be transferred easily via
moving charge. Copper wire is a common material used in electrical circuits, where
electrons carrying the fundamental negative charge are easily transferred from one copper
atom to another. In a circuit, the flow of charge is called electrical current (I) and is
calculated as the amount of charge moving past a point in the circuit per unit time. The
unit for current is the Ampere, commonly called the Amp (A).
For more information about electrical current, go to
http://www.facstaff.bucknell.edu/mastascu/eLessonsHTML/Basic/Basic2i.html
Figure 2. A hand crank generator that is part of an electrical circuit. (From
http://demo.physics.uiuc.edu/lectdemo/scripts/demo_descript.idc?DemoID=109)
Electrical loads are placed along the circuit to convert the electrical energy to some useful
form. For example, a light bulb is a device which converts electrical energy from the
circuit into light and thermal energy. Other examples of electrical loads are kitchen
appliances, such as mixers and can openers that can convert energy from the circuit to
kinetic energy in a rotating electric motor to stir and cut. When loads are plugged into a
household outlet, they become part of the greater electrical circuit that is more commonly
called the power grid.
A summary of the most common household items that use electricity is found at
http://www.eia.doe.gov/emeu/recs/recs2001/enduse2001/enduse2001.html
Electrical loads oppose the current in a circuit, a phenomenon more commonly called
electrical resistance (R). In many cases, the resistance of an electrical load is equal to the
voltage drop across the load (V) divided by the electrical current (I), a relationship called
Ohm’s Law.
R
V
I
To learn more about Ohm’s Law, go to
http://www.sciencejoywagon.com/physicszone/lesson/otherpub/wfendt/ohmslaw.htm
There are two basic types of circuits: (1) series circuits and (2) parallel circuits. In a
series circuit, the energy source (generator or battery) and electrical loads are connected
end to end so that the current has only one path. Because the electrical loads are
connected in series, the voltage in the circuit drops (decreases) across each load and the
electrical current is the same over the whole circuit (the amount of current is determined
by the total resistance of the circuit and the applied voltage). The total electrical
resistance in a series circuit is simply the sum of the individual resistances provided by
each electrical load.
RTOT  R1  R2  ...  Rn
In a series circuit, if one of the electrical loads fails (e.g., a light bulb filament breaks),
the circuit no longer has a closed loop and electrical current stops.
Figure 3. Diagrams of simple series (right) and parallel (left) circuits.
The top figures are cartoons showing the wires, switch, bulb, and
battery. The bottom figures are electric schematic diagrams using
symbols to represent the circuit components. (From:
http://www.curriculum.edu.au/sciencepd/electricity/circ_circ.htm)
Alternatively, in a parallel circuit, the electrical current takes multiple paths. This means
that the total current in the circuit is simply the sum of the currents through each path.
Additionally, by nature of the parallel paths all branching from one location and joining
back at another location, the voltage drop across each parallel path is the same and is
equal to the total voltage drop across the circuit. The total electrical resistance in a
parallel circuit is found by adding up the reciprocals of individual resistance values, and
therefore, the total resistance in a parallel circuit is less than the lowest resistance value of
an individual resistor.
1
RTOT

1
1
1

 ... 
R1 R2
Rn
In a parallel circuit, if one of the electrical loads fails, only the electrical current through
that path fails. In the rest of the circuit, electrical current continues.
More information about series and parallel circuits is found at
http://physics.bu.edu/py106/notes/Circuits.html
There are two ways in which electrical current is transferred moves through a circuit: (1)
direct current, known as DC, and (2) alternating current, AC. In DC, the energy is
transferred through the circuit in one direction only and the potential energy source is
commonly a battery. DC circuits are found, for example, in mobile phones and
computers.
To learn more about DC, go to http://www.ndted.org/EducationResources/HighSchool/Electricity/directcurrent.htm
Although DC circuits are widely used, most commercial power is delivered via AC
circuits. In AC, the electrical energy is transferred in both directions through the circuit.
In fact, in the United States, the direction changes sixty times per second (60 cycles per
second or 60 Hz) in commercial AC circuits.
To learn more about AC, go to http://www.allaboutcircuits.com/vol_2/chpt_1/1.html
Although there are both DC and AC electrical generators, AC was selected early in the
20th Century as the preferred transmission mode because AC allows energy to be
transmitted long distances with little loss. In a circuit, the power (normally expressed
using the symbol P and equal to the energy per unit time) consumed is equal to the
resistance in the transmitting wire times the square of the electrical current.
P  I 2R
Because the resistance in the conducting wire is already minimized by using certain types
of material, such as copper, reducing power losses means reducing the electrical current
through the wire—and the best way to reduce electrical current is to increase the voltage
in the circuit, according to the following equation.
P  VI
AC circuits allow generator-induced voltages to be increased easily within a circuit using
a “step-up” transformer, making high voltage transmission lines common and resulting
energy losses small. Transformers are also used to “step down” voltages to less energetic
values that are safer to use at homes and other locations.
More details about transformers can be found at
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/transf.html
Performance Benchmark P.12.C.6
Students know electricity is transferred from generating sources for consumption and
practical uses. I/S
Common misconceptions associated with this benchmark:
1. Students incorrectly think generators and batteries create electricity.
Generators and batteries do not create “electricity.” Electricity is generally understood to
be the transfer of energy through moving electrical charges. But these charges are not
created in the generators and batteries; rather, the charges lie within the conducting
material. The generator and battery provide the potential energy for these charges to
begin moving through the conducting material in a manner that they can interact in an
electrical load, allowing the energy to be transformed in a useable form.
To learn more about this and other misconceptions associate with electricity, go to
http://amasci.com/miscon/elect.html
2. Students incorrectly believe that charges move through circuits at very high
speeds.
In a circuit, individual charges move at relatively slow speeds. The rate of individual
charge movement is called drift velocity, with typical drift velocities a few millimeters
per second. However, a phenomenon called electrical signal moves very rapidly through
the circuit, at roughly the speed of light through the conductive material. The electrical
signal is essentially a wave mechanism for transferring energy. The signal initiates when
an electrical potential energy is established by a generator or battery, allowing an
electrical current to move through the circuit virtually instantaneously.
This misconception, as well as others associated with electrical circuits, are found at
http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/circuits/u9l2e.html
3. Students incorrectly think that, in all circuits, charges move all the way around.
Charges (mostly “free” electrons) move relatively slowly through an electrical circuit. In
a DC circuit, some electrons may make a complete path from (1) the point of origin
somewhere in the conductive material, (2) through the DC generator or battery, (3)
through all the electrical loads, and (4) back to the original point. However, in AC
circuits, where the electrical current changes direction rapidly, the “free” electrons jiggle
back and forth, roughly within the same area, and do not move completely through the
circuit.
Students should understand that the charges lie all along the electrical circuit and come
into useful motion when an electrical signal is applied to the circuit via a generator or
battery. The signal, which results from these potential energy sources, travels through the
entire circuit extremely rapidly, and in the case of AC circuits, travels around the circuit
in both directions several times each second.
More information about this misconception, as well as other common physical science
misconceptions held by students, are found at
http://phys.udallas.edu/C3P/Preconceptions.pdf
Performance Benchmark P.12.C.6
Students know electricity is transferred from generating sources for consumption and
practical uses. I/S
Sample Test Questions
1. What role do electrical generators or batteries have in circuits?
a. They are the source of electrons that moves through a circuit.
b. They are the source of potential energy in a circuit.
c. They control how hard it is for electricity to pass through a circuit.
d. They regulate the electrical current by storing then releasing charge.
2. What function does a circuit serve in your home?
a. It protects your home against lightning strikes.
b. It increases the voltage from the power lines outside your house.
c. It provides a complete path through which electrical energy can flow.
d. It provides a barrier against electromagnetic radiation from the outside.
3. What type of electrical current is mostly commonly used to transfer electrical
energy from commercial electrical generating stations to your home?
a. Intermittent current
b. Impulse current
c. Direct current
d. Alternating current
4. In an electrical circuit,
a. energy in the form of an electric signal moves very rapidly through the
circuit, but electrons move relatively slowly.
b. electrons move from the generator or battery through circuit at nearly the
speed of light.
c. electrons jump from atom to atom in the wire very rapidly to create a
voltage.
d. electrons are created in the generator or battery and converted to energy in
the electrical load.
5. In an alternating current (AC) circuit,
a. voltages can be easily increased and decreased within the circuit allowing
for lower energy losses during transmission.
b. voltages change direction several times a second to generate electrical
charge more rapidly.
c. rapid current changes regulate the ease at which electricity to passes
through the circuit.
d. current most often is high to prevent energy losses during transmission,
but can be low to decrease the voltage in the circuit.
6. An electrical load is generally used to
a. generate a voltage to create charges that will be used in the circuit.
b. control the direction of the electrical current within the circuit.
c. transform electrical energy in the circuit to a useful form.
d. provide an electromotive force that resists charge in the circuit.
7. The following figure is an example of what type of electrical circuit?
Figure 4. From http://iss.cet.edu/electricity/pages/a14.xml.
a.
b.
c.
d.
Indirect circuit
Series circuit
Alternating circuit
Parallel circuit
8. The following is an example of what type of electrical circuit?
Figure 5. From
http://iss.cet.edu/electricity/pages/a14.xml
a.
b.
c.
d.
Indirect circuit
Series circuit
Alternating circuit
Parallel circuit
Performance Benchmark P.12.C.6
Students know electricity is transferred from generating sources for consumption and
practical uses. I/S
Answers to Sample Test Questions
1.
2.
3.
4.
5.
6.
7.
8.
(b)
(c)
(d)
(a)
(a)
(c)
(d)
(b)
Performance Benchmark P.12.C.6
Students know electricity is transferred from generating sources for consumption and
practical uses. I/S
Intervention Strategies and Resources
The following list of intervention strategies and resources will facilitate student
understanding of this benchmark.
1. The OhmZone Online
The Leading English Education and Research Network (LEARN) has several interactive
lessons that are useful to classrooms with Internet access. The OhmZone is one such
interactive lesson where students can build virtual circuits. Once these circuits are built,
students can calculate the current and voltage in different parts of the circuit. The
OhmZone is particularly useful for classrooms where students have access to the Internet
and sufficient computers for students to work in groups, and also, for classrooms that lack
the lab resources and equipment to do electricity laboratories.
You can access the OhmZone at http://www.article19.com/shockwave/oz.htm
2. Energy Infobooks
The National Energy Education and Development (NEED) Project has developed Energy
Infobooks that provide background information for educators and students at all grade
levels. Specifically, they have developed a Secondary Energy Infobook that is
appropriate for high school students. Besides science concepts related to energy, the
books also have content connections to social studies, mathematics, and language arts.
The “electricity” chapter of the Secondary Infobook discusses generation and
transmission of electrical energy and can be found at
http://www.need.org/needpdf/infobook_activities/IntInfo/Elec1I.pdf
3. Power Transfer Activity
In this activity from the Physics Teacher Research Association (PTRA) Manual: "Role of
the Laboratory in Teaching Introductory Physics", students are asked to find the
condition for maximum power transfer from a power supply to a load resistor. By adding
an "internal resistor" to a power supply, it can be made a variable in a typical power
transfer laboratory activity. Students who have studied calculus can do a maximumminimum calculation to check the results of this activity, but the activity does not require
students to understand calculus to be successful.
To download the activity, go to
http://www.compadre.org/precollege/document/ServeFile.cfm?ID=3815&DocID=123.
Note: This activity is located on the comPADRE education Web site. You may have to
register on the site to get this activity; however, registration is free.
4. Electricity and Circuit Simulations
The University of Colorado has created many simulations for physical science and they
have a whole series devoted to electricity and circuits. These simulations require a
classroom with computers connected to the Internet. The best of these simulations allow
students to virtually construct various kinds of circuits and measure current and voltage at
various locations across the circuit. In addition, both DC and AC circuits may be
constructed. Another simulation allows students to conduct experiments with a virtual
electrical generator.
These simulations can be accessed at http://phet.colorado.edu/web-pages/simulationsbase.html