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Performance Benchmark P.12.C.6
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 Physical Science Matter Force and Motion Energy P.12.C.1 P.12.C.2 P.12.C.3 P.12.C.4 P.12.C.5 P.12.C.6 Content Areas Nature of Science (NOS) Life Science Earth Science Physical Science

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.

 (Click Image to Enlarge) 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).

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).

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.

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.

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.

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.

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.

http://www.ndt-ed.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.

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.

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.

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

Content 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.

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.

http://phys.udallas.edu/C3P/Preconceptions.pdf

Content Benchmark P.12.C.6

Students know electricity is transferred from generating sources for consumption and practical uses. I/S

Sample Test Questions

1st Item Specification: Describe various methods for generating electricity.

Depth of Knowledge Level 1

1. Which of the following correctly describes how water is used to generate electricity?
1. Moving water turns a turbine which turns a generator.
2. A turbine moves water which turns a generator.
3. A generator moves water to turn a turbine.
4. Still water enters a turbine which turns a generator.
1. Which of the following is NOT a method for generating electricity?
1. Creating a chemical reaction.
2. Wires moving relative to a magnetic field.
3. Sunlight on a photovoltaic cell.
4. Pumping water from a reservoir to a field.
1. Use the diagram of a waterfall to answer the question below. Where

Where would placing a generator produce the largest amount of energy?

1. Arrow 1
2. Arrow 2
3. Arrow 3
4. Arrow 4

Depth of Knowledge Level 2

1. Which of the following describes how a dam creates electricity?
1. Water is heated up and creates steam to turn the generator to create electricity.
2. Gravitational potential energy transfers to kinetic energy which turns a generator.
3. Electrons are removed from the water as it passes and creates electricity.
4. Water flows over wires in the dam which creates current from the water.
1. How does natural gas create electricity?
1. Chemical reactions within the gas transfers directly into electricity.
2. Electrons from the bonds within the gas are transferred through wires
which creates electricity.
3. Chemical reactions within the gas transfers into thermal energy which
creates steam to turn the turbine.
4. Electrons are stored in batteries transfers directly into the electricity.

2nd Item Specification: Identify the processes by which various forms of energy
(e.g., chemical, mechanical, and electromagnetic) are converted to electricity.

Depth of Knowledge Level 1

1. An electrical load is generally used to
1. Generate a voltage to create charges that will be used in the circuit.
2. Transform electrical energy in the circuit to a useful form.
3. Control the direction of the electrical current within the circuit.
4. Provide an electromotive force that resists charge in the circuit.
1. What role do electrical generators or batteries have in circuits?
1. They are the source of potential energy in a circuit.
2. They are the source of electrons that moves through the circuit.
3. They control how hard it is for the electricity to pass through a circuit.
4. They regulate the electrical current by storing then releasing charge.

Depth of Knowledge Level 2

1. Which of the following BEST describes how chemical energy is converted into electrical energy?
1. Coal is burned which creates heat which is then used to ionize electrons.
2. Coal is crushed and then directly used to create electrical current.
3. Coal is burned which creates steam which then causes a turbine
generator to turn.
4. Coal is melted and then is used to turn a turbine used for electricity.
1. Which of the following BEST explains how electromagnetic energy is converted to electrical energy?
1. Photons from the Sun hit the photovoltaic cell and create voltage via the
photoelectric effect.
2. Electromagnetic waves heat up the photovoltaic cell which frees electrons
to move through the wires.
3. Electromagnetic waves from the Sun create a chemical reaction within the
photovoltaic cell and then used to create steam.
4. Vibrations from the electromagnetic waves cause electricity to flow
through the wires as alternating current.

3rd Item Specification: Use a diagram to trace the transfer of electricity from generating sources to end uses by consumers.

Depth of Knowledge Level 1

1. What function does a circuit serve in your home?
1. It protects your home against lightning strikes.
2. It increases the voltage from the power lines outside your house.
3. It provides a complete path through which electrical energy can flow.
4. It provides a barrier against electromagnetic radiation from the outside.
1. What type of electrical current is most commonly used to transfer electrical energy from commercial electrical generating stations to your home?
1. Intermittent current
2. Impulse current
3. Direct current
4. Alternating current

Depth of Knowledge Level 2

1. In an electrical circuit,
1. energy in the form of an electric signal moves very rapidly through the circuit, but electrons move relatively slowly.
2. electrons move from the generator or battery through circuit at nearly the speed of light.
3. electrons jump from atom to atom in the wire very rapidly to create a voltage.
4. electrons are created in the generator or battery and converted to energy in the electrical load.
1. In an alternating current (AC) circuit,
1. voltages can be easily increased and decreased within the circuit allowing for lower energy losses during transmission.
2. voltages change direction several times a second to generate electrical charge more rapidly.
3. rapid current changes regulate the ease at which electricity passes through the circuit.
4. current most often is high to prevent energy losses during transmission, but can be low to decrease the voltage in the circuit.

Content Benchmark P.12.C.6

Students know electricity is transferred from generating sources for consumption and practical uses. I/S

1. A, DOK Level 1
2. D, DOK Level 1
3. D, DOK Level 1
4. B, DOK Level 2
5. C, DOK Level 2
6. B, DOK Level 1
7. A, DOK Level 1
8. C, DOK Level 2
9. A, DOK Level 2
10. C, DOK Level 1
11. D, DOK Level 1
12. A, DOK Level 2
13. A, DOK Level 2

Content 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 is a list of intervention strategies and resources that 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 maximum-minimum calculation to check the results of this activity, but the activity does not require students to understand calculus to be successful.