Content Benchmark P.12.B.2
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Students know laws of motion can be used to determine the effects of forces on the motion of objects. E/S

Electromagnetic force is one of the four fundamental forces of the universe. It is a force that involves the interactions between electrically charged particles that occur due to their charge and for the emission and absorption of photons. An electromagnetic force generates an electromagnetic field, which exerts on electrically charged particles. Electricity and magnetism are two aspects of a single electromagnetic force. On the macroscopic scale, both electric and magnetic forces behave differently, even though they are identical at the subatomic scale, where moving charges create both electrical and magnetic fields.

Scottish physicist James Clerk Maxwell was able to deduce that electricity and magnetism are mutual manifestations of the same force involving the exchange of photons. By means of his mathematical equations, he was able to integrate light and wave phenomena into electromagnetism; illustrating how electric and magnetic fields travel together through space as waves of electromagnetism with changing fields reciprocally sustaining one another.

To learn more about James Clerk Maxwell, go to http://www.clerkmaxwellfoundation.org/

Excluding gravity, electromagnetic forces are responsible for nearly all the phenomena encountered in daily life. It is a force that acts on electrically charged particles, such as protons and electrons. Electrically charged particles are influenced by and create electromagnetic fields. Consequently, electric and magnetic forces may be acknowledged in regions called electric and magnetic fields. The interaction between a moving charge and the electromagnetic field is the primary source of the electromagnetic force. Thus electricity and magnetism are ultimately inextricably linked. However, in many cases, one aspect may dominate, and the separation is meaningful.

To learn more about the physics of electromagnetic forces and fields, go to http://www.chemistrydaily.com/chemistry/Electromagnetism

Electric Force
The heart of the electric force lies with charge, which like mass, is an intrinsic property of matter. However, unlike mass, there are two kinds of charges, commonly referred to as positive and negative. In the 1900s, Ernest Rutherford and Niels Bohr proposed a simple model of the atom illustrating that ordinary matter is made up of atoms, which have positively charged nuclei and negatively charged electrons surrounding them.

An electron has a fundamental negative charge and a proton has a fundamental positive charge. The unit of electric charge is the Coulomb, which is 6.24 x 1018 natural units of electric charge (i.e., 6.24 x 10¹⁸ times greater than the charge on an electron or proton). Therefore, charges on an electron are negative and very small (-1.6 x 10^-19 Coulombs) and charges on a proton are positive and very small (+1.6 x 10^-19 Coulombs). A positive charge can join with a negative charge and result in a net charge of zero. Most importantly, charge is always conserved in a system. In other words, charge cannot be created or destroyed, and the net charge in an isolated system will not change.

Figure 1. Charge Interactions (from http://www.glenbrook.k12.il.us/gbssci/phys/Class/estatics/u8l1c.html)

The ancient Greeks discovered that by rubbing amber together, it attracted small, light objects. Greek philosopher, Thales of Miletus, believed that amber had a soul as well as another Greek philosopher three centuries later, Theoprastus. Though little progress in the study of electricity occurred within the 2,000 year period after Theoprastus; however, an English physician, William Gillbert published in which declared many other substances other than amber could be charged by rubbing as well. He coined these substances with a Latin name electrica, which is derived from the Greek word elektron, which means “amber”. In 1646, English writer and physician Sir Thomas Browne, first used the word electricity. A common day example of electric charge being transferred between two objects would be by rubbing them together plastic and fur. This would result in electrons from the fur being rubbed off onto the plastic and leaving the fur positively charged, meanwhile the plastic negatively charged.

Figure 2. Electric Charge (from http://www.physics.sjsu.edu/becker/physics51/elec_charge.htm)

The fundamental rule at the base of all electrical phenomena is that “like charges repel and unlike charges attract.”

Figure 3. Charge Interactions (from http://www.glenbrook.k12.il.us/gbssci/phys/Class/estatics/u8l1c.html)

The law that describes how strongly charges push and pull each other is called Coulomb’s Law. The equation consists of two charges Q₁ and Q₂ separated by a distance r with the magnitude of the force proportional to the charges, and, as with gravitation, inversely proportional to the square of the distance between them. Between any two charged particles, electric force is infinitely greater than the gravitational force. Most observable forces such as those exerted by a coiled spring or friction may be directed to electric forces acting between atoms and molecules. The electric force, in particular, is responsible for most of the physical and chemical properties of atoms and molecules.

Figure 4. Illustration of Coulomb’s Law (from http://www.sciencemadesimple.com/static.html)

To learn more about electric forces, go to
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elefor.html

Electric Field
Fields of electric forces are a common way to depict the effects that charges have on one another. Instead of looking at the force between two charges, we look at how a charge creates a force "field" in the empty space around it. For example, an electric field will surround an isolated positive change and a negative charge placed anywhere in this force field is attracted toward the positive charge. Similarly, a positive charge placed in identical location will be repelled. Furthermore, the motion of an individual charge may be affected by its interaction with the electric field and, for a moving charge, the magnetic field. Hence, a moving electric charge will produce a magnetic field and a charge moving in a magnetic field will experience an electric force.

Figure 5. Electric Field: Positive and Negative Fields (from http://www2.glenbrook.k12.il.us/gbssci/phys/Class/estatics/u8l4c.html)

The strength of an electric field E at any point is defined as the electric force F exerted per unit positive electric charge q at that point, or E = F/q. An electric field collectively has direction and magnitude and can be characterized by lines of forces, or field lines, that start on positive charges and expire on negative charges. The electric field is stronger where the field lines are close together than where they are farther apart. The value of the electric field has dimensions of force per unit charge and is measured in units of Newton’s per Coulomb (N/C).

Figure 6. Electric Field: Field lines near equal but opposite charges (from http://www.iop.org/Our_Activities/Schools_and_ Colleges/Teaching_Resources/Teaching%20Advanced%20Physics/Fields/ Electrical%20Fields/page_4802.html)

To learn more about electric fields, go to
http://www.colorado.edu/physics/2000/waves_particles/wavpart3.html

Magnetism and Electromagnetism
Magnetism is another aspect of an electromagnetic force. Recall that an electric field acting on a charge occurs from the presence of other charges and from a varying magnetic field. Reversely, the magnetic field acting on a moving charge arises from the motion of other charges and from an alternating electric field. Though they may be interrelated, they behave quite differently.

Magnetic Force
A magnetic force is an attraction or repulsion that occurs between electrically charged particles that are in motion. Whilst only electric forces exist among stationary electric charges, both electric and magnetic forces reside among moving electric charges. The magnetic force between two moving charges is the force exerted on one charge by a magnetic field created by the other. This force is zero if the second charge is traveling in the direction of the magnetic field due to the first and is greatest if it travels at right angles to the magnetic field. Magnetic force is responsible for the action of electric motors and the attraction between magnets and iron.

Figure 7. Magnetic Force: force between small permanent bar magnets (from http://www.swe.org/iac/images/NewMagnet.jpg)

Figure 8. Magnetic force acting on a charged particle that is moving perpendicular to a magnetic field. (from http://www.windows. ucar.edu/physical_science/magnetism/images/ force_charge_vel_mag_field_vectors.jpg)

The magnetic field is the resultant of moving electrically charged particles or intrinsic within magnetic objects such as a magnet. In a magnet, the atomic structure is such that the magnetic fields around individual atoms (due to moving electrons) are aligned together to create an overall additive effect. Because of this additive effect, a magnet is an object that demonstrates a strong magnetic field and will attract materials like iron. Magnets are dipoles, having two poles called the north seeking pole (N) and south seeking pole (S). Two magnets will be attracted by their opposite poles, and each will repel the like pole of the other magnet. The north and south magnetic poles of a magnetic object are related to the Earth's north and south magnetic poles. The magnetic flux is defined as moving from North to South. Magnetism has countless uses in modern life such as: a can opener, a navigational compass, refrigerator magnets, motors, computer diskettes, speakers, VCR/VHS tape, refrigerator, clothes dryer, etc.

Figure 9. Magnetic Flux (from http://www-spof.gsfc.nasa.gov/Education/wmfield.html)

To learn more about magnetism, go to
http://www-spof.gsfc.nasa.gov/Education/Imagnet.html

Magnetic Field
Magnetic forces can be detected in regions called magnetic fields. A changing electric field may produce a magnetic field and vice versa, independent of exterior change. A magnetic field is part of an electromagnetic field that exerts a force on a moving charge. A magnetic field is a region around a magnet, moving charge such as an electric current or by a changing electric field. The effects of such forces are unmistakable in the deflection of an electron beam in a cathode-ray tube and the motor force on a current-carrying conductor. In addition, magnetic fields such as that of Earth can cause magnetic compass needles and other permanent magnets to line up in the direction of the field.

Figure 10. Magnetic field or lines of flux of a moving charged particle. (from http://www.school-for-champions.com/science/magnetism.htm)

Electromagnetic Waves
The basis of electromagnetism lies with Maxwell’s equations, stating that “an electric field is created when a magnetic field changes,” “a magnetic field is created when an electric field changes,” and “the direction of the created magnetic field is perpendicular to the changing electric field.” Anytime an electron is accelerated, an electric field is created, thus beginning the process of creating sustained electromagnetic fields which propagate energy even in the vacuum of deep space. For convenience, we call these electromagnetic waves or simply light. Visible light represents only a small part of the electromagnetic spectrum, but is most common to use because we observe visible light with our eyes. Other portions of electromagnetic spectrum include radio waves, microwaves, infrared radiation, ultraviolet radiation, X-rays, and gamma rays.

To learn more about electromagnetic waves, go to http://www.colorado.edu/physics/2000/waves_particles/index.html

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Performance Benchmark P.12.B.2

Students know magnetic forces and electric forces can be thought of as different aspects of electromagnetic force. I/S

Common misconceptions associated with this benchmark:

1. Students incorrectly assume that neutral objects have no charge.

Matter is commonly referred to only having the passing relation to electrical effects. Yet, the nature of matter itself also encompasses physical substances such as molecules, atoms, and positive and negative electric charges. Since matter contains electric charges being a foremost component of all atoms, it is electrical. Neutral objects have a net charge of zero, a net charge being the sum of an equal exchange of positive protons and negative electrons. All neutral objects are an exchange of cancelled electric charges; equally between positive and negative charges, yielding material substance of neutral atoms. The electrical interaction is a force which can be analyzed using a free-body diagram, utilizing Newton’s Laws of Motion, and Coulomb’s Law. In essence, physical objects or material substances are the charge, a charge not necessarily holding new protons and neutrons in the material, but two neutral objects gaining a positive or negative charge, thus changing from a no net charge to some net charge.

To learn more about how neutral objects having a charge, go to http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/estatics/u8l1b.html

2. Students incorrectly think that the magnetic pole of the earth in the northern hemisphere is a north pole, and the pole in the southern hemisphere is a south pole.

Diagrams found in many textbooks illustrate a bar magnet extending beneath the earth’s surface. These diagrams are depicting earth’s magnetic field lines to be radiating from spots on the earth’s surface. Actually, the earth’s magnetic poles radiate deep within the earth, down inside the core. The earth’s magnetic field does not originate from a giant bar magnet nor do any magnetic fields surface near the earth’s North Pole and South Pole. The Geomagnetic “poles” on the earth’s surface are not places where the field is stronger but points on the landscape where the field lines are vertical. Instead students should be shown magnetic field lines radiating from the poles inside the earth’s core and field lines around the northern and southern area of the earth’s surface vertical and parallel, not radial or at specific points on the earth’s surface. The magnetic and geographic north pole of the earth is not located in the same place; opposite poles attract.

To learn more about Earth’s magnetic poles, go to http://www.phy6.org/earthmag/demagint.htm

3. Students incorrectly think that for an object to become positively charged it only gains protons.

All objects are made of atoms consisting of protons, electrons, and neutrons. Each atom contains positive charges in the center which are surrounded by negative charges. Most often the numbers of two charges in the atom is equivalent and therefore balance each other out. An object with identical numbers of positive and negative charges is said to be neutral. We have learned that protons carry positive charges whilst electrons carry negative charges. It is possible for electrons to transfer from one material to another when placed in contact with each other and separated. If an object receives extra electrons, it will become negatively charged. As a result, the object losing the electrons will become positively charged.

To learn more about charged objects and the imbalance of protons and electrons, go to
http://www.glenbrook.k12.il.us/gbssci/Phys/Class/estatics/u8l1b.html

4. Students incorrectly think that the magnetic and geographic north pole of the earth is located at the same place.

The magnetic and geographic north pole of the earth is not located in the same place. Opposite poles attract. For example, hold two bar magnets near each other, the “N” pole of one magnet is attracted by the “S” pole of another. If the bar magnet is balanced by a thread, however, then the “N” pole of that magnet will point toward the Earth’s north. So why does this occur? Physicists classify “N” magnetic poles as being north-pointing ends of magnets and compasses which is a characteristic of Maxwell’s equation. For example, wind an electromagnetic coil, and observe which end points towards the Earth’s North Pole. The end that points to the earth’s North Pole is the “N” pole of the electromagnet. Hence, the magnetic pole located inside the northern hemisphere of the Earth is actually a south-type magnetic pole. In other words, the Earth’s northern magnetic pole is the “S” pole. This is necessary, otherwise it would not attract the “N” pole of a compass.

To learn more about how to identify the True (Geographic) North, go to
http://www.windows.ucar.edu/tour/link=/spaceweather/location_mag_poles.html

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Performance Benchmark P.12.B.2

Students know magnetic forces and electric forces can be thought of as different aspects of electromagnetic force. I/S

Sample Test Questions

1. Which of the diagrams below best represents the charge distribution on a metal sphere when a positively-charged plastic tube is placed nearby?

2. What does the movement of an electric charge produce?
a. A magnetic field
b. An electron or proton
c. An electric field
d. Movement of a magnetic charge

3. Electrical forces _____________.
a. have no affect on objects.
b. can cause objects to only attract each other.
c. can cause objects to attract or repel each other.
d. can cause objects to only repel each other.

4. A physics student observes two balloons suspended from the ceiling upon entering the classroom. He notices that instead of hanging straight down vertically, the balloons appear to be repelling each other. He conclusively says …

a. one balloon is charge positively and the other negatively.
b. both balloons have a negative charge.
c. both balloons are charged with the same type of charge.
d. both balloons have a positive charge.

5. Two balloons are charged as shown below. Balloon X will _______ balloon Y.

a. attract b. repel c. not affect d. first attract then repel

6. Balloons X, Y and Z are suspended from strings as shown below. Negatively charged balloon X attracts balloon Y and Balloon Y attracts balloon Z. Balloon Z _____.

a. May be positively-charged
b. May be negatively-charged
c. Must be positively-charged
d. Must be neutral

7. When magnets are broken into small bits,
a. the bits themselves can become small magnets.
b. the bits themselves can become larger magnets.
c. the bits themselves will no longer have a magnetic pole.
d. the bits themselves will lose their magnetic field.

8. Where is the magnetic north pole of this magnet?

a. Top
b. Bottom
c. Left
d. Right

Students know magnetic forces and electric forces can be thought of as different aspects of electromagnetic force. I/S

Answers to Sample Test Questions
1. (d)
2. (a)
3. (c)
4. (c)
5. (b)
6. (b)
7. (a)
8. (c)

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Performance Benchmark P.12.B.2

Students know magnetic forces and electric forces can be thought of as different aspects of electromagnetic force. 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. Earth’s North Magnetic Pole Interactive
This is an interactive program online illustrating how a compass is used to find north. It as well demonstrates how a compass may allow for false readings because the North Magnetic Pole is not exactly located in the same place as the Geographic North Pole. The students will need to drag the compass around the map in order to see how it identifies the Geographic North. If unable to view this site’s animation, students may need to download the latest Flash player.

To access this site, go to
http://www.windows.ucar.edu/tour/link=/physical_science/
magnetism/north_mag_pole_interactive.html

2. Electric Force Field
This is an applet illustrating the concepts of electric force fields and lines of force. Students will click on “test” electrons using the mouse to observe which direction the field points and how strong it is. The line will point in the direction that the electron will move as its length of the line will denote the strength of the force at its location. They will be able drag the mouse to place the electrons down. The lines in the patterns created by the students are known as “lines of forces.” The force field lines coming out of the positive charge entering the negative charge are both connected by field lines.

To access this site, go to Electric Force Field applet
http://www.colorado.edu/physics/2000/waves_particles/wavpart3.html

Another interactive site that allows the students to manipulate the atoms by adding test charges and observe how the forces change is found at http://www.colorado.edu/physics/2000/applets/nforcefield.html

3. Magnetic Interactions with Moving Charge
This is a site which provides students the opportunity to choose an active graphic which will illustrate and explain the following:

• Positive charge moving through magnetic field –
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/forchg.html#c1

• Positive charge moving through a stationary wire in a magnetic field. –
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/forwir.html#c1

• Wire moved through magnetic field by external force –
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/genwir.html#c1

To access the site, go to
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magint.html#c1

4. Magnetism/Electromagnetism (Quia.com) – Flashcards
Students will be able to practice vocabulary terms for magnetism/electromagnetism.
You can view this flashcards at http://www.quia.com/jfc/313985.html
For more Quia activities:

• Concentration – students will uncover matching pairs of cards
http://www.quia.com/cc/313985.html

• Matching – students will find the matching squares
http://www.quia.com/mc/313985.html

 

5. Electrostatics PhysicsQuest
This site developed by Dolores Gende for physics online investigations is the ultimate educational resource, which provides students with numerous physicsquests (web quests) for studying high school physics. Specifically, the following links deal with Electricity and Magnetism:

• Electrostatics- students will investigate various applications of electrostatics.
http://physicsquest.homestead.com/quest13.html

• Electricity – students will take a look at the basic elements of circuits and how they function and the hazards of electricity and the various factors affecting them.
http://physicsquest.homestead.com/quest14.html

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