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
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%20Adva
nced%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/mag
netism/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
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
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?
a.
c.
b.
d.
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.
b.
c.
d.
one balloon is charge positively and the other negatively.
both balloons have a negative charge.
both balloons are charged with the same type of charge.
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.
b.
c.
d.
May be positively-charged
May be negatively-charged
Must be positively-charged
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.
b.
c.
d.
Top
Bottom
Left
Right
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
Answers to Sample Test Questions
1.
2.
3.
4.
5.
6.
7.
8.
(d)
(a)
(c)
(c)
(b)
(b)
(a)
(c)
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_inter
active.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