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Transcript
```Lesson Plan 1
The Four Fundamental Forces
Author: Brittany Krutty
Date lesson will be taught: Friday, October 21st, 2011, and Monday, October 24th, 2011
Knowledge Package:
Lesson Source: http://www.learner.org/resources/series42.html;
http://science.howstuffworks.com/environmental/earth/geophysics/question2322.htm;
http://csep10.phys.utk.edu/astr162/lect/cosmology/forces.html;
http://library.thinkquest.org/27930/forces.htm; http://science.howstuffworks.com/magnet.htm;
http://science.howstuffworks.com/magnet4.htm;
http://aether.lbl.gov/elements/stellar/strong/strong.html; http://www.physorg.com/news/2011-01weak-nuclear.html
Concepts: The four fundamental forces of nature are the gravitational force, the electromagnetic
force, the strong nuclear force, and the weak nuclear force. All other forces can be attributed to
these four fundamental forces, or interactions. The gravitational force is responsible for the
attraction between any two objects with mass. The electromagnetic force acts between
electrically charged particles. The strong force binds quarks together and binds protons and
neutrons together into nuclei. The weak force plays a role in the radioactive disintegration of
certain nuclei and also in particle decay.
Objectives:
Students will be able to:
∙ Describe the four fundamental forces.
∙ Identify which fundamental forces are responsible for a given common phenomenon.
∙ Compare and contrast the fundamental forces
Kansas Science and/or Mathematics Standards, Benchmarks, and Indicators:
12.1.1.1. Actively engages in asking and evaluating research questions.
12.2.3.1. There are four fundamental forces in nature: strong nuclear force, weak nuclear force,
electromagnetic force, and gravitational force.
12.4.3.1. Understands gravitational attraction of objects in the solar system keeps solar system
objects in orbit
12.7.1.3. Recognizes the universality of basic science concepts and the influence of personal and
cultural beliefs that embed science in society.
12.7.2.1. Understands scientific knowledge describes and explains the physical world in terms of
matter, energy, and forces. Scientific knowledge is provisional and is subject to change as new
evidence becomes available.
Materials list, advance preparation, and handouts:
For each group of 3 students (there will be 9 groups):
1 whiteboard
1 whiteboard marker
1. 1st day: The teacher’s name and particleadventure.org will already be written on the
board, along with the words FUNDAMENTAL and FORCE. Students will be sitting with
their groups. Have pre-tests printed and upside-down on students’ desks.
2. 2nd day: The video will already be loaded and the projector will be set up. Have post-tests
printed and ready to hand out.
Accommodations:
All students should be able to participate unless there is a particular physical disability.
Any student unable to physically complete any portion of the lesson may require additional
assistance from a Para-professional, a classroom assistant, or a student assistant. ELL students
and students who are hard-of-hearing will benefit from the “big ideas” presented in this lesson
and the visuals drawn on the board or presented in the video. The teacher should ask appropriate
questions for the stage of language acquisition and use nonverbal signals to emphasize the
meaning of instructions and subject matter. The teacher should write down key terms and
concepts on the board and perhaps even provide additional written materials on the fundamental
forces to students who are hard-of-hearing.
Safety:
Be sure that students are using the materials appropriately. Do not allow students to
continue the experiment if they are mistreating the materials. Ensure that students feel the
classroom is a safe environment in which to ask questions, make mistakes, and learn.
Five-E Plan – Day 1
Teacher Does
Engage:
Probing Questions Student responses
Critical questions that will
connect to prior knowledge
Expected Student
and create a need to know.
Responses/Misconceptions
Learning Experience(s)
Time: ___15___minutes
write on the board. The
rest of the class will tell
1. What is a force?
1. A push or pull. The
effect on a particle due to
the presence of other
fundamental and force.
This strategy is used by the
teacher to gauge prior
understanding. If no one
questions, treat the
questions as “Think-PairShare” questions, where
particles. A thing that is
passed between two objects.
2. Do all forces have to be
contact forces?
2. No.
3. What are some forces
3. Gravity, normal force,
friction, torque, magnetic
force, electricity,
centrifugal/centripetal force,
etc.
4. Changes an object’s
motion, deforms an object
in some way, etc.
4. What does a force do?
5. What does fundamental
mean?
6. A force is fundamental if
it can’t be attributed to any
other force. All “forces” in
our world can be attributed
to the fundamental forces.
6. What makes a force
fundamental?
to their seats. Discuss the
difference between a force
and an interaction, and
write the four fundamental
forces on the board.
5. Central, important, not
more basic.
1. A lot of times physicists
use the words force and
interaction interchangeably.
What does interaction
mean?
1. When two things act
upon each other or affect
each other.
2. What do you think
interaction means in
physics?
2. The effect on a particle
due to the presence of
another particle. A thing
that is passed between two
objects.
Give students the pre-test
(at the end of this
document).
Teacher Decision Point
Based on class participation and the answers to the
Assessment:
questions, do the students understand what fundamental
force means? If so, then move on to the Explore section.
If not, then revisit this concept.
Teacher Does
Probing Questions
Student responses
Explore:
Learning Experience(s)
Time: __10____minutes
Have five of the groups
write everything they know
whiteboards, and the other
four groups write
electromagnetism on their
whiteboards. Use questions
to lead them in the right
direction. Have the groups
put the whiteboards up on
the whiteboard marker rails
around the classroom when
they are done.
Critical questions that will
guide students to a common
set of experiences.
Expected Student
Responses/Misconceptions
1. What does the force do?
1. Gravity: attracts objects,
makes stuff fall.
Electromagnetism: attracts
or repels charged objects.
2. How strong is the force?
2. Gravity: VERY strong,
Electromagnetism: Stronger
when you are closer to the
3. What are some examples
of this force in action?
3. Gravity: object falling,
planetary orbits, etc.
Electromagnetism:
lightning, magnets, electric
circuits, etc.
5. Is it weaker or stronger
than other forces you have
5. Gravity: weaker.
Electromagnetism: stronger.
6. What types of objects
experience this force?
6. Gravity: all objects that
have mass.
Electromagnetism: magnets,
protons and electrons, etc.
Teacher Decision Point
When the students are done writing on the boards, move on
Assessment:
to the Explain section.
Teacher Does
Explain:
Learning Experience(s)
Probing Questions
Student responses
Critical questions that will
help students clarify their
understanding and
introduce information
Expected Student
Responses/Misconceptions
related to the lesson
concepts/skills.
Time: ___20___minutes
Discuss and explain gravity
and electromagnetism,
starting with the groups’
whiteboards in front of the
class. Draw a chart on the
board (pictured below) to
help organize the four
forces.
Gravity: Students should
understand that gravity is
the weakest interaction, and
every object in the universe
exerts a gravitational force
on everything else. It acts
on all particles having
mass, and the gravitational
force is equal between the
two masses. It has an
infinite range, and is always
attractive. It affects
everything from the
structure of galaxies to
planetary orbits to everyday
objects on Earth.
Electromagnetism: Students
should understand that the
electromagnetic force acts
between electrically
charged particles. The
electrostatic force acts on
charged particles at rest,
and the electric and
magnetic forces act on
charged particles moving
relative to each other.
Electromagnetism also has
infinite range. The residual
electromagnetic force
(draw a picture on the
board) explains why I don’t
fall through the floor and
even explains friction. The
electromagnetic force
causes oppositely charged
objects to attract and likecharged objects to repel
each other.
Electromagnetism and
gravitation are both
described by similar
equations (the inverse
square law).
Discuss and explain the
strong nuclear force and
weak nuclear force. Use the
chart on the board (pictured
below) to help organize the
four forces.
Strong nuclear force:
Students should understand
that this force holds quarks
together through gluons,
and that the residual strong
force holds the nucleus
together. It has a very short
range.
Weak nuclear force:
Students should understand
that it is responsible for
some nuclear phenomena
like beta decay. It can
change one type of quark or
other fundamental particle
into another. It is a
fundamental force, even
though it does not have to
do with things pushing or
pulling each other. Above
certain energies, the
electromagnetic force and
the weak force merge into
the electroweak force.
Force/Interaction:
Current theory:
Relative strength:
Strong
Quantum
Chromodynamics
1038
Electromagnetic
Quantum Electrodynamics
Weak
Electroweak
Gravitation
General Relativity
1036
1025
1
Long-distance
behavior:
Range (meters):
10-15
1/r2
infinity
10-18
1/r2
infinity
Teacher Decision Point
Based on class participation and the answers to the
Assessment:
questions, do the students understand the difference
between the fundamental forces? If so, then move on to
the Extend/Elaborate section. If not, then revisit this
concept.
Teacher Does
Extend / Elaborate:
Learning Experience(s)
Time: ___5___minutes
all the fundamental forces.
Compare and contrast the
fundamental forces, and
discuss unification of the
forces if time permits.
Probing Questions
Student responses
Critical questions that will
help students extend or
apply their newly acquired
concepts/skills in new
situations.
Expected Student
Responses/Misconceptions
1. Why doesn’t the
electromagnetic force
dominate gravity on the
scale of our solar system,
since it’s so much stronger
and has the same range?
1. There is no huge
concentration of positive
and negative charge, since
they “cancel each other
out.” However, there are
huge concentrations of
mass.
2. Why doesn’t the weak
nuclear force or strong
nuclear force dominate
gravity on the scale of our
solar system?
3. What force holds you and
me together? What force
holds all matter together?
4. Scientists have concluded
that the moon is held in its
gravity and not by the
electrical force. Why would
they think that?
5. Many physicists strive to
unite the four fundamental
2. They do not have as large
a range as gravity.
3. Electromagnetic force
(technically, the residual
electromagnetic force).
4. The earth and the moon
are essentially neutral and,
therefore, have no electrical
force between them.
5. Other forces have been
united in the past:
“chemical” force and
electric force (battery),
forces of nature. What
evidence would cause them
to feel this may be possible?
6. How are the
electromagnetic and
gravitational forces similar?
How are they different?
When is each most
dominant?
electricity and magnetism,
electromagnetism and weak,
etc. At high energies, the
equations describing the
forces look similar. The
inverse square law applies
to gravity and
electromagnetism.
6. They both have infinite
square law, but gravity is
only an attractive force
while electromagnetism can
be attractive or repulsive.
The electromagnetic force is
dominant on small scales,
and the gravitational force is
dominant on large scales.
Have students write one
question they still have
forces. This is their “exit
slip.”
Teacher Decision Point
Based on class participation and the answers to the
Assessment:
questions, do the students understand why gravity
dominates on a large scale? If not, then revisit this concept.
Teacher Does
Evaluate:
Include formative and
summative evaluation
below.
Time: _______minutes
Probing Questions
Student responses
students to demonstrate
their understanding of the
concepts and process skills.
These questions must
directly relate to the
lesson’s performance
objectives.
Expected Student
Outcomes
Formative evaluation, day 1: Determine a reasonable point to move on after each concept based
on student responses and participation. During group work in the Explore section, walk around
Five-E Plan – Day 2
Teacher Does
Engage:
Probing Questions Student responses
Critical questions that will
connect to prior knowledge
Expected Student
and create a need to know.
Responses/Misconceptions
Learning Experience(s)
Time: ___2___minutes
Conduct a review of
gravity using Barbie and
Ken dolls (this should be
funny!).
1. Does Mr. Strawderman
have mass?
1. Yes.
2. Does Barbie have mass?
2. Yes.
3. Is Mr. Strawderman
attracted to Barbie?
3. Yes.
4. And is Barbie attracted to
Mr. Strawderman?
4. Yes.
5. So it’s a mutual
attraction! Who is attracted
to whom more?
6. Is Mr. Strawderman
attracted to other things,
too?
7. Uh-oh, Barbie’s going to
be jealous! Are they
attracted to him?
8. Show of hands: who is
attracted to Ken?
5. Neither. The force of
gravity is equal for both of
them.
6. Yes.
7. Yes.
8. (Everyone raises his/her
hand)
Teacher Decision Point
Based on class participation and the answers to the
Assessment:
questions, do the students understand that the gravitational
force is equal and attractive between two objects of
different mass? If so, then move on to the Explain section.
If not, then revisit this concept.
Teacher Does
Explore:
Learning Experience(s)
Time: __38____minutes
Watch the video found at
http://www.learner.org/reso
urces/series42.html,
questions and check for
understanding.
Probing Questions
Student responses
Critical questions that will
guide students to a common
set of experiences.
Expected Student
Responses/Misconceptions
1. The strong nuclear force
“binds protons and neutrons
into nuclei.” – How?
1. Quarks in the protons and
neutrons are attracted to
each other due to the
residual strong nuclear force
by exchanging gluons.
2. On the range of the
strong force: “12 zeros
express it numerically as a
fraction of a centimeter.”
The range of the strong
force is 10-15 m (about the
diameter of a medium-sized
nucleus). The weak force
“has effects that are by no
means weak” – then why is
it called the weak force?
2. Its range is 10-18 (about
1/1000 the diameter of a
proton), and its strength is
much less than the
electromagnetic force and
strong nuclear force.
3. Gravity: it “makes the
moon fall, … it is a
fundamental force of the
Universe.” – What do they
mean the moon “falls”?
3. It experiences the force of
gravity due to the earth’s
mass. Since the earth’s
mass is much larger than the
moon’s, we say the moon is
“falling.” When you jump,
you “fall” towards the earth
and the earth “falls” towards
you, but the earth is much
more massive.
4. “Gravity reaches out and
shakes hands with the entire
Universe.” – What do they
mean by this?
4. An object with mass is
attracted to everything else
in the Universe. (Some may
refer to the Barbie example
from the Engage section.)
5. On electricity: “It binds
together everything in the
5. The protons in one atom
are attracted to the electrons
Universe. Everything from
soup to nuts.” – How?
in surrounding atoms due to
the residual electromagnetic
force.
6. “When people shake
hands, why don’t they fry
each other like potatoes?” –
Why don’t we feel the
effects of electricity in our
bodies?
6. We have an about equal
amount of positive and
negative charges in our
bodies, so they are canceled
out and we are left with no
electromagnetic force
between us and other
people.
7. When the video is going
through the equations for
If the strength of both of
these forces has the same
relationship with distance
(1/r2), then why does
gravity dominate over large
distances?
7. There is no huge
concentration of positive
and negative charge, since
they “cancel each other
out.” However, there are
huge concentrations of
mass.
8. “Found in the debris of
nuclear collisions, these
fragments suggest a deep
underlying connection
between the electric force
and the weak nuclear force.”
– This is the basis for
electroweak theory. Some
textbooks even consider
there to be 3 fundamental
forces instead of 4. At very
short distances or very high
energies, the strengths of
the electromagnetic force
and the weak force are
comparable.
9. “Gravity – it’s the force
that makes some objects fall
and keeps others from
spinning off the face of the
earth.” – What does this
mean? Can you think of an
example of each?
9. Objects with mass, such
as balls, pencils, and people,
fall when dropped from any
height above the surface of
the earth. Very massive
objects, such as buildings,
are kept from spinning off
the face of the earth by
gravity. (Think of a mouse
trying to hang on to a
the force of gravity between
the objects is weak enough
to be ignored.)
10. “Gravity – it keeps the
moon orbiting the earth, and
the planets orbiting the
sun.” – How?
10. The moon experiences
the force of gravity due to
the earth’s mass, keeping it
in orbit. Planets experience
the force of gravity due to
the sun’s mass, keeping
them in orbit.
11. “When two objects
come into contact, their
positive and negative
charges interact, and their
surfaces are, at least
temporarily, bonded
together.” Can anyone
visualize this? Draw what
you think this looks like on
12. “Seeing the positive
charge just up ahead, the ion
feels attracted and
accelerates.” – What do they
mean when they say the ion
“sees” the positive charge?
13. “Carbon ions are
directed through an
evacuated beam pipe toward
the target, helium.” – What
do they mean by evacuated?
Why is this important?
12. They mean that the
electromagnetic force
between the ion and the
positive charge becomes
stronger and stronger as the
ion approaches the positive
charge.
13. Evacuated means the
beam pipe is a vacuum;
there are very few particles
in the beam pipe for the ions
to interact with.
14. “And we’ll get on with
those problems next time.”
– These topics lead to the
Standard Model of physics,
the Grand Unified Theory,
and the Theory of
Everything.
Teacher Decision Point
Based on class participation and the answers to the
Assessment:
questions, do the students understand the difference
between the fundamental forces? If so, then move on to
the Extend/Elaborate section. If not, then revisit this
concept.
Teacher Does
Explain:
Learning Experience(s)
Time: ___2___minutes
Explain the video,
elaborate, and review from
yesterday.
Probing Questions
Student responses
Critical questions that will
help students clarify their
understanding and
introduce information
related to the lesson
concepts/skills.
Expected Student
Responses/Misconceptions
1. Which fundamental force
is responsible for friction?
1. Electromagnetism
2. Nuclear bonding?
2. The strong force
3. Planetary orbits?
3. Gravity.
4. Which interactions act on
the protons in you? Why?
4. All of them because they
have mass, electric charge,
and they are on the scale of
the nucleus of an atom.
5. Which force is most
fundamental? Why?
5. The four fundamental
forces are all equally
fundamental because they
cannot be attributed to any
other forces.
Teacher Decision Point
Based on class participation and the answers to the
Assessment:
questions, do the students understand where different
forces apply? If so, then move on to the Extend/Elaborate
section. If not, then revisit this concept.
Teacher Does
Extend / Elaborate:
Probing Questions
Student responses
Critical questions that will
help students extend or
Expected Student
Responses/Misconceptions
Learning Experience(s)
Time: ___2___minutes
based on the video, that tie
concepts concerning the
fundamental forces.
apply their newly acquired
concepts/skills in new
situations.
1. Which forces dominate
over large distances? Small
distances?
1. In the nucleus of the
atom, the strong force
dominates. On the atomic
scale, electricity dominates.
On very large scales,
gravity dominates.
Go through the “exit slips”
from the day before,
even having the students
questions.
1. What questions do you
forces?
Teacher Decision Point
Based on class participation and the answers to the
Assessment:
questions, do the students understand the main concepts of
the lesson? If so, then move on to the Evaluate section. If
not, revisit the main points.
Teacher Does
Evaluate:
Include formative and
summative evaluation
below.
Time: __6____minutes
Probing Questions
Student responses
students to demonstrate
their understanding of the
concepts and process skills.
These questions must
directly relate to the
lesson’s performance
objectives.
Summative assessment (below)
Expected Student
Outcomes
Name ______________________
Fundamental Forces Pre-Test
1. What are the four fundamental forces of nature?
Gravitational, electromagnetic, strong nuclear, weak nuclear
2. Which force holds all matter together?
Electromagnetic
3. In some models of the atom, electrons orbit the nucleus in much the same way that
planets orbit the sun. Why can this analogy be made?
a. Both forces depend inversely on the square of the distance.
b. Both forces are proportional to the masses of the attracting bodies.
c. The electromagnetic force, like gravity, depends on mass.
d. The electromagnetic force is really gravity acting at small distances.
4. Which one of the following is not a consequence of the gravitational force?
a. The formation of galaxies.
b. The organization of the solar system.
c. A stone falling.
d. The structure of the nucleus.
5. Particle decays are a manifestation of which force?
Weak nuclear
6. Scientists have concluded that the moon is held in its orbit about the earth by gravity
and not by the electrical force. Which of the following statements gives the reason for
that conclusion?
a. Only gravity depends inversely on the square of the distance between two
bodies.
b. The electrical force is canceled out by the strong force between the earth and
the moon.
c. The earth and the moon are essentially neutral and, therefore, have no electrical
force between them.
d. The gravitational constant is larger than the electrical constant in the respective
force laws.
Name ______________________
Fundamental Forces Post-Test
7. What are the four fundamental forces of nature?
Gravitational, electromagnetic, strong nuclear, weak nuclear
8. Which force holds all matter together?
Electromagnetic
9. In some models of the atom, electrons orbit the nucleus in much the same way that
planets orbit the sun. Why can this analogy be made?
a. Both forces depend inversely on the square of the distance.
b. Both forces are proportional to the masses of the attracting bodies.
c. The electromagnetic force, like gravity, depends on mass.
d. The electromagnetic force is really gravity acting at small distances.
10. Which one of the following is not a consequence of the gravitational force?
a. The formation of galaxies.
b. The organization of the solar system.
c. A stone falling.
d. The structure of the nucleus.
11. Particle decays are a manifestation of which force?
Weak nuclear
12. Scientists have concluded that the moon is held in its orbit about the earth by gravity
and not by the electrical force. Which of the following statements gives the reason for
that conclusion?
a. Only gravity depends inversely on the square of the distance between two
bodies.
b. The electrical force is canceled out by the strong force between the earth and
the moon.
c. The earth and the moon are essentially neutral and, therefore, have no electrical
force between them.
d. The gravitational constant is larger than the electrical constant in the respective
force laws.
Backup plan (mainly in case video doesn’t work)/ potential lesson for 3rd day:
Assign a topic related to the fundamental forces to each group and hand each
person in a group an information sheet (below) corresponding to their
fundamental force.
Have students write the main points on their group’s whiteboard.
Let each group present their fundamental force in front of the class.
Review the topics that would have been covered in the video
Unification of the Forces
Properties of the Fundamental Forces
• The strong interaction is very strong, but very short-ranged. It acts only over ranges of order 10-13
centimeters and is responsible for holding the nuclei of atoms together. It is basically attractive,
but can be effectively repulsive in some circumstances.
• The electromagnetic force causes electric and magnetic effects such as the repulsion between like
electrical charges or the interaction of bar magnets. It is long-ranged, but much weaker than the
strong force. It can be attractive or repulsive, and acts only between pieces of matter carrying
electrical charge.
• The weak force is responsible for radioactive decay and neutrino interactions. It has a very short
range and, as its name indicates, it is very weak.
• The gravitational force is weak, but very long ranged. Furthermore, it is always attractive, and
acts between any two pieces of matter in the Universe since mass is its source.
Unification of the Forces of Nature
Although the above discussion indicates that the fundamental forces in our present Universe are distinct
and have very different characteristics, the current thinking in theoretical physics is that this was not
always so. There is a rather strong belief (although it is yet to be confirmed experimentally) that in the
very early Universe when temperatures were very high compared with today, the weak, electromagnetic,
and strong forces were unified into a single force. Only when the temperature dropped did these forces
separate from each other, with the strong force separating first and then at a still lower temperature the
electromagnetic and weak forces separating to leave us with the 4 distinct forces that we see in our
present Universe. The process of the forces separating from each other is called spontaneous symmetry
breaking.
There is further speculation, which is even less firm than that above, that at even higher temperatures (the
Planck Scale) all four forces were unified into a single force. Then, as the temperature dropped,
gravitation separated first and then the other 3 forces separated as described above. The time and
temperature scales for this proposed sequential loss of unification are illustrated in the following table.
Loss of Unity in the Forces of Nature
Characterization
All 4 forces unified
Forces Unified
Gravity, Strong,
Electromagnetic, Weak
Strong, Electromagnetic,
Weak
Time Since
Beginning
Temperature (GeV)*
~0
~infinite
Gravity separates (Planck
10-43 s
1019
Scale)
Strong force separates (GUTs
Electromagnetic, Weak
10-35 s
1014
Scale)
Split of weak and
None
10-11 s
100
electromagnetic forces
Present Universe
None
1010 y
10-12
*
13
Temperature Conversion: 1 GeV = 1.2 x 10 K
Theories that postulate the unification of the strong, weak, and electromagnetic forces are called Grand
Unified Theories (often known by the acronym GUTs). Theories that add gravity to the mix and try to
unify all four fundamental forces into a single force are called Superunified Theories. The theory that
describes the unified electromagnetic and weak interactions is called the Standard Electroweak Theory, or
sometimes just the Standard Model.
Grand Unified and Superunified Theories remain theoretical speculations that are as yet unproven, but
there is strong experimental evidence for the unification of the electromagnetic and weak interactions in
the Standard Electroweak Theory. Furthermore, although GUTs are not proven experimentally, there is
strong circumstantial evidence to suggest that a theory at least like a Grand Unified Theory is required to
make sense of the Universe.
Messenger Particles
There is a unique messenger particle associated with each of the four fundamental forces. These particles
can be considered the "smallest amount" of each force than can exist in nature. Experiments have
confirmed the existence of three of the four particles, but the graviton has yet to be discovered.
Calculations show that it should be massless. The weak gauge bosons come in two separate varieties with
different masses.
Force
Particle
Mass
Gravitational Force
Graviton
0
Electromagnetic Force
Photon
0
Weak Nuclear Force
Weak Gauge Bosons
86, 97
Strong Nuclear Force
Gluon
0
Gravity
The messenger particle of gravity is the graviton. It has not been experimentally verified, mainly because
it is extremely hard to find the smallest denomination of the weakest force. Recent calculations show that
it will likely be massless. Interestingly, all versions of modern string theory incorporate gravity (unlike
previous quantum theories) and not only allow but require a particle with the properties of the graviton.
Its discovery will likely represent a major victory for string theory, since previous quantum theories based
on the model of point particles give illogical, infinite answers when gravity is incorporated.
Electromagnetism
The electromagnetic force is actually second in effective strength only to the strong force, but it is listed
out of order here because it, like gravity, is more familiar to most people. Its strength is less than 1% of
that of the strong force, but it, like gravity, has infinite range. However, unlike gravity, the
electromagnetism has both attractive and repulsive properties that can combine or cancel each other out.
Whereas gravity is always attractive, electromagnetism comes in two charges: positive and negative. Two
positive or two negative things will repel each other, but one positive and one negative attract each other.
This can be neatly illustrated by magnets: two of the same "pole" will repel each other, but two opposite
poles attract each other.
This is the principle that keeps atoms together: the positively charged nucleus and the negatively charged
electrons attract each other. This is also the principle of atomic size: more electrons have greater repulsive
force, so atoms with more electrons are larger because of the electrons' mutual repulsion. Similarly, atoms
with larger nuclei and the same number of electrons are smaller overall because they exert a greater
attractive force on the electrons.
The messenger particle of electromagnetism is the photon, a massless particle that logically (since light is
a manifestation of electromagnetism) travels at the speed of light (299 792 458 m/s or 299 972 km/s).
The Weak Nuclear Force
The weak nuclear force is one of the less familiar fundamental forces. It operates only on the extremely
short distance scales found in an atomic nucleus. The weak force is responsible for radioactive decay. In
actuality, it is stronger than electromagnetism, but its messenger particles (W and Z bosons) are so
massive and sluggish that they do not faithfully transmit its intrinsic strength.
The Strong Nuclear Force
The strong nuclear force is the other unfamiliar fundamental force. Like the weak force, its range is
limited to subatomic distances. Its "duties" are keeping quarks together inside protons and neutrons, and
keeping protons and neutrons inside atomic nuclei. Its messenger particle is the massless gluon, so named
because it "glues" elementary particles together.
Einstein's Gravity
Albert Einstein, who won the Nobel Prize in Physics in 1921, contributed an alternate theory of
gravity in the early 1900s. It was part of his famous General Theory of Relativity, and it offered
a very different explanation from Newton's Law of Universal Gravitation. Einstein didn't believe
gravity was a force at all; he said it was a distortion in the shape of space-time, otherwise known
as "the fourth dimension" (see How Special Relativity Works to learn about space-time).
Basic physics states that if there are no external forces at work, an object will always travel in the
straightest possible line. Accordingly, without an external force, two objects travelling along
parallel paths will always remain parallel. They will never meet.
But the fact is, they do meet. Particles that start off on parallel paths sometimes end up colliding.
Newton's theory says this can occur because of gravity, a force attracting those objects to one
another or to a single, third object. Einstein also says this occurs due to gravity -- but in his
theory, gravity is not a force. It's a curve in space-time.
According to Einstein, those objects are still travelling along the straightest possible line, but due
to a distortion in space-time, the straightest possible line is now along a spherical path. So two
objects that were moving along a flat plane are now moving along a spherical plane. And two
straight paths along that sphere end in a single point.
Still more-recent theories of gravity express the phenomenon in terms of particles and waves.
One view states that particles called gravitons cause objects to be attracted to one another.
Gravitons have never actually been observed, though. And neither have gravitational waves,
sometimes called gravitational radiation, which supposedly are generated when an object is
accelerated by an external force [source: Scientific American].
Gravitons or no gravitons, we know that what goes up must come down. Perhaps someday, we'll
know exactly why. But until then, we can be satisfied just knowing that planet Earth won't go
hurdling into the sun anytime soon. Gravity is keeping it safely in orbit.
How Magnets Work
It all started when we went shopping for a magnet for a demonstration on liquid body armor. We
wanted to show that a magnetic field could cause certain liquids to behave as solids. Along with
the petri dishes and iron filings we needed, the Steve Spangler Science catalog had a neodymium
magnet it described as "super strong." We ordered our supplies, hoping that the magnet would be
powerful enough to create an effect we could capture on film.
The magnet didn't just transform our iron-and-oil fluid into a solid -- sometimes, its pull on the
fluid cracked the petri dish holding it. Once, the magnet unexpectedly flew out of a
videographer's hand and into a dish full of dry filings, which required considerable ingenuity to
remove. It also adhered itself so firmly to the underside of a metal table that we had to use a pair
of locking pliers to retrieve it. When we decided it would be safer to keep the magnet in a pocket
between takes, people wound up momentarily stuck to the table, a ladder and the studio door.
Around the office, the magnet became an object of curiosity and the subject of impromptu
experiments. Its uncanny strength and its tendency to suddenly and noisily jump from unwary
grips to the nearest metal surface got us thinking. We all knew the basics of magnets and
magnetism -- magnets attract specific metals, and they have north and south poles. Opposite
poles attract each other while like poles repel. Magnetic and electrical fields are related, and
magnetism, along with gravity and strong and weak atomic forces, is one of the four fundamental
forces in the universe.
But none of those facts led to an answer to our most basic question. What exactly makes a
magnet stick to certain metals? By extension, why don't they stick to other metals? Why do they
attract or repel each other, depending on their positioning? And what makes neodymium magnets
so much stronger than the ceramic magnets we played with as children?
Iron filings (right) align along the magnetic field lines of a cylindrical neodymium magnet.
To understand the answers to these questions, it helps to have a basic definition of a magnet.
Magnets are objects that produce magnetic fields and attract metals like iron, nickel and cobalt.
The magnetic field's lines of force exit the magnet from its north pole and enter its south pole.
Permanent or hard magnets create their own magnetic field all the time. Temporary or soft
magnets produce magnetic fields while in the presence of a magnetic field and for a short while
after exiting the field. Electromagnets produce magnetic fields only when electricity travels
through their wire coils.
Iron filings (right) align along the magnetic field lines of a cubical neodymium magnet.
Until recently, all magnets were made from metal elements or alloys. These materials produced
magnets of different strengths. For example:
• Ceramic magnets, like the ones used in refrigerator magnets and elementary-school
science experiments, contain iron oxide in a ceramic composite. Most ceramic magnets,
sometimes known as ferric magnets, aren't particularly strong.
• Alnico magnets are made from aluminum, nickel and cobalt. They're stronger than
ceramic magnets, but not as strong as the ones that incorporate a class of elements known
as rare-earth metals.
• Neodymium magnets contain iron, boron and the rare-earth element neodymium.
Samarium cobalt magnets combine cobalt with the rare-earth element samarium. In the past few
years, scientists have also discovered magnetic polymers, or plastic magnets. Some of these are
flexible and moldable. However, some work only at extremely low temperatures, and others pick
up only very lightweight materials, like iron filings.
Magnet Myths
Every time you use a computer, you're using magnets. A hard drive relies on magnets to store data, and
some monitors use magnets to create images on the screen. If your home has a doorbell, it probably uses
an electromagnet to drive a noisemaker. Magnets are also vital components in CRT televisions, speakers,
microphones, generators, transformers, electric motors, burglar alarms, cassette tapes, compasses and car
speedometers.
In addition to their practical uses, magnets have numerous amazing properties. They can induce current in
wire and supply torque for electric motors. A strong enough magnetic field can levitate small objects or
even small animals. Maglev trains use magnetic propulsion to travel at high speeds, and magnetic fluids
help fill rocket engines with fuel. The Earth's magnetic field, known as the magnetosphere, protects it
from the solar wind. According to Wired magazine, some people even implant tiny neodymium magnets
in their fingers, allowing them to detect electromagnetic fields [Source: Wired].
Magnetic Resonance Imaging (MRI) machines use magnetic fields to allow doctors to examine patients'
internal organs. Doctors also use pulsed electromagnetic fields to treat broken bones that have not healed
correctly. This method, approved by the United States Food and Drug Administration in the 1970s, can
mend bones that have not responded to other treatment. Similar pulses of electromagnetic energy may
help prevent bone and muscle loss in astronauts who are in zero-gravity environments for extended
periods.
Magnets can also protect the health of animals. Cows are susceptible to a condition called traumatic
reticulopericarditis, or hardware disease, which comes from swallowing metal objects. Swallowed
objects can puncture a cow's stomach and damage its diaphragm or heart. Magnets are instrumental to
preventing this condition. One practice involves passing a magnet over the cows' food to remove metal
objects. Another is to feed magnets to the cows. Long, narrow alnico magnets, known as cow magnets,
can attract pieces of metal and help prevent them from injuring the cow's stomach. The ingested magnets
help protect the cows, but it's still a good idea to keep feeding areas free of metal debris. People, on the
other hand, should never eat magnets, since they can stick together through a person's intestinal walls,
blocking blood flow and killing tissue. In humans, swallowed magnets often require surgery to remove.
Cow magnets
Photo courtesy Amazon
Some people advocate the use of magnet therapy to treat a wide variety of diseases and conditions.
According to practitioners, magnetic insoles, bracelets, necklaces, mattress pads and pillows can cure or
alleviate everything from arthritis to cancer. Some advocates also suggest that consuming magnetized
drinking water can treat or prevent various ailments. Americans spend an estimated \$500 million per year
on magnetic treatments, and people worldwide spend about \$5 billion. [Source: Winemiller via
NCCAM].
Proponents offer several explanations for how this works. One is that the magnet attracts the iron found in
hemoglobin in the blood, improving circulation to a specific area. Another is that the magnetic field
somehow changes the structure of nearby cells. However, scientific studies have not confirmed that the
use of static magnets has any effect on pain or illness. Clinical trials suggest that the positive benefits
attributed to magnets may actually come from the passage of time, additional cushioning in magnetic
insoles or the placebo effect. In addition, drinking water does not typically contain elements that can be
magnetized, making the idea of magnetic drinking water questionable.
Some proponents also suggest the use of magnets to reduce hard water in homes. According to product
manufacturers, large magnets can reduce the level of hard water scale by eliminating ferromagnetic hardwater minerals. However, the minerals that generally cause hard water are not ferromagnetic. A two-year
Consumer Reports study also suggests that treating incoming water with magnets does not change the
amount of scale buildup in a household water heater.
The Strong Nuclear Force (also referred to as the strong force)…
is one of the four basic forces in nature (the others being gravity, the electromagnetic force, and
the weak nuclear force). As its name implies, it is the strongest of the four. However, it also has
the shortest range, meaning that particles must be extremely close before its effects are felt. Its
main job is to hold together the subatomic particles of the nucleus (protons, which carry a
positive charge, and neutrons, which carry no charge. These particles are collectively called
nucleons). As most people learn in their science education, like charges repel (+ +, or - -), and
unlike charges attract (+ -).
If you consider that the nucleus of all atoms except hydrogen contain more than one proton, and
each proton carries a positive charge, then why would the nuclei of these atoms stay together?
The protons must feel a repulsive force from the other neighboring protons. This is where the
strong nuclear force comes in. The strong nuclear force is created between nucleons by the
exchange of particles called mesons. This exchange can be likened to constantly hitting a pingpong ball or a tennis ball back and forth between two people. As long as this meson exchange
can happen, the strong force is able to hold the participating nucleons together. The nucleons
must be extremely close together in order for this exchange to happen. The distance required is
about the diameter of a proton or a neutron. If a proton or neutron can get closer than this
distance to another nucleon, the exchange of mesons can occur, and the particles will stick to
each other. If they can't get that close, the strong force is too weak to make them stick together,
and other competing forces (usually the electromagnetic force) can influence the particles to
move apart. This is represented in the following graphic. The dotted line surrounding the nucleon
being approached represents any electrostatic repulsion that might be present due to the charges
of the nucleons/particles that are involved. A particle must be able to cross this barrier in order
for the strong force to "glue" the particles together.
In the case of approaching protons/nuclei, the closer they get, the more they feel the repulsion
from the other proton/nucleus (the electromagnetic force). As a result, in order to get two
protons/nuclei close enough to begin exchanging mesons, they must be moving extremely fast
(which means the temperature must be really high), and/or they must be under immense pressure
so that they are forced to get close enough to allow the exchange of meson to create the strong
force. Now, back to the nucleus. One thing that helps reduce the repulsion between protons
within a nucleus is the presence of any neutrons. Since they have no charge they don't add to the
repulsion already present, and they help separate the protons from each other so they don't feel as
strong a repulsive force from any other nearby protons. Also, the neutrons are a source of more
strong force for the nucleus since they participate in the meson exchange. These factors, coupled
with the tight packing of protons in the nucleus so that they can exchange mesons creates enough
strong force to overcome their mutual repulsion and force the nucleons to stay bound together.
The preceding explanation shows the reason why it is easier to bombard a nucleus with neutrons
than with protons. Since the neutrons have no charge, as they approach a positively charged
nucleus they will not feel any repulsion. They therefore can easily "break" the electrostatic
repulsion barrier to being exchanging mesons with the nucleus, thus becoming incorporated into
it.
Weak nuclear force is less weak
January 13, 2011 By Phillip F. Schewe
The force that governs some of the reactions that keep our sun shining is not quite as weak as
scientists had previously thought. As a consequence, our estimation of how energetic the sun
actually is just went up by a tiny amount.
The evidence for this weak nuclear force comes from the decay of muons, essentially heavier cousins of
the electron, one of the building blocks of atoms.
Just as biologists sometimes study the tiniest and most ephemeral of organisms such as fruit flies, which
live for barely a day, to learn things about human disease, so physicists often study the properties of
particles that last a fraction of a second to learn about the universe.
The muon lives only about 2 millionths of a second -- 2 microseconds -- far from the realm of human
sensation but long enough for scientists to make detailed measurements. The state of digital electronics is
so advanced that measurements far shorter than this, even down to trillionths of a second or less, can
Researchers then gathered a fine spray of muons, directed them and stopped them in their own metal
target which was surrounded by a detector that could track the muons' demise. The decay of over 2 trillion
muons provided the best yet value for the average muon lifetime. It comes out to 2.1969803
microseconds.
"This is the most precise lifetime determination of any state in the atomic or subatomic world," said
David Hertzog, one of the leaders of the experiment and a professor at the University of Washington in
Seattle.
This lifetime, known to an uncertainty of one part per million, is so precise that it can be used to make a
new determination of the intrinsic strength of the weak nuclear force, which operates over only a very
short range inside the nucleus of atoms.
Scientists know of four physical forces. Gravity, a form of mutual attraction, keeps the Earth going
around the sun and keeps us from floating into space. The electromagnetic force is responsible for holding
atoms together, for bonding atoms into molecules, for impelling the movement of electrons through wires
in the form of electricity, and for light waves. The strong nuclear force holds nuclei together and is
responsible for some kinds of radioactivity.
The weak nuclear force, the fourth and last force to be discovered by physicists in the twentieth century,
helps to turn protons into neutrons inside the sun, a necessary step in converting those protons into
heavier elements like helium and releasing the radiant energy that makes its way to Earth. The weak force
also acted billions of years ago inside exploding stars known as supernovas to make the elements such as
oxygen and carbon found in our own bodies and other natural things on Earth.
The strength of the weak force is encapsulated in a number called the Fermi constant, named for the
Italian-American scientist Enrico Fermi. Hertzog said that the new value for the Fermi constant is about
0.00075 percent greater than the previous value. Thus the weak force is just a tiny bit stronger than we
thought…
Another expert on the weak force, University of Wisconsin professor Michael Ramsey-Musolf, considers
the muon experiment to be a tour-de-force piece of work. The important thing for him is that the
uncertainty of the muon lifetime has now dropped by a factor of ten. But he also said that a more precise
lifetime and a more precise knowledge of the strength of the weak nuclear force tells us just a bit more