Long Range Repulsion,
Short Range Attraction
Part of a Series of Activities in Plasma/Fusion Physics
to Accompany the chart
Fusion: Physics of a Fundamental Energy Source
Teacher's Notes
Robert Reiland, Shady Side Academy, Pittsburgh, PA
Chair, Plasma Activities Development Committee of the
Contemporary Physics Education Project (CPEP)
Editorial assistance: G. Samuel Lightner, Westminster College, New Wilmington, PA
and Vice-President of Plasma/Fusion Division of CPEP
Advice and assistance: T. P. Zaleskiewicz, University of Pittsburgh at Greensburg,
Greensburg, PA and President of CPEP
Prepared with support from the Department of Energy, Office of Fusion Energy Sciences,
Contract #DE-AC02-76CH03073.
©2002 Contemporary Physics Education Project (CPEP
Preface
This activity is intended for use in high school and introductory college courses to
supplement the topics on the Teaching Chart, Fusion: Physics of a Fundamental
Energy Source, produced by the Contemporary Physics Education Project (CPEP).
CPEP is a non-profit organization of teachers, educators, and physicists which develops
materials related to the current understanding of the nature of matter and energy,
incorporating the major findings of the past three decades. CPEP also sponsors many
workshops for teachers. See the homepage www.CPEPweb.org for more information on
CPEP, its projects and the teaching materials available.
The activity packet consists of the student activity and these notes for the teacher. The
Teacher’s Notes include background information, equipment information, expected
results, and answers to the questions that are asked in the student activity. The student
activity is self-contained so that it can be copied and distributed to students. Teachers
may reproduce parts of the activity for their classroom use as long as they include the title
and copyright statement. Page and figure numbers in the Teacher’s Notes are labeled
with a T prefix, while there are no prefixes in the student activity.
Developed in conjunction with the Princeton Plasma Physics Laboratory and funded
through the Office of Fusion Energy Sciences, U.S. Department of Energy, this activity
has been field tested at workshops with high school and college teachers.
We would like feedback on this activity. Please send any comments to:
Robert Reiland
Shady Side Academy
423 Fox Chapel Road
Pittsburgh, PA 15238
e-mail: robreiland1@comcast.net
voice: 412-968-3049
Long Range Repulsion, Short Range Attraction
Teacher’s Notes
Part of a Series of Activities in Plasma/Fusion Physics
to Accompany the chart
Fusion: Physics of a Fundamental Energy Source
Introduction:
One of the harder ideas to grasp in fusion is that nuclei, which strongly repel at long range
(compared to the size of a nucleus), can strongly attract at short range. This activity produces a
magnetic example of repulsion switching to attraction as two objects get closer together. It does
not work the same way as the forces between nuclei work; electrical repulsive force
overwhelmed by short range (residual strong) attractive nuclear force. However, it does provide
an analogous example of forces (magnetic in this case) switching in direction as the separation
distance in reduced.
This activity can be used directly in a unit on nuclear physics or fusion and plasma physics.
However, it is just as likely to fit in a unit on magnetism in which it will help students to use a
magnetic pole model of magnetic forces to explain magnetic forces between permanent magnets
and ferromagnetic materials such as iron or steel. Then its application to understanding fusion
can be brought in as an extra topic.
Materials:
pair of neodymium or cobalt-samarium disc magnets*
steel rod*
machine nuts*
steel ball bearing*
Optional: iron filings
* Comment on Dimensions:
What works best is for the ball bearings and machine nuts to have similar diameters to the
disc magnets, but the effect works for a range of diameters of at least a factor of two.
Please see the discussion in the next section for more detail.
Background:
The basic ideas behind the model are that like poles, such as two north poles, will repel one
another, unlike poles attract, and a pole of a permanent magnet will induce two poles in a nearby
piece of unmagnetized steel. Specifically, a north pole brought near a piece of steel will induce
the development of a south pole on the side of the steel closest to the magnet and a north pole
farther away. If the piece of steel is small, it will simply become “polarized” and act as an
extension of the magnet (see Figure T1).
Long Range Repulsion, Short Range Attraction – Page T2
(b)
(a)
S N
S N
(c)
S
N
S
N
The steel becomes
Induced poles
like an extension
appear in the
steel as it gets
of the magnet
closer to the
magnet
Figure T1: As the unmagnetized piece of steel shown in (a) is moved toward the permanent
magnet on its left, magnetic north and south poles begin to be induced in it (b). These will
cause it to act like an extension of the permanent magnet once the piece of steel reaches the
permanent magnet (c).
Unmagnetized steel too far
away to show polarization
Permanent
magnet
If it is long, as is a steel rod, the end nearest the magnet will develop one pole, but the other end
will be too far away to be affected. Instead, the second pole will be inside the rod and relatively
close to the permanent magnet that induces the poles (see Figure T2).
S
N
Permanent
magnet
S
N
Long steel
rod
Figure T2: Induced south pole on the near end of a long steel rod is close to the inducing
north pole of the permanent magnet while he induced north pole is just a short distance inside
the rod. The far end of the rod is not magnetized by the permanent magnet.
Since the induced pole closest to the magnet affecting the piece of steel is opposite in type to the
pole inducing it, this induced pole will be attracted by the permanent magnet. At the same time
(as expected from Newton’s Third Law) this closest induced pole will attract the permanent
magnet. The repulsive forces between the original pole and the induced like pole acts at a
greater distance than does the above mentioned attractive force and so is weaker. Thus the net
force between a permanent magnet and a previously unmagnetized piece of steel is always
attractive (see Figure T3).
Long Range Repulsion, Short Range Attraction – Page T3
F1 on 4
1
S
2
N
3
S
F2 on 3
N
4
F2 on 4
F1 on 3
(a) Forces exerted by poles 1 and 2 (S and N in
permanent magnet) on poles 3 and 4 (induced
S and N in steel object on the right)
F1 on 4
F1 on 3
F2 on 3
F2 on 4
Fnet
(b) Vector sum of these
forces on the steel object
Figure T3: The net force on the steel object on the right (by Newton’s Third Law opposite to
the net force on the permanent magnet) shown in (b) is the vector sum of the forces exerted
by the permanent poles (1 and 2) of the magnet on the induced poles (3 and 4) shown in (a).
This vector sum is shown to be attractive in (b). This illustration shows a typical situation in
which the net force is dominated by the closest poles and when they are opposite as shown,
that will be attractive.
The odd effects that will be seen with a ball bearing and with two machine nuts between the like
poles of two permanent magnets can be explained by the production of two poles by each of the
two permanent magnets. This is a quadrupole with two like poles in the center of the ball
bearing (see Figure T4 following answer to procedure question 4) or two like poles pushing the
machine nuts apart (see Figure T5 following answer to procedure question 5). With a typical
pair of neodymium or cobalt-samarium disc magnets the steel rod should be at least ten
centimeters (four inches) long, the two steel machine nuts at least half a centimeter (one quarter
inch) thick and the steel ball bearing of about one centimeter (nearly one half inch) diameter.
However, all this can vary, and you should try different sizes of ball bearings and machine nuts
to find what works best with your magnets before presenting this to students.
With a little guidance, most students will be able to figure out how the model explains what they
see. But expect a period of surprise before they get into problem solving.
Answers to questions in procedures:
1. Holding on very tightly to the two disc magnets, one in each hand, bring the flat sides of the
two magnets toward each other. You should feel the effects of either an attractive or a
repulsive force. Reverse one of the magnets to reverse the effect. In the case of the repulsive
effect, slowly bring the magnets toward each other until they touch. You will have to hold
them very firmly to do this. Does the effect ever become attractive?
Answer: No.
2. While firmly holding the two magnets, locate two sides that repel as in the previous
procedure. Hold the two magnets far apart, but keep the repelling sides (called like magnetic
poles) toward each other. Maintaining this alignment, place the steel rod between the two
magnets so that each end of the bar is near a magnet (see Figure 1). Slowly bring the
magnets closer to the ends of the rod until one, then the other, touches the rod. Does the
effect ever become repulsive?
Long Range Repulsion, Short Range Attraction – Page T4
Answer: No. The two magnets are too far away from each other to repel. Each polarizes its
end of the bar and is attracted to the induced pole nearest to it.
3. Again locate two repelling poles of the magnets. Separate these poles, but keep them toward
each other. Maintaining this alignment, place the ball bearing against one of the repelling
poles while the other magnet is several centimeters away from the ball bearing (see Figure 2).
Slowly and carefully bring the second magnet toward the ball bearing until contact is made.
Do this several times to be sure of what is happening. How is the result like that of the
repulsive part of Procedure 1? How is it like that of Procedure 2? Can the results be
explained in terms of what happened in the previous procedures?
Answers: It is like the result of Procedure 1 in that repulsive forces are felt initially when the
second magnet is relatively far from the ball bearing. It is like the result of Procedure 2 in
that both magnets end up being attracted to the ball bearing like they were attracted to the
steel rod.
The initial repulsion indicates that the side of the ball bearing away from the
magnet that it is initially touching and held to is like an extension of that magnet. Just as that
magnet would have repelled the other, as the two are oriented, the ball bearing repels the
second magnet. The final attraction indicates that the second magnet has been able to
polarize its side of the ball bearing, and each magnet is symmetrically producing polarization
of the ball bearing in the same way that each magnet had polarized the steel rod (see Figure
T4).
SN
S N
N
S
Fnet
Fnet
(a) Net force on polarized ball
bearing and on permanent
magnet when they are not
very near (long range).
N S N NS
Fnet
N
N SNNS N
Fnet
(b) Right magnet starting
to affect the ball
bearing as they get
closer.
(c) Both magnets affecting the
ball bearing equally resulting
in symmetrical induction of
poles. The dominant forces
are those between each
inducedatsouth
and(a)
theto
Figure T4: “Pole” representations of how forces go from repulsive
longpole
range
nearest
north
pole
attractive at short range (b) and (c). For simplicity, only the north poles of the magnets are
illustrated in (b) and (c).
4. Again locate two repelling poles. Separate these poles, but keep them toward each other.
Maintaining this alignment, place both machine nuts flat against one of the repelling poles
while the other magnet is several centimeters away from the machine nuts (see Figure 3).
Slowly and carefully bring the second magnet directly toward the machine nuts until contact
is made. Were there any surprising results?
Answer: Yes. One of the machine nuts suddenly jumps away from the other and onto the
second magnet (see Figure T5).
Long Range Repulsion, Short Range Attraction – Page T5
Magnet
nuts
Machine
N S N S
N S N
S NN S
N
N S N
N S N
N
(c) Once the right magnet
dominates the magnetic
polarization of the machine
nut nearest it, this machine
nut is repelled by the other
machine nut and attracted
to the right magnet.
Figure T5: “Pole” representation of how the machine nuts come to repel one another as the
second magnet (right one) changes the polarization of the machine nut nearest to it. For
simplicity only the north poles of the permanent magnets are illustrated.
(a) Right magnet is far
away (not in figure)
(b) Right magnet is starting to affect the
machine nut nearest it.
If not, try again, and move more slowly. Would the same forces which acted within the ball
bearing in the previous procedure cause the machine nuts to separate in this one?
Answer: Yes, with one exception. In the center of the ball bearing will be two like poles that
repel each other just as the like poles induced on the contact sides of the machine nuts
repelled each other. The difference is that within the ball bearing there are attractive forces
from atomic bonding that keep it together.
Answers to Questions:
1. How many magnetic poles are induced in a ball bearing in contact with a single magnet?
Answer: Two. In classical electrodynamics there is no way to produce a single pole, i.e.,
what is called a monopole. Even though the production of monopoles is not forbidden within
quantum electrodynamics, no monopole has ever been observed. Consequently, it currently
seems that magnetic poles must be produced in pairs, as illustrated in Figures T1 through T5.
If the magnet had a north pole against the ball bearing, it would induce a south pole on the
near side of the ball bearing and a north pole on the far side. It would in effect polarize the
ball bearing so that the bearing would seem like an extension of the magnet.
2. How many poles were induced in the steel rod when both magnets were in contact with its
ends? Where along the rod might these poles be located?
Answer: Four poles. Each magnet would polarize the side of the rod it was closest to. If
north poles of the magnets were both against the rod, each would induce a south pole next to
itself and a north pole a little further away in the rod.
Long Range Repulsion, Short Range Attraction – Page T6
3. What evidence do you have that the same number of poles were at times induced in the ball
bearings and in the pair of machine nuts as in the steel rod? Where would these poles be in
the ball bearings and in the pair of machine nuts?
Answer: Two things happened to support this. The first is that both magnets ended up being
attracted to the ball bearing and to the machine nut closest to it -- just as they were both
attracted to the steel rod (see Figures T2, T4, and T5). The second is that the machine nuts
repelled each other when the second magnet got close to them. This indicates that there were
two like poles on the parts of the machine nuts furthest from the magnets and closest to each
other. To see that there are poles induced in the steel rod when the magnets are at both ends
of the rod, put a piece of paper over the rod and magnets, and sprinkle some iron filings over
the paper. You will see filings concentrating around the magnets and at two locations away
from the magnets toward the center of the rod.
4. Would you view the ball bearing with both magnets against it as a tripole or as a quadrupole?
Why?
Answer: A quadrupole (see Figure T4(c)). As indicated in the answer to Question 1,
magnetic poles are always formed in pairs. The center of the ball bearing acts like a single
pole opposite to in type but of twice the strength as each of the poles that form next to the
magnets. The fact that it is of twice the strength as the other poles is consistent with thinking
of it as two poles very close together.
5. For nuclear fusion to take place, the long range electrical repulsion between nuclei must be
overcome by a short range nuclear force attraction. If this didn’t happen:
a. could stars come into existence gravitationally?
b. could stars radiate large amounts of energy fairly continuously for billions of years?
c. could massive elements form in stars?
Answers:
a. Yes. The gravitational force responsible for the condensation of interstellar gases and
dust into stars is always attractive and not dependent upon nuclear or electrical forces.
b. No. Stars could convert gravitational potential energy into radiant energy for millions of
years while shrinking in size, but the rate would decrease over time and become
relatively insignificant after a billion years.
c. No. Stars produce elements up to iron through nuclear fusion. If there were no short
range force to allow lower mass nuclei to stay together to form higher mass nuclei,
hydrogen would be the only element in the universe. No nucleus with more than one
proton would be stable. In supernovae, elements with atomic number beyond that of
iron are formed in energy absorbing (endothermic) nuclear reactions.
6. What would the night sky look like if objects that repel at some separations did so at all
separations?
Answer: Very, very dark. Actually there would be no Earth as such and no day or night.
Earth is made mostly of elements that would not form in a universe in which nuclear fusion
couldn’t occur.
Long Range Repulsion, Short Range Attraction – Page T7
APPENDIX
Alignment of the Activity
Long Range Repulsion, Short Range Attraction
with
National Science Standards
An abridged set of the national standards is shown below. An “x” represents some
level of alignment between the activity and the specific standard.
National Science Standards (abridged)
Grades 9-12
A. Science as Inquiry
Abilities necessary to do scientific inquiry
X
Understandings about scientific inquiry
X
B. Physical Science Content Standards
Structures of atoms
X
Motions and forces
X
Conservation of energy
X
Interactions of energy and matter
X
D. Earth and Space
Origin and Evolution of the Universe
X
E. Science and Technology
Understandings about science and technology
G. History and Nature of Science
Nature of scientific knowledge
X
Long Range Repulsion, Short Range Attraction – Page T8
Alignment of the Activity
Long Range Repulsion, Short Range Attraction
with
AAAS Benchmarks
An abridged set of the benchmark is shown below. An “x” represents some level
of alignment between the activity and the specific benchmark.
AAAS Benchmarks (abridged)
Grades 9-12
1. THE NATURE OF SCIENCE
B. Scientific Inquiry
X
2. THE NATURE OF MATHEMATICS
B. Mathematics, Science, and Technology
X
3. THE NATURE OF TECHNOLOGY
C. Issues in Technology
4. THE PHYSICAL SETTING
A. The Universe
X
D. The Structure of Matter
E. Energy Transformations
X
F. Motion
G. Forces of Nature
X
11. COMMON THEMES
A. Systems
X
B. Models
X
C. Constancy and Change
X
D. Scale
X
12. HABITS OF MIND
B. Computation and Estimation