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CHAPTER 3
Scientists’ Ideas
INTERACTIONS, SYSTEMS, AND POTENTIAL ENERGY
In this Chapter you developed some ideas involving three different ‘action-at-adistance’ interactions, in which the objects involved exert forces on each other without
touching. You also saw how a group of interacting objects could be considered as a
‘system’ and how applying the idea of energy conservation you developed in Chapter 1
allowed you to deduce that there were changes in different forms of potential energy
within such systems.
Below we summarize some of the general ideas developed by scientists involving
interacting systems, potential energy, and ‘action-at-a-distance’. Following these are
scientists’ ideas about each of the types of interaction you examined in this chapter,
including a brief historical account of the development of some of those ideas. For each
of the scientists’ ideas listed that is not just a definition, you should think about the
evidence from your own experiments that would support that idea. You should also be
able to draw I/O energy diagrams for the systems involved in each interaction.
Systems, Potential Energy, and ‘Action-at-a-Distance’
In the first half of the 19th century, when scientists first began to think about energy as a
useful concept in the description of interactions, they concentrated on easily perceptible
forms of energy, such as kinetic energy and thermal energy. Early ideas about the
conservation of energy were confined to these types and energy was only regarded as
being conserved under certain specific circumstances. However, it was recognized that
some objects had the potential to develop ‘real’ energy, such as an object held above the
ground and then released, which develops kinetic energy as it falls. In 1853 William
Rankine first used the term ‘potential energy’ to signify energy that a system has the
power to acquire, rather than energy it already has. In 1867 Rankine further defined
potential energy as ‘energy of configuration’, that is that the ‘real’ energy developed by
a system depended on how the system was arranged to start with. However it was only
in the years that followed that the idea of conservation of energy as a powerful
universal law began to be recognized, and with it the idea that potential energy is a real
form of energy that must be taken into account.
Ancient Greeks scientists, such as Plato and Aristotle, knew of the phenomenon of
‘action-at-a-distance’ as demonstrated by magnets and static electricity. They explained
it in a number of ways, including supernatural intervention and the idea that some
objects have a natural tendency or ‘desire’ to be in certain places. However, the favored
idea was that some invisible substance (called the ether) filled the space between objects
and transmitted their influence. This latter idea was supported by Francis Bacon in the
11th century and further developed by Rene Descartes in the 17th century. While Isaac
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Chapter 3
Newton recognized that his ideas about gravity also represented ‘action-at-a-distance’
he was not convinced there was enough evidence to support the idea that the ether
actually existed. In the mid-19th century Michael Faraday proposed the idea of ‘lines of
force’ to explain magnetic interactions and this idea, which can also be thought of as a
‘field of influence’ quickly proved useful in also explaining electric charge and
gravitational interactions.
Idea S1 - Definition of a System
A System is a group of two or more interacting objects. The objects within the
system may, or may not, interact with objects that are outside the system, as well
as with each other.
If the only interactions that occur are between objects
that are both themselves components of the system then
there are no energy inputs to or outputs from the system.
(Scientists say that such a system is closed with respect to
energy.) In this case the Law of Conservation of Energy,
applied to the system as a whole, takes on the form:
Energy Changes = 0
This means that any increase in one type of energy in the
system must be compensated for by an equal decrease in
another type of energy (or more than one type
combined) in order for the total change to be zero.
Evidence/examples:
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Objects in the
system
Energy increases
and decreases in
the system:
add up to zero
Scientists’ Ideas: Interactions, Systems, and Potential Energy
Idea S2 – Potential Energy in Systems
Potential Energy is energy that a system has because of
the way the interacting objects within the system are
arranged. When the objects within the system are
rearranged, the amount of potential energy in the system
may change. If there is no energy input or output for the
system (a closed system), any change in potential energy
will also result in a change in another form of energy
within the system (usually kinetic energy). According to
the conservation of energy, if the potential energy in
such a closed system increases (decreases), then the other
type of energy in the system will decrease (increase).
Different specific types of potential energy are associated
with different types of interactions between the objects in
a system and are discussed below.
Objects in the
system
Decrease
(increase) in
potential energy
Increase
(decrease) in
kinetic energy
Evidence/examples:
Idea S3 – Potential Energy in Systems with attractive and repulsive forces
If the mutual interactions between the components of a system are attractive,
when the average separation between the components increases, the potential
energy of the system increases also.
If the mutual interactions between the components of a system are repulsive,
when the average separation between the components increases, the potential
energy of the system decreases.
Evidence/examples:
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Magnetic Interactions
Magnetism is one of the earliest known physical phenomena. The ancient Greeks
studied naturally occurring magnets (called lodestones) and the basic properties of
magnetic interactions were discovered before 600 BC. However many contributions to
the understanding of such magnetic interactions were made later, by such scientists as
William Gilbert (16th century), Charles Coulomb (18th century), Michael Faraday and
James Clerk Maxwell (both 19th century)
Idea M1 - Magnetic Interactions between two magnets:
A magnetic interaction occurs between a magnet and another nearby magnet.
Two magnets will either attract or repel each other, depending on which ends
face each other. Scientists call the two ends the North and South poles. Two
magnets with like poles facing each other will repel. Two magnets with unlike
poles facing each other will attract.
Evidence/examples:
Idea M2 - Magnetic interaction between a magnet and a ferromagnetic object:
A magnet will always attract a nearby ferromagnetic object. Ferromagnetic
objects include iron, nickel and cobalt. Other metals, as well as non-metals, will
not interact with a magnet.
Evidence/examples:
Idea M3 – Action at a distance:
A magnet can exert forces on another magnet, or a ferromagnetic object, without
touching it. (Scientists call this ‘action-at-a-distance’.) These forces can be
represented on a force diagram in the same way as any other forces acting on the
magnet: (on the left are unlike poles)
Force exerted on
Magnet B by hand
Force exerted on
Magnet B by M agnet A
Magne t A
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Magne t B
B
Scientists’ Ideas: Interactions, Systems, and Potential Energy
(This phenomenon of ‘action at a distance’ can be accounted for by the idea of an
invisible magnetic ‘field of influence’ that extends around a magnet. Any other
magnets within this magnetic field will feel attractive and repulsive forces due to
the influence of the field on them.)
Evidence/examples:
Idea M4 – Magnetic Potential Energy:
In any system of magnets (or magnets and
ferromagnetic objects) there is magnetic potential
energy, the amount of which depends on how the
magnets (and ferromagnetic objects) are arranged
with respect to each other. When the magnets (and
ferromagnetic objects) are rearranged this magnetic
potential energy may change. When this happens in
a system with no energy inputs or outputs (a closed
system), then, according to the Law of Conservation
of Energy, if the magnetic potential energy in the
closed system increases (decreases), then the kinetic
energy of the objects in the system will decrease
(increase), and vice versa.
For example, when two carts with magnets attached
push each other apart, the energy diagram for this
system would be like this:
Both
Magnet-carts
Decrease in
magneti c
potenti al energy
Increase in
kineti c energy
During time that both
magnet-carts are pushing
each other further apart
Other evidence/examples:
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Electric Charge Interactions
Many of the same scientists who studied magnetic interactions also studied electric
charge interactions. In fact, we owe the very word ‘electric’ to the ancient Greeks, who
studied electric charge interactions, as well as magnetic interactions. In their studies
they rubbed samples of fossilized tree resin (which we call amber) with fur to charge
them. The Greek word for amber is ‘elektron’! As early as the 4th century B.C. Plato
wrote about the effects of rubbed amber and magnets.
Observations of electrical effects continued well into the 16th century when scientists
such as William Gilbert and others noted many similar effects with many other types of
materials and the effect of repulsion was also added to the list of observed electrical
phenomena.
Idea EC1 - Electric Charge Interactions between charged objects:
An electric charge interaction occurs between two nearby charged objects. Two
like-charged objects will repel. Two unlike-charged objects will attract. (Scientists
call the two types of charge positive and negative.)
Evidence/examples:
Idea EC2 - Electric Charge Interaction between charged and uncharged objects:
A charged object will always attract a nearby uncharged object, regardless of the
material of which the uncharged object is made.
Evidence/examples:
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Scientists’ Ideas: Interactions, Systems, and Potential Energy
Idea EC3 - Action at a distance:
A charged object can exert forces on another
object (both charged and uncharged) without
touching it. Such forces can be represented on a
force diagram in the same way as any other
forces acting on the object.
(This can be accounted for by the idea of an
invisible electrostatic ‘field of influence’ that
extends around a charged object. Any other
objects within this electrostatic field will feel
either an attractive or repulsive force due to the
influence of the field on them.)
Evidence/examples:
Force exerted o n Tape T1
by Tape T2
T1
Idea EC4: Electrostatic Potential Energy.
In any system that includes at least one charged
object there is electrostatic potential energy, the
amount of which depends on how the objects are
arranged with respect to each other. When the objects
in the system are rearranged this electrostatic
potential energy may change. When it does so in a
closed system then, according to the Law of
Conservation of Energy, if the electrostatic potential
energy in the closed system increases (decreases),
then the kinetic energy of the objects in the system
will decrease (increase), and vice versa.
Both
Charged tapes
Decrease in
electrostatic
potenti al energy
Increase in
kineti c energy
For example, when two charged tapes are attracted
and start to move toward each other, the energy
During time that the
diagram for this system would be like this:
charged tapes are attracting
Other evidence/examples:
and moving towards each
other
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Idea EC4 - Model of Static Electricity:
The process by which an object becomes charged with static electricity can be
explained in terms of a model of charges in materials. This model assumes that
inside all materials are tiny charges of two separate types, called positive and
negative. It is further assumed that the negative charges are free to move, while
the positive charges stay fixed in place.
In an uncharged material there are equal numbers
of positive and negative charges, so the object has
no overall charge. However, by rubbing objects
together (or other means) it is possible to move
negative charges from one object to another. In
this process one object gains extra negative
charges, and so has an overall negative charge,
while the other object has a deficit of negative
charges, and so is left with an overall positive
charge.
Other examples:
When an uncharged object is brought near a charged
object, it becomes electrically polarized, with the two
sides of the object becoming oppositely charged. This
happens because the negative charges in the
uncharged object redistribute themselves under the
influence of the force exerted on them by the charged
object. This causes the area of the uncharged object
closest to the charged object to have the opposite type
of charge and so the two attract.
For example, when a negatively charged balloon is
brought close to a wall, the negative charges in the
wall are repelled, leaving the surface of the wall
positively charged. Thus, the negatively charged
balloon is attracted to the wall and may stick to it.
Other examples:
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Scientists’ Ideas: Interactions, Systems, and Potential Energy
Gravitational Interactions and Potential Energy
In the late 16th and early 17th centuries Galileo did much of the important early work on
the motion of falling objects, showing that they speed up as they fall. Though seemingly
unrelated, the next important step was the work of Kepler, who developed three laws
that accurately describe the motion of the planets in their orbits around the sun.
Sir Isaac Newton realized that he could explain both of these motions with a single force
(gravity), if the strength of that force depended on the masses of both the interacting
objects, and on the distance between them. Newton’s Law of Universal Gravitation was
his crowning achievement. His Laws of Motion, which you have already encountered,
were developed along the path to his explanation for gravity. However, Newton was
very concerned about one aspect of his ideas, that of ‘action-at-a-distance’. To him the
idea that one object could influence another without touching it seemed too much like
‘magic’ and while he thought there must be some unseen agent that transmitted the
influence, he did not feel there was enough evidence to be convinced of the reality of
the ether.
Idea G1 - Gravitational Interactions:
A gravitational interaction occurs between any
two objects that have mass even though they are
not touching. During this interaction the two
objects always exert attractive forces on each
other. (This can be accounted for by the idea of
an invisible gravitational ‘field’ that extends
around all objects. Any other objects within this
gravitational field will feel an attractive force due
to the influence of the field itself on them.)
This gravitational force can be represented on a
force diagram (along with any other forces acting
on the object).
Evidence/examples:
Gravitational force
exerted on apple by
the Earth
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Idea G2 - Strength of the Gravitational Force:
The strength of the gravitational attraction between two objects is determined by
the amount of mass of both of the objects involved and by the distance between
their centers.
The larger either of the masses, the stronger the gravitational attraction between
them. However, with “normal sized” objects (like people, cars, buildings, etc.)
the gravitational interaction between them is so small that it is not noticeable.
Only when at least one of the objects is very, very massive (like the Earth), does
the strength of the gravitational interaction become significant and cause
noticeable effects. Also, the further apart the centers of the objects are, the
weaker the strength of the gravitational interaction between them.
Since the strength of the gravitational force on an object is proportional to its
mass, then all objects fall with the same increasing rate of speed, independent of
their mass. (This assumes that no other forces are affecting the objects.)
Evidence/examples:
Idea G3 – Gravitational Potential Energy:
In any system of objects there is gravitational
potential energy, the amount of which depends on
how the objects are arranged with respect to each
other. When the objects are rearranged this
gravitational potential energy may change. When it
does so in a closed system, there will also be a change
in the kinetic energy of the objects within the system.
(When one of the objects involved in a gravitational
interaction is very massive, like the Earth, the change
in its kinetic energy is imperceptible.)
For example, when a pencil falls because of the
gravitational interaction between it and the Earth, the
energy diagram for this system would be like this:
Evidence/examples:
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Pencil and
Earth
Decrease in
gravitati onal
potenti al energy
Increase in
kineti c energy
Scientists’ Ideas: Interactions, Systems, and Potential Energy
Idea G3 – Effect of moving through air:
Here on the Earth the way an object moves (including how it falls) can be
affected by an opposing force exerted by the air. Scientists call this force ‘air
resistance’ or ‘drag’.
This force affects light objects with a large surface area much more than small,
heavy objects. Because the strength of this force also increases as the speed of an
object increases, this can lead to a situation in which the forces acting on an
accelerating object eventually become balanced when it reaches a certain speed.
If this happens, the speed will then remain constant and is referred to as the
object’s terminal velocity.
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