CHAPTER 1
Scientists’ Ideas
INTERACTIONS AND ENERGY
In this chapter you first learned a way of representing the motion of an object using
speed-time graphs. Then you developed some ideas involving interactions and energy.
You studied several types of interactions, energy transfers and energy changes.
Types of Interactions: Contact Push/Pull, Heat Conduction, Infrared, Light and Electric
Circuit
Types of Energy Transfer: Mechanical, Heat, Light and Electrical
Types of Energy Changes (within an object or system): Kinetic, Chemical potential and
Thermal (‘Eye-Brain System’ energy was also included as a special type of energy
associated with vision.)
Below, after summarizing some ideas regarding the representation of motion, we
summarize some of the ideas developed by scientists involving each of the types of
interactions mentioned above. Below each type, we include 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 or examples from your own
experiments that would support that idea.
Ideas Involving the Representation of Motion
Idea RM1 – Representing Motion on a Speed-Time Graph
The ordinate value on a speed-time graph indicates the speed of the object at the given
instant in time.
A horizontal straight line on a speed-time graph indicates the object is moving at a
constant speed. An upward-sloped straight line indicates the object is speeding up. A
downward-sloped straight line indicates the object is slowing down.
Evidence/examples:
Idea RM2 – Representing Motion with Speed Arrows
The motion of an object can be represented with speed arrows. The direction of the
arrow indicates the direction the object is moving, and the length of the arrow is
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proportional to the speed of the object. By convention, the arrow is drawn above the
object and with a half arrowhead.
Object moving with
lower speed
Object moving with
higher speed
Ideas Involving the Energy Description of Interactions
Idea E1 – Interactions can be described in terms of a Source/Receiver (S/R) energy
diagram.
During an interaction two objects act on or influence each other to cause some effect.
One object is the energy source (where the energy comes from) and the other object is
the energy receiver (where the energy goes). During the interaction, energy is
transferred from the source to the receiver; there is a decrease in energy within the
source and an increase in energy within the receiver. The S/R energy description of an
interaction can be represented using the following diagram.
By convention, the names of the interacting objects are included within rectangles, the
type of energy transferred is included in a broad arrow, and the energy changes within
the objects are included in ovals.
Evidence/examples:
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Idea E2 – Interactions can be described in terms of an Input/Output (I/O) energy
diagram.
When a single object or system (of objects) is involved in more than one interaction
with surrounding objects, it is convenient to focus only on the single object or system
and show only the types of its interactions, the energy transfers (as a result of those
interactions) into and out from the system, and the energy changes within the object or
system. In these I/O energy diagrams, we do not show the other interacting objects.
Evidence/examples:
Ideas Involving Contact Push/Pull Interactions
When scientists study the natural world they focus their attention on the interactions
between objects and how they act on or influence each other during these interactions.
Starting around the 17th century, ideas about energy were developed by many scientists.
However, it was not until the mid-19th century that these various ideas were brought
together. This was possible due to the work of James Prescott Joule, who showed that
the increases in thermal energy produced in a friction-type contact push/pull
interaction could be directly related to the change in the kinetic energy of the objects
involved.
Idea ME1 - Definition of a Contact Push/Pull Interaction
A contact push/pull interaction occurs when any two touching objects push or pull each
other. In the absence of an equally strong opposing contact push/pull interaction there
is a change in the speed (and/or direction) of at least one of the objects involved. In
terms of energy, during the contact push/pull interaction there is a transfer of
mechanical energy from one object to the other. The object that outputs mechanical
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energy will simultaneously decrease in some type(s) of energy. For the object that
receives the mechanical energy, there will be an increase in some type(s) of energy.
Evidence/examples:
Idea ME2 – Some Possible Energy Changes during a Contact Push/Pull Interaction
The type of energy associated with the speed of an object is called kinetic energy. During
a contact push/pull interaction, an object’s kinetic energy may increase or decrease,
depending on whether mechanical energy is transferred into it or out from it. If a
human person is the source of the contact push/pull interaction, then as mechanical
energy is transferred out from the person, the chemical potential energy in the person
decreases.
As an example, here is an S/R energy diagram describing the interaction between a
person’s hand and a cart, causing the cart to speed up. We are ignoring the effects of
friction.
Contact push/pull Interaction
We can also describe this interaction with a pair of I/O energy diagrams.
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Contact Push/Pull Interaction
Contact Push/Pull Interaction
Idea ME3 - Contact Push/Pull Interactions involving friction
A contact push/pull interaction occurs when two surfaces rub against each other. The
evidence of such a friction-type contact push/pull interaction is that at least one of the
objects involved slows down and the temperature of both the surfaces increases.
In terms of energy, during a friction-type contact push/pull interaction there is a
decrease in the kinetic energy of at least one of the objects involved and an increase in
the thermal energy of both objects. As an example of an I/O energy diagram, consider
a block sliding along a surface. There is a friction-type contact push/pull interaction
between the block and surface and, as a result, both the block and table surface warm
up. Also, as will be discussed below, there will be heat conduction/IR interactions
between the block and table and the surroundings. If we treat the block and surface
together as a system, the I/O energy diagram would look like this:
Ideas Involving Light Interactions
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In the 17th century, Sir Isaac Newton introduced a theory to explain the behavior of
light. Newton (and many other scientists) thought that light consisted of streams of
some sort of particles (which they called corpuscles) emanating from light sources. The
corpuscle theory was effective in explaining most observable phenomena, such as
observations involving pinholes and mirrors (the corpuscles were thought to act like
tennis balls bouncing off a wall at a certain angle). But it became very complicated
when Newton had to start introducing more and more different types of corpuscles in
order to explain his observations involving different colors of light. Christian Huygens
criticized Newton’s corpuscular theory of light because of its complexity, and in 1678 he
argued in favor of a different theory of light. He argued that light is not the
transference of substance made of corpuscles but instead the transference of energy.
Both theories were effective in explaining most observations but it wasn’t until the early
19th century that evidence that light was energy instead of a substance grew more
persuasive. The experiments of Augustin Jean Fresnel, Thomas Young and others
revealed many phenomena that can be understood on the basis of an energy
explanation but not with the corpuscular model.
Idea L1 - A light interaction occurs when a source of light illuminates an object
In terms of energy, during a light interaction, light energy is transferred from the source
(e.g. a bulb) to the receiver (e.g. an eye).
Evidence/examples:
Idea L2 – Light travels through a single material (like air) in straight lines.
A light ray shows the direction that light travels. A light ray diagram is a description (a
“story”) of how light travels as it goes from a source to a receiver (usually an eye),
including how the light behaves when striking shiny and/or non-shiny surfaces
between the source and receiver.
Evidence/examples:
Idea L3 - When light interacts with a shiny object, the light is reflected in a particular
direction so that the angle at which the light reflects from the surface equals the
angle at which the light strikes the surface.
Evidence/examples:
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Idea L4 - When light interacts with a white, non-shiny object, the light is reflected in
all directions away from its surface.
Evidence/examples:
Idea L5 – When you look at a light source, a mirror reflection or an illuminated, nonshiny surface, light enters your eye to enable you to see the object, reflection or
surface.
When light energy is transferred into the eye, enabling vision, there is an increase in
Eye-Brain System energy. (This term is unique to this course.)
Evidence/examples:
Here are some examples of light ray diagrams and the corresponding S/R energy
diagrams.
Light ray diagram (seeing flashlight)
Flashlight
Eye
S/R Energy diagram (seeing flashlight)
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Light ray diagram (shiny object)
Light ray diagram (white, non–shiny object)
S/R Energy diagram (shiny or non-shiny object)
Idea L6 - When light interacts with a pure black object, all the light energy is
absorbed in the object and none is reflected:
The temperature of the black object increases, which means that its thermal energy
increases. (With real black objects there usually is a small percentage of light that is
reflected. For simplicity, however, we will ignore this small percentage of reflected
light.)
Here is an I/O energy diagram for a black card illuminated by light from some source.
Because the card warms up, it transfers heat energy to the surroundings.
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When you look at a black object, no light enters your eye. When you look towards any
object, and no light from that object enters your eye, the object will appear to be black.
Evidence/examples:
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Ideas Involving Electric-Circuit Interactions
By the 18th century experiments with electricity were the rage and scientists such as
Stephen Gray and Charles Dufay began to notice the effect of connecting electrical
materials together using a wire or a thread.
A vital experimental finding in the history of the electric circuit interaction was
discovered largely by accident by Luigi Galvani who produced an electrical convulsion
in a frog’s leg. This accidental discovery led Alessandro Volta to construct the first
“voltaic cells” or batteries in the early 19th century. Experiments with this device led to
the discovery that a noticeable quantity of heat was generated in the wire connecting
the battery to a device. This discovery was astonishing from the standpoint of energy
because no external energy was being supplied to the battery. Therefore, scientists had
to generate an understanding of energy transformations in order to explain the source
of this heat.
Idea EC1 - An electric-circuit interaction occurs when a source of electrical energy is
connected in a closed path of conductors to an energy receiver:
If the path is opened, or if a non-conductor (insulator) is placed in the direct path, then
the electric-circuit interaction will cease occurring.
Evidence/examples:
Idea EC2 - Each device in an electric circuit is two-ended; and each end must be
directly connected in the circuit:
(If only one end of a device is connected in the circuit, then the device or circuit will not
work.)
Evidence/examples:
Idea EC3: Electric-circuit interactions can be described in terms of electrical energy:
During an electric-circuit interaction, electrical energy is transferred from the energy
source to the energy receiver. Below are separate I/O energy diagrams for a battery
and a bulb connected together in an electric circuit.
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Idea EC4 - Rate of electrical energy transferred into a bulb and its brightness
The rate of electrical energy transferred into a bulb is the amount of electrical energy
transferred into the bulb each second. The greater the rate, the brighter the bulb. The
rate is measured in units called watts. (1 watt = 1 joule/sec)
Evidence/examples:
Idea EC5 – Series and Parallel Circuits
Bulbs can be connected to a source of electrical energy (e.g. battery or generator) in two
different ways: parallel or series.
In a parallel circuit, each bulb is connected in its own loop with the energy
source. Each bulb glows with the same brightness it would have if it were
connected by itself to the energy source. If one bulb is removed, the other bulbs
continue to glow with the same brightness.
In a series circuit, all the bulbs are connected together in a single loop with the
energy source. Although all bulbs glow with the same brightness, each bulb is
dimmer than it would be if it were connected to the energy source by itself. If
one bulb is removed, the circuit becomes open and all bulbs go out.
Evidence/examples:
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Ideas Involving Heat Conduction and Infrared Interactions
It should come as no surprise that the idea of fluids or substances was also used to
explain the concept of heat in the 16th and 17th centuries. The process of heat transfer
was formerly thought to be a flow of an invisible, weightless fluid called caloric. It was
believed that in thermal interactions the caloric fluid flowed from one object to another.
In 1798, Count Rumford rejected the idea of heat as an invisible, weightless fluid. He
argued that the caloric model of heat could not account for the enormous amount of
heat produced in a friction-type mechanical interaction, and that heat was really just
another form of energy. It was James Prescott Joule’s experiments that finally
convinced the scientific community of this. Keep in mind also that around the same
time period, many of the other fluid-type theories (e.g. for electricity, magnetism, and
light) were being challenged, modified and/or replaced by other models that were
more effective in explaining observations. Experiments such as those conducted by
James Joule, Sadi Carnot (1842) and Rudolph Clausius (1850) led to the adoption of the
idea that heat flow is really a transfer of energy.
Even though large-scale adoption of heat as a transfer of energy did not occur until the
mid-19th century, many scientists did experiments involving heat and temperature long
before then. In 1800 Sir Fredrick William Herschel began to study the heat properties of
the rainbow of light that is created when sunlight is passed through a glass prism. He
measured the temperature of each color and found not only that the temperature
increased from the violet to the red part of the spectrum but also that the temperature
continued to increase beyond the red part, where he could see no light. He found that
this region had the highest temperature of all. Herschel referred to the energy in this
region as “colorific rays,” now known as infrared radiation.
Idea H1 - Objects can interact due solely to their temperature differences.
During this interaction, heat energy is transferred from the warmer object to the cooler
object. The warmer object decreases in thermal energy (and therefore decreases in
temperature) and the cooler object increases in thermal energy (and therefore increases
in temperature). The interaction stops when the two objects reach the same
temperature.
Evidence/examples:
Idea H2 - A heat conduction interaction occurs between any two objects that are in
contact (touching) and have different temperatures.
Evidence/examples:
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Idea H3 - An infrared interaction occurs between any two objects that are near one
another and have different temperatures.
Evidence/examples:
Idea H4 - Heat energy and thermal energy are different types of energy, and they do
not mean the same thing.
Thermal energy is a property of an object, and is related to the temperature of the object.
It changes during most types of interactions, and when it does, the temperature of the
object changes. Heat energy is a type of energy transfer between objects. It is not a
property of a single object. It only exists when two objects at different temperatures are
involved in either a heat conduction or an infrared interaction.
Evidence/examples:
Idea H5 - During any interaction involving the transfer of any type of energy into an
object, the thermal energy of the object usually increases, and some heat energy is
almost always transferred from the object or system to the surroundings (that is,
nearby and touching objects).
The amount of heat energy transferred to the surroundings may be small or it may be
large. The greater the temperature difference between the object and its surroundings,
the greater the rate at which heat energy is transferred from the object to its
surroundings. Usually, when an object is warmer than its surroundings, there is both
heat conduction and infrared interactions between the object and surroundings.
Especially when drawing I/O energy diagrams, it is often useful to include both types
of interactions together, since they both cause heat energy transfers to the surroundings.
As a shorthand notation, we can write HC/IR Interactions to mean Heat Conduction and
Infrared Interactions.
For example, when a hand is holding a hot cup of coffee, the interactions between the
cup and the surroundings, and between the hand and the surroundings, can be
described with the following I/O energy diagrams.
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Ideas Involving Energy Conservation
Up until the 18th century, many of the models of electric, magnetic, and light
interactions were largely based on the idea of forces between tiny particles that made
up a substance that flows from one object to another. It was not until the middle of the
19th century that energy became a primary concept on which physics was based.
Throughout the 19th century, Sir James Prescott Joule spent much of his time trying to
understand the relationship between mechanical energy and the changes in
temperature that occur as a result of mechanical interactions. He developed a simple
apparatus that allowed him to measure the relationship between changes in motion
energy and changes of the temperature of water that was stirred through a mechanical
interaction. He found that the change in mechanical energy during the experiment was
equal to the heat energy necessary to change the temperature of the water from its
initial to its final value. Later work led to the recognition that these two forms of
energy, mechanical and heat energy, were only two of many forms of energy. Sadi
Carnot (1842) and Rudolph Clausius (1850) actually demonstrated the transformation of
energy from one form to another.
In a lecture in 1846, William Thompson (Lord Kelvin), referring to the work of Sir James
Joule, announced that in his view, energy had become the primary concept on which
physics was to be based. In 1847, Hermann von Helmholtz used mathematics to
express that mechanical, light, heat, electricity, and magnetism were different
manifestations of energy. In 1852 and 1855, W. J. Rankine declared that the term
‘energy’ could be applied to “ordinary motion and mechanical power, chemical action,
heat, light, electricity magnetism, and other powers, known or unknown, which are
convertible or commensurable with these.”
Just a few years later, Michael Faraday published an essay called, ‘On the conservation
of force.’ He understood this to mean the transformability and indestructibility of
natural powers. In the essay, Faraday discussed the ambiguities of the phrase because
he understood that force can be applied and removed. Rankin argued that a better way
to express what Faraday was trying to say is the phrase ‘conservation of energy’ which
was not ambiguous, for it is energy that is not created or destroyed.
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In drawing energy diagrams that can illustrate energy conservation, it is best to use the
Input/Output types of energy diagrams.
Idea C1 - Law of Conservation of Energy
Energy cannot be created or destroyed, but only changed from one form to another. If,
over a period of time, you keep track of the total amount of energy transferred into an
object or system, the total amount of energy transferred out of the object or system, and
the total energy change (which may be positive or negative) in the object or system,
then:
Energy Input = Energy Changes + Energy Output
Total Energy
INPUT
Object or System
Total Energy
OUTPUT
Energy
CHANGES
Evidence/examples:
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