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, Heat Conduction and Infrared Types of Energy Transfer: Mechanical Energy and Heat Energy Types of Energy Changes (within an object): Potential and Thermal Kinetic, Chemical Potential, Elastic Below, after giving 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. You should also be able to draw energy diagrams to describe these interactions. Ideas Involving the Representation of Motion Idea RM1 Representing Motion on a Speed-Time Graph A speed-time graph shows how the speed of the object behaves over time. A horizontal straight line on a speed-time graph indicates the object is moving at a constant speed. An upward-sloped line indicates the object is speeding up. A downward-sloped line indicates the object is slowing down. Idea RM2 Representing Motion with Speed Arrows The motion of an object can also be represented with speed arrows. The direction of the arrow indicates the direction the object is moving, and the length of the arrow is proportional to the speed of the object. By convention, the arrow is drawn above the object and with a half arrowhead. 2007 PET 1-119
Chapter 1 Object moving with lower speed Object moving with higher speed Ideas Involving the Energy Description of Interactions Idea E1 Interactions between objects 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 to). 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 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. 1-120
Scientists Ideas: Interactions and Energy A chain of interactions involves a sequence of interactions, where some objects are an energy receiver for one interaction and an energy source for another. Idea E2 Interactions engaged in by an object can be described in terms of an Input/Output (I/O) energy diagram. When a single object is involved in one or more interactions with surrounding objects, it is sometimes convenient to focus only on that single object 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 itself. In these I/O energy diagrams, we do not show the other interacting objects. 1-121
Chapter 1 Ideas Involving Contact 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 17 th century, ideas about energy were developed by many scientists. However, it was not until the mid-19 th 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 interaction could be directly related to the change in the kinetic energy of the objects involved. Idea C1a - Definition of a Contact Interaction: A Contact Interaction occurs when any two touching objects push or pull each other. In the absence of an equally strong opposing contact interaction there is a change in the speed (and/or direction) of at least one of the objects involved. Idea C1b - Duration of Contact Interactions and changes in speed: Contact interactions have a definite duration in time, and changes in speed take place gradually and continuously all the time the interaction continues. Idea C1c Energy transfer and changes during a Contact Interaction: In terms of energy, during a contact interaction there is a transfer of mechanical energy 1 between the interacting objects. (Mechanical energy is defined as the type of energy transfer that occurs when two objects push or pull on each other.) The type of energy associated with an object s motion is called kinetic energy, thus when the speed of an object changes during a contact interaction, so does its kinetic energy. 1 Scientists often refer to the mechanical energy transferred between objects during a contact interaction as the work done by one object on the other. 1-122
Scientists Ideas: Interactions and Energy Scientists identify three specific types of contact interactions, which are described in ideas C2, C3, and C4. These ideas assume that the particular interaction in question is acting in isolation. That is, there is no opposing contact interaction of equal strength that would cause there to be no change in speed (and/or direction) of the objects involved. Idea C5 deals with the situation of opposing interactions. Idea C2 - Contact Interactions involving rigid objects: One type of contact interaction occurs when two touching non-elastic (i.e. rigid or stiff) objects push or pull on each other. In such contact interactions, the speed (and/or direction) of at least one of the non-elastic objects changes while they are touching and push or pull on each other. As an example here is a S/R energy diagram describing the interaction between a person s hand and a cart, that causes the cart to speed up. (Assuming the effects of friction can be ignored.) This could also be represented using two I/O energy diagrams, one for each of the interacting objects. Notice that in this case one object (the hand) has no energy input, while the other object (the cart) has no energy output. 1-123
Chapter 1 In terms of energy, there is a change in the kinetic energy of at least one of the objects involved. If a human person is the source of the contact interaction, then as mechanical energy is transferred out from the person, the chemical potential energy in the person decreases. Idea C3 - Contact Interactions involving elastic objects: Another type of contact interaction occurs when two touching objects push or pull on each other and at least one of them is elastic (stretchy or compressible). In such contact interactions while the interacting objects are touching, there is a change in the speed (and/or direction) of at least one of the objects involved while the amount of compression or extension of at least one of them also changes. In terms of energy, during an elastic-type contact interaction there is transfer of mechanical energy between the interacting objects, and a change in the kinetic energy of at least one of them. There is also a change in the elastic potential energy of the elastic object involved. 1-124
Scientists Ideas: Interactions and Energy Idea C4 - Contact Interactions involving friction: A contact interaction also occurs when two surfaces rub against each other. The evidence of such a friction-type contact 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 interaction there is transfer of mechanical energy between the interacting objects and a decrease in the kinetic energy of at least one of the objects involved. There is also an increase in the thermal energy (which scientists usually call internal energy) of both objects. Though friction can never be totally eliminated, in some situations its effects can be neglected for simplicity. This is especially true during periods when other contact interactions (such as strong pushes and pulls) are having a strong enough effect on an object that including the effects of friction would make little or no difference. (Scientists refer to this idea as the impulse approximation.) Idea C5 - Opposing contact interactions of equal strength: It is possible for an object to be involved in more than one contact interaction at the same time. If, when acting alone, these two interactions would tend to have opposite effects on the speed (or direction) of the object, then it is possible for their effects to cancel each other, and thus the speed and direction of the object remain constant. (This also applies to a stationary object, which will remain stationary, with a constant speed of zero, all the time such conditions persist.) 1-125
Chapter 1 In terms of energy, a moving object involved in opposing interactions would be the energy receiver in one contact interaction and the energy source in another. If the mechanical energy transferred to the object in one interaction is exactly balanced by the mechanical energy transferred from the object in the second interaction, then the kinetic energy of the object will remain constant. 2 Ideas Involving Heat Conduction and Infrared Interactions It should come as no surprise that the idea of fluids or substances was used to explain the concept of heat in the 16 th and 17 th 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 interactions between objects with different temperatures 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 contact 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-19 th 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. 2 Movement must occur for mechanical energy transfer to take place. For an object that remains at rest, due to two opposing contact interactions, it is true that its kinetic energy remains constant (at zero), but in this particular case no mechanical energy is transferred to or from the object. 1-126
Scientists Ideas: Interactions and Energy 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. When such interactions occur in isolation, the warmer object decreases in thermal energy (hence, it decreases in temperature) and the cooler object increases in thermal energy (hence, it increases in temperature). The interaction stops when the two objects reach the same temperature. (However, if energy is being transferred into the warm object at the same time, via another interaction, it is possible that it will not decrease in thermal energy.) Idea H2 - A heat conduction interaction occurs when heat energy is transferred between any two objects that are touching and have different temperatures. Idea H3 - An infrared interaction occurs when heat energy is transferred between any two objects that are near one another and have different temperatures. 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 its value depends on 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. 1-127
Chapter 1 Idea H5 - During any real-world contact interaction between objects, the thermal energy (and hence temperature) of the objects usually increases, due to the effects of friction. Because of this some heat energy is almost always transferred from the warm objects to the surroundings (that is, nearby and touching objects), allowing the warm objects to cool down. The amount of heat energy transferred to the surroundings may be small (even negligible) 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 a warm object to its surroundings. Usually, when an object is warmer than its surroundings, there are both heat conduction and infrared interactions between the object and those 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. For example, when a hand is holding a hot cup of coffee, the interactions between the hand and cup, and between the cup and surroundings, can be described with the following I/O energy diagrams. 1-128
Scientists Ideas: Interactions and Energy Ideas Involving Energy Conservation Up until the 18 th 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 19 th century that energy became a primary concept on which physics was based. Throughout the 19 th 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 contact 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 contact 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 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. Rankine 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. 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 and transferred between objects. If, over a period of time, you keep track of the total amount of energy transferred into an object, the total amount of energy transferred out of an object, and the total energy change within the object, then: 1-129
Chapter 1 Energy Input = Change in Energy + Energy Output Total Energy INPUT Object or System Total Energy OUTPUT Energy CHANGES 1-130