CHAPTER 3 Scientists’ Ideas INTERACTIONS, SYSTEMS, AND POTENTIAL ENERGY In this Chapter you developed some ideas involving three different ‘action-at-a- distance’ 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 Newton recognized that his ideas about gravity also represented ‘action-at-a-distance’ ©2007 PET 3-87 Chapter 3 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: 3-88 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. 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: 3-89 Chapter 3 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: Force exerted on Force exerted on Magnet B by Magnet A Magnet B by hand Magnet A Magnet B B 3-90 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 Both energy, the amount of which depends on how the Magnet-carts 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 Decrease in a system with no energy inputs or outputs (a closed magnetic system), then, according to the Law of Conservation potential energy of Energy, if the magnetic potential energy in the closed system increases (decreases), then the kinetic Increase in energy of the objects in the system will decrease kinetic energy (increase), and vice versa. For example, when two carts with magnets attached During time that both push each other apart, the energy diagram for this magnet-carts are pushing system would be like this: each other further apart Other evidence/examples: 3-91 Chapter 3 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: 3-92 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.