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Voltage Sources: Batteries are designed to act as a change in electric altitude. Batteries kind of act like ski lifts--a positive charge that comes in the negative terminal of the battery is boosted to a higher altitude. The source of energy for a battery's ski lift comes from chemical processes. Those of you interested in the exact nature of this chemical process can read starting on Page 490 of the textbook; however, this is not recommended until after we've examined chemistry to an extent. Electric Current: A flow of charges along any path is referred to as a current. Electric currents can be likened to having a stream of skiers going down a slope. The current isn't necessarily constant and it can even be zero sometimes; it doesn't even have to flow through wires. However, it's perhaps best to start imagining it as a constant stream of skiers; the path for the skiers is like the wire through which current can flow. Electric current is measured in Ampéres, Amps for short. It is equal to the amount of charge per second flowing through a given point, so 1 A = 1 C/s. The skiing analogy fails somewhat in the consideration of alternating current, which is what you get from a typical electrical outlet. In ac circuits, the charges that are flowing are literally moving back and forth; the skiing equivalent would be having the mountainside oscillate between upslope and downslope. This might sound counterproductive, but work is being done on the charges and some of this work is being transferred into mechanical or heat energy. Alternating current is preferred because it allows for the transmission of electrical power with much less loss of energy. Electrical Resistance: Continuing our analogy of electrical circuits as skiers (or snowboarders, I'm not picky), we run into the concept of resistors. All materials except superconductors exhibit some degree of resistance to the flow of electric current. The skiing analogy would be requiring the skiers to go through a particular path. If the path is narrow, few skiers can pass through at any given time. If the path is narrow and long, few skiers will be completing the path during a given period of time. And if the path is really bumpy, the skiers will have to slow down in order to navigate their way through. These three properties of the path have analogies with electrical resistors. Resistors that are narrow have high resistance. Resistors that are long also have high resistance. And resistors have a third property, called resistivity, that is similar to a bumpy ski path. For instance, copper is very non-bumpy, so it has a low resistivity. Other materials, such as rubber, are extremely bumpy, meaning that they have high resistivity. These three properties combine to form the resistance of a particular object. It is important that you understand the difference between resistance and resistivity. All materials made of copper have the same resistivity because they're made of copper. But one copper wire might have a different resistance from another copper wire if the wires have different lengths and/or widths. Resistors have units of Ohms (), named for Georg Simon Ohm, a German physicist. With our understanding of voltage and resistance and current in place, we can develop Ohm's Law. This is actually an equation that applies to a lot of materials but not to all materials. Nevertheless, it is very useful because it applies to a lot of commonly-used materials such as copper. This law states that the amount of current you get along a path is equal to the voltage across the path divided by the resistance of the path: I = V/R. Thus, if I increase voltage, I increase current. If I increase resistance, I decrease current. The skiing analogy would say that if I were to keep the resistance constant, then I'm keeping the length of the skiing path the same, so if I increase the altitude, the only way I could do it would be to make the path steeper. This results in skiers going faster and coming out of the other end of the path more quickly. If I were to keep the change in altitude constant but increase the resistance, I increase the difficulty of getting through the path, so the number of skiers decreases. Magnetic Poles: Initial observations of magnets go back to Thales of Miletus, who first marked upon their attraction to each other. In fact, the word "magnet" derives its name for the region known as "Magnesia", where lodestone (naturally occurring magnets) was commonly found. In approximately 83 A.D, the Chinese made the first magnetic compass using lodestone. Pierre de Maricourt made observations of magnets and compasses in the 13th century. This work eventually led Gerardus Mercator in the mid16th century to the conclusion that the Earth acted as a gigantic magnet; in 1581, Robert Norman was able to experimentally verify this. In 1751, John Michell examined the phenomenon of inducing magnetic properties in another substance and came up with a force law between two magnetic fields. Also in 1751, Ben Franklin was the first person to really see a relationship between electricity and magnetism when he found that electricity could magnetize iron needles. This observation was more fully realized in 1820 when André Ampére found that there was a force on an electric current within a magnetic field and also when Hans Christian Oersted found that an electric current deflected a compass needle (Oersted's discovery actually came while he was giving a classroom demonstration and so this is widely regarded as the only thing ever learned in a class). Further knowledge of magnetism and its relationship with electricity was garnered over the course of the rest of the 19th century, culminating in James Clerk Maxwell's theory of electromagnetism (bolstered by the important experimental work of Michael Faraday and Heinrich Hertz). Magnets have an important distinction from electric charges--they always have a north pole and a south pole. This would be equivalent to all charges actually being composed of a positive charge and a negative charge separate but bound together. Magnets are therefore natural dipoles, unlike electrical charges, which can come as monopoles. This is true even if a magnet were cut in half--the result is two weaker magnets with independent north and south poles. However, magnets share a similar characteristic with electric charges: like poles repel and opposite poles attract. Magnetic Fields: Because magnets are natural dipoles, the shape of the magnetic field is identical to the shape of the electric field of two equal but opposite electric charges in proximity to each other. In a magnetic field, the lines point from the north pole to the south pole, making the north pole analogous to a positive charge. Thus, when a magnet is placed in an external magnetic field (such as a compass in the Earth's external magnetic field), it will align itself so that the north pole will point in the direction of the magnetic field (because that will bring it closer to the south pole); the south pole also has a desire to point towards the north pole of the external magnetic field. Because of these mutual tendencies, if a magnet is placed so as to no be in alignment with an external magnetic field, it will oscillate around until it's aligned properly. Magnetic fields arise from the motion of charge. Since a current is composed of moving charges, that means that currents produce magnetic fields. The magnetic field produced by a moving current circles around the current itself. This begins the explanation of why magnetic fields have north poles and south poles. Because if you view the current as coming straight towards you, then the magnetic field appears to circle around counterclockwise. But if the current is moving directly away from you, the magnetic field appears to be clockwise. This can be contrasted with a positive electric charge: if I view it from one side, it looks like a positive electric charge and if I view it from the other side it still looks like a positive electric charge. Magnetic fields arise from moving charges. Suppose we choose to have the charges moves about in a loop of wire (such as a solenoid, which is a long sequences of loops of wire through which current can flow). If we examine the direction of the magnetic field generated by a loop of current, we see that the magnetic field lines consistently come in through one side of the loop and come out through the other side of the loop. The side into which the magnetic field lines point would therefore by the south pole and the side from which the magnetic field lines emanate become the north pole. It is thus impossible for a current to generate a magnetic monopole.