Download Observations of electricity go back to the discovery of static cling

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.