03-01-2013, 12:26 PM
Lessons In Electric Circuits, Volume I - DC
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BASIC CONCEPTS OF ELECTRICITY
Static electricity
It was discovered centuries ago that certain types of materials would mysteriously attract one another
after being rubbed together. For example: after rubbing a piece of silk against a piece of glass, the
silk and glass would tend to stick together. Indeed, there was an attractive force that could be
demonstrated even when the two materials were separated:
Glass and silk aren't the only materials known to behave like this. Anyone who has ever brushed
up against a latex balloon only to ¯nd that it tries to stick to them has experienced this same phe-
nomenon. Para±n wax and wool cloth are another pair of materials early experimenters recognized
as manifesting attractive forces after being rubbed together:
STATIC ELECTRICITY 3
after being rubbed could be classed into one of two distinct categories: attracted to glass and repelled
by wax, or repelled by glass and attracted to wax. It was either one or the other: there were no
materials found that would be attracted to or repelled by both glass and wax, or that reacted to
one without reacting to the other.
More attention was directed toward the pieces of cloth used to do the rubbing. It was discovered
that after rubbing two pieces of glass with two pieces of silk cloth, not only did the glass pieces repel
each other, but so did the cloths. The same phenomenon held for the pieces of wool used to rub the
wax:
Conductors, insulators, and electron °ow
The electrons of di®erent types of atoms have di®erent degrees of freedom to move around. With
some types of materials, such as metals, the outermost electrons in the atoms are so loosely bound
that they chaotically move in the space between the atoms of that material by nothing more than
the in°uence of room-temperature heat energy. Because these virtually unbound electrons are free
to leave their respective atoms and °oat around in the space between adjacent atoms, they are often
called free electrons.
In other types of materials such as glass, the atoms' electrons have very little freedom to move
around. While external forces such as physical rubbing can force some of these electrons to leave
their respective atoms and transfer to the atoms of another material, they do not move between
atoms within that material very easily.
This relative mobility of electrons within a material is known as electric conductivity. Conduc-
tivity is determined by the types of atoms in a material (the number of protons in each atom's
nucleus, determining its chemical identity) and how the atoms are linked together with one another.
Materials with high electron mobility (many free electrons) are called conductors, while materials
with low electron mobility (few or no free electrons) are called insulators.
Electric circuits
You might have been wondering how electrons can continuously °ow in a uniform direction through
wires without the bene¯t of these hypothetical electron Sources and Destinations. In order for the
Source-and-Destination scheme to work, both would have to have an in¯nite capacity for electrons
in order to sustain a continuous °ow! Using the marble-and-tube analogy, the marble source and
marble destination buckets would have to be in¯nitely large to contain enough marble capacity for
a "°ow" of marbles to be sustained.
Voltage and current
As was previously mentioned, we need more than just a continuous path (circuit) before a continuous
°ow of electrons will occur: we also need some means to push these electrons around the circuit.
Just like marbles in a tube or water in a pipe, it takes some kind of in°uencing force to initiate °ow.
With electrons, this force is the same force at work in static electricity: the force produced by an
imbalance of electric charge.
If we take the examples of wax and wool which have been rubbed together, we ¯nd that the
surplus of electrons in the wax (negative charge) and the de¯cit of electrons in the wool (positive
charge) creates an imbalance of charge between them.