Electric Current
The Basics - SOURCE
Ohm's Law
For many conductors
of electricity, the electric
current which will flow through them is directly proportional to the
voltage
applied to them. When a microscopic
view of Ohm's law is taken, it is found to depend upon the fact that
the drift velocity of charges through the material is proportional to
the electric field in the conductor. The ratio of voltage to current is
called the resistance,
and if the ratio is constant over a wide range of voltages, the material
is said to be an "ohmic" material. If the material can be
characterized by such a resistance, then the current can be predicted
from the relationship:
Data can be entered into any of the boxes below. Specifying any two of
the quantities determines the third. After you have entered values for
two, click on the text representing to third in the active illustration
above to calculate its value.
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Voltage Law
The voltage
changes around any closed loop must sum to zero. No matter what path you
take through an electric
circuit, if you return to your starting point you must measure the
same voltage, constraining the net change around the loop to be zero.
Since voltage is electric potential energy per unit charge, the voltage
law can be seen to be a consequence of conservation
of energy.
The voltage law has great practical utility in the analysis of
electric circuits. It is used in conjunction with the current
law in many circuit analysis tasks.
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Current Law
The electric
current in amperes which flows into any junction in an electric
circuit is equal to the current which flows out. This can be seen to
be just a statement of conservation of charge.
Since you do not lose any charge during the flow process around the
circuit, the total current in any cross-section of the circuit is the
same. Along with the voltage law, this law is a
powerful tool for the analysis of electric circuits.
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Electric Current
Electric current is the rate of charge flow past a
given point in an electric circuit, measured in coulombs/second which is
named amperes. In most DC
electric circuits, it can be assumed that the resistance
to current flow is a constant so that the current in the circuit is
related to voltage
and resistance by Ohm's
law.
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Electric Charge
The unit of electric charge is the coulomb. Ordinary matter is made
up of atoms which have positively charged nuclei and negatively charged
electrons surrounding them. Charge is quantized as a multiple of the
electron or proton charge: 
The influence of charges is characterized in terms of the forces between
them (Coulomb's
law) and the electric field and voltage produced by them. One
coulomb of charge is the charge which would flow through a 120 watt
lightbulb (120 volts AC) in one second. Two charges of one coulomb each
separated by a meter would repel each other with a force
of about a million tons!
The rate of flow of electric charge is called electric
current and is measured in amperes.
In introducing one of the fundamental properties of matter, it is
perhaps appropriate to point out that we use simplified sketches and
constructs to introduce concepts, and there is inevitably much more to
the story. No significance should be attached to the circles
representing the proton and electron, in the sense of implying a
relative size, or even that they are hard sphere objects, although
that's a useful first construct. The most important opening idea,
electrically, is that they have a property called "charge"
which is the same size, but opposite in polarity for the proton and
electron. The proton has 1836 times the mass of the electron, but
exactly the same size charge, only positive rather than negative. Even
the terms "positive" and "negative" are arbitrary,
but well-entrenched historical labels. The essential implication of that
is that the proton and electron will strongly attract each other, the
historical archtype of the cliche "opposites attract". Two
protons or two electrons would strongly repel each other. Once you have
established those basic ideas about electricity, "like charges
repel and unlike charges attract", then you have the foundation for
electricity and can build from there.
From the precise electrical neutrality of bulk matter as well as
from detailed microscopic experiments, we know that the proton and
electron have the same magnitude of charge. All charges observed in
nature are multiples of these fundamental charges. Although the standard
model of the proton
depicts it as being made up of fractionally charged particles called quarks,
those fractional charges are not observed in isolation -- always in
combinations which produce +/- the electron charge.
An isolated single charge can be called an "electric
monopole". Equal positive and negative charges placed close to each
other constitute an electric
dipole. Two oppositely directed dipoles close to each other are
called an electric
quadrupole. You can continue this process to any number of poles,
but dipoles and quadrupoles are mentioned here because they find
significant application in physical phenomena.
One of the fundamental symmetries of nature is the conservation
of electric charge. No known physical process produces a net change
in electric charge.
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Conventional Electric Current
Although it is electrons which are the mobile charge
carriers which are responsible for electric current
in conductors
such as wires, it has long been the convention to take the direction of
electric current as if it were the positive charges which are moving.
Some texts reverse this convention and take electric current direction
as the direction the electrons move, an obviously more physically
realistic direction, but the vast majority of references use the
conventional current direction and that convention will be followed in
most of this material. In common applications such as determining the
direction of force
on a current carrying wire, treating current as positive charge
motion or negative charge motion gives identical results. Besides the
advantage of agreeing in direction with most texts, the conventional
current direction is the direction from high voltage to low voltage,
high energy to low energy, and thus has some appeal in its parallel to
the flow of water from high pressure to low (see water
analogy).
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