Most of
us are familiar with magnetic fields. Small magnets are surrounded with an
invisible "magnetic field" which pulls on iron, and which can attract or
repel other magnets. Obtain a small compass, and you'll see the little
compass pointer twist and align with the magnetic field of the earth. That's
magnetism. There is another type of invisible field besides magnetism.
It is called the "electric field" or "e-field." This second kind of field is
a lot like magnetism: it's invisible, it has lines of flux, and it can
attract and repel objects. However, it is not magnetism, it is something
separate. It is voltage.
Whenever a negative charge attracts a positive charge,
invisible fields of voltage must exist between the charges. Voltage causes
the attraction between opposite charges; the voltage fields reach across
space. In reality, "static" electricity has nothing to do with motion (or
with being static), instead it involves high voltage.
Whenever charges flow
through a wire, they only move because they're being driven along by a
voltage-field which runs through the length of the wire. "Voltage" causes
electric currents in wires. Electric current is caused by "static
electricity," and "static electricity" is not necessarily static. The
connection between voltage and "static" electricity is not explained in the
books, and that's one main reason why voltage seems so complicated and
mysterious.
Voltage is a way of using
numbers to describe an electric field. Electric fields or "E-fields" are
measured in volts over a distance; volts per centimeter for example. A
stronger e-field has more volts per centimeter than a weaker field. Voltage
and e-fields are basically the same thing. E-fields can exist in the
air, and so can voltage. If you have a high voltage across a short distance,
you have strong e-fields. When an e-field is attracting or repelling an
object, we instead could say that the object is being driven by the voltage
in the space around the object.
Voltage is closely connected with electrical energy.
Electrical kinetic energy appears whenever positive charges flow through
negative charges. We call this "electric current," and it causes magnetism.
Electrical potential energy appears when positive charges are yanked away to
a distance from their corresponding negative charges. We call this "net
electrostatic charge," and it causes voltage. Electrical KE is associated
with current, and electrical PE is associated with voltage.
So, to simplify our mental picture of
electric currents, we intentionally define electric current as being a flow
of exclusively positive particles flowing in one particular direction. We
don't care about the real polarity of the particles. We don't care about
their speed, and we don't care about their number. We ignore the chemical
effects and the effects of the velocity and direction moving particles. We
ignore the collisions between positive and negative particles. All we care
about is the total charge which moves past a particular point in the
circuit.
Once we start ignoring the speed and direction of the
charges, we can then build "amp meters," which measure the electric current
in terms of the magnetism it creates, or by the voltage drop which appears
across a resistor, or by the temperature rise being created in a calibrated
piece of resistance wire. In nearly every situation they will tell us
all we could ever want to know about flows of charged particles in any
circuit. An amp-meter might not be appropriate when used in an exotic
physics experiment. But for more than 99% of electricity and electronics,
the direction of the particles is irrelevant, and an ammeter tells us the
"real" current.
We do cause some problems in choosing to simplify
"Electric Current" in this way. For example, what if we think in terms of
simplified electric current for many, many years? Couldn't we all eventually
come to believe that this oversimplified concept of electric current is
REAL? Yet it's not real, it is simply one aspect of flowing particles. For
this reason, we might start to see "Electric Current" as a sort of abstract,
invisible, difficult-to-visualize thing. We might lose track of the facts
that electric current is an actual flow of matter, or that there are real,
visible particles flowing along inside that circuit, or that they have a
particular average speed, mass, and direction.
Electric current in copper wire is a flow of electrons,
but these electrons are not supplied by batteries or generators. They
come from the wire. In copper wire, copper atoms supply the flowing
electrons. The electrons in a circuit were already there before the battery
was connected. They were even there before the copper was mined and made
into wires! Batteries and generators do not create these electrons, they
merely pump the electrons. In order to understand electric circuits,
we must imagine that all the wires are pre-filled with a sort of "liquid
electricity."
To clarify this, get rid of the battery. Instead, use a
hand-cranked generator as your power supply. Ask yourself exactly where the
flowing "electricity" comes from when a generator powers a light bulb. A
generator takes electrons in from one terminal and simultaneously moves them
out the other one. At the same time, the generator pushes electrons through
the moving coil of wire inside itself. It also pushes them through the rest
of the circuit. Unlike the situation with a battery-powered circuit, all we
have here is wires. Where is the source-point of this flowing "electricity?"
By including the generator in the circuit, we find that the circuit is a
continuous closed loop, and we can find no single place where the
"electricity" originates. A generator or battery is like a
closed-loop pump, but it does not supply the substance being pumped. We were taught during grade-school that "batteries and generators
create Current Electricity." A better statement would be:
"Batteries and generators cause
electric charge to flow."
To complete
the picture, add:
"all conductors are full of
a movable charge."
That's what
a conductor is, it's a material which contains a movable charge.
When a generator stops, or when a switched circuit is opened, all the
electrons stop where they are, and the wires remain filled with electric
charges because the copper wires were full of vast quantities of charges in
the first place. For example in copper, electric current is the flow of electrons. Many
books claim that these electrons flow at the speed of light, but this is
incorrect. Electrons actually flow quite slowly, at speeds on the order of
centimeters per minute. It's the energy in the circuit which flows fast, not
the electrons. When the electrons at one point in the circuit are pumped,
electrons in the entire loop of the circuit are forced to flow, and energy
spreads almost instantly throughout the entire circuit. This happens even
though the electrons move very slowly.
