What is Voltage?
(c)1998
William
J. Beaty
![[magnetic field around bar magnet, electric field around electric charges]](http://amasci.com/graphics/voltage.gif)
Of several electricity concepts, the
idea of "voltage" or "electrical
potential" is probably the hardest
to understand.
It's also really tough to explain.
It's a headache for both the student and
the teacher. <GRIN!> To understand
voltage, it helps if you first
understand a little about its nearest
relative, magnetism.
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. The magnetic field
causes oblong magnetic objects (such as
iron rods, or iron powder) to twist and
align to follow particular directions.
Put a bar magnet under a piece of paper,
sprinkle on some iron filings, and the
filings line up and show the general
shape of the invisible field. 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 "electrostatic
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.
Most people know about magnetic
fields but not about e-fields or
"voltage fields." In part, this is
because magnetism is explained in
school, but for some reason voltage
fields are hidden away under the name
"static electricity," and they're never
mentioned in beginner's science
textbooks. This is odd, since voltage
and "static electricity" go together.
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. Scuff across a
rug, and you charge your body to several
thousand volts. When you take a wool
sock out of the clothes dryer and all
the fibers stand outwards, the fibers
are following the invisible lines of
voltage in the air. Fibers are the "iron
filings" that make the voltage patterns
visible. And 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
dryer-cling, but it also causes electric
currents in wires. Another way to say
it: 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.
The Simple Math Behind "Voltage"
To be a bit more specific, 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: if e-fields
are like the slope of a mountainside,
then the volts are like the various
heights of each different spot on the
mountain. The slope of a mountainside
can make a boulder start rolling. So can
the differing heights of the different
points on the mountain, it's just
another way to describe the same thing.
"Voltage" and "e-fields" are two ways to
describe the same basic concept. When you have e-fields, you have voltage. 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.
How High is my Voltage?
Can an object have a certain voltage?
No. Why not? Well, please tell what my distance is. What is my distance? It's a
ridiculous question, because I didn't
say my distance FROM WHAT. Voltage is a
bit like length, it is a measurement
made BETWEEN two things. My distance is
300ft above sea level, but is also 1cm
from the floor (since I'm not barefoot,)
and it's also 93 million miles from the
sun. My voltage might be -250 Volts in
relation to the earth, but it also might
be billions of volts when compared to
the moon. Volts are always measured
along the flux lines of electric field,
therefore voltage is always measured
between two charged objects. If I start
at the negative end of my flashlight
battery, I can call that end "zero
volts", and so the other end must be
positive 1.5 volts. However, if I start
at the POSITIVE end instead, then the
positive battery terminal is zero volts,
and the other terminal is negative 1.5
volts. Or, if I start half way between
the battery terminals, then one terminal
is -.75 volts, and the other terminal is
+.75 volts. OK, what is the REAL voltage
of the positive battery terminal: zero,
or +1.5, or +.75 volts? Nobody can say.
The terminal can have several voltages
at the same time. But this is no big
deal, because neither can anyone tell
you the battery's distance! We can
easily imagine the distance between two
points, and we can also imagine the
voltage between two points. But single
objects don't "have distance", and
single objects also don't "have
voltage."
Un-twisting the Terminology
You've probably heard of
"electromagnetic fields" and
"electromagnetism." In the word
Electromagnetism, the term "electro"
does not refer to electricity. Instead
it refers ...to voltage!
Electromagnetism is the study of
e-fields and magnetic fields:
electro/magnetism. Charge flow (electric
current) is intimately associated with
magnetism, and separated opposite
charges are intimately associated with
voltage. A flow of electromagnetic
energy along a cable is composed half of
electric current, and half of voltage.
It is "voltagecurrent," it is
electrostatic/magnetostatic, it's
electro-magnetism. Electromagnetism is a
two-sided coin, so what is voltage? It's
one side of EM (the other side being
magnetism.) Besides not being found in elementary school science books, Voltage is
also missing from our everyday language.
If we have no common words to describe
something, we tend to never talk about
it. We even have trouble believing it
exists. For example, we have the word
"magnetism", and most people have heard
of magnetic fields. ELECTRIC fields
exist too. Unfortunately "electri-cism"
is not an English word. Everyone can
discuss magnetism, but nobody ever talks
about "electricism." Without the word
"electricism," we have a hard time
talking about electric fields and
electric attraction/repulsion forces,
and we never realize that they are
important in electric circuits. Yet
there is a word we could use instead of
"Electricizm." We don't have to coin
some weird new term.
If magnetism is "that which involves
magnetic fields", then what is "that
which involves electric fields?"
Voltage!
Pick up some nails with a magnet, and
that's an example of magnetism, then
pick up some bits of paper with a
fur-rubbed balloon, and that's an
example of voltage. What are the three
kinds of invisible field? Gravity,
magnetism... and voltage! Perhaps we should change the word "Electromagnetism" into
"Voltagemagnetism?" (grin!)
|
VOLTAGE SURROUNDS TWO ELECTRIC
CHARGES |
MAGNETISM SURROUNDS A MAGNET'S
POLES |
Electromagnetic Duality
Voltage and magnetism form a pair of
twins; they are two halves of a duality.
Physicists and engineers even use the
word "dual" to describe them: voltage is
the "dual" of magnetism, and magnetism
is the "dual" of voltage. This duality
raises its head in many places in the
physical sciences. One small analogy: A
spinning flywheel can store energy. So
can a compressed spring. In electrical
physics, a superconductor ring can store
energy in the form of magnetism, and a
capacitor can store energy in the form
of voltage. A coil of wire and a
capacitor are the "duals" of each other,
since one involves magnetism, and the
other is based on voltage. Voltage Energy
Voltage is intimately connected with
electrical energy. So is magnetism. We
can even say that electrical energy is
the fundamental object of our study,
while voltage and magnetism are the two
faces it displays to the outside world.
Another analogy: in mechanical physics,
both the Kinetic energy (KE) and the
Potential energy (PE) are part of
matter: relative motion of an object has
Kinetic Energy, and stretched or
compressed objects (e.g. springs or
rubber bands) have Potential Energy. In
a similar way, 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. If
electrical energy is the same as
Electromagnetism, then maybe we should
be more sensible and name it
"VoltageCurrent-ism." Potential Energy vs. "Potential"
Voltage is also called "electrical
potential." So... is voltage a type of potential energy? Close, but not totally
accurate. Think of it like this. If you
roll a big boulder to the top of a hill,
you have stored some potential energy.
But after the boulder has rolled back
down, THE HILL IS STILL THERE. The hill
is like voltage: the height of the hill
has "Gravitational Potential." But the
hill is not *made* of Potential Energy,
since we need both the hill *and* the
boulder before we can create potential
energy. The situation with voltage is
similar. Before we can store any
ELECTRICAL potential energy, we need
some charges, but we also need some
voltage-field through which to push our
charges. The charges are like the
boulder, while the voltage is like the
hill (volts are like height in feet.
Well, sort of...) But we wouldn't say
that the Potential Energy is the
boulder, or we wouldn't say the hill is
the PE. In the same way, we should not
say that electric charges are Potential
Energy, neither should we say that
voltage is Potential Energy. However,
there is a close connection between
them. Voltage is "electric potential" in
approximately the same way that the
height of a hill is connected with
"gravitational potential." You can push
an electron up a voltage-hill, and if
you let it go it will race back down
again.
Currents don't have Voltage
Voltage is not a characteristic of
electric current. It's a common mistake
to believe that a current "has a
voltage" (and this mistake is probably
associated with the 'current
electricity' misconception, where people
believe that 'current' is a kind of
substance that flows). Voltage and
current are two independent things. It
is easy to create a current which lacks
a voltage: just short out an
electromagnet coil. It is also easy to
create a voltage without a current:
flashlight batteries maintain their
voltage even when they are sitting on
the shelf in the store. Water analogy:
Think of water pressure without a flow.
That's like voltage alone. Now think of
water that's coasting along; a water
flow without a pressure. That's like
electric current alone.
"Kinds" of Electricity?
Grade-school textbooks wrongly teach
that electricity comes in two types:
static electricity and current
electricity. These textbooks would be
much closer to the truth if they instead
said this:
The two halves of "electricity"
are "voltage electricity" and
"current electricity."
Still misleading, since the meaning of
the word "electricity" is not clearly
defined. It would be better if they said
that electrical energy has two main
characteristics: voltage and current.
But the above statement is not nearly as
bad as the stuff they teach about
"static vs. current." For one thing, the stillness of the charges is not important. "Static"
electricity is NOT electricity which is
static. Instead, "static charge" really
means "separated opposite charges". We
should not be surprised to learn that
"static electricity" is able to flow
from place to place without losing any
of its characteristics. Maybe it's not
"static" anymore, but this doesn't
matter, since a separation of charge can
move along. It's the IMBALANCE between
opposite charges that's important, and
their "static-ness" is not.
NOTE: Do you see how K-6 textbook
authors could be playing a game of
'telephone?' In this game, words are
progressively distorted by errors in
communication. In K-6 textbooks the
science concepts become more and more
distorted over the years. Authors are
taught from earlier textbooks, and often
they get their information directly from
modern textbooks. Then they write new
ones. If authors make mistakes, what
will happen? Start out by saying
"electromagnetism has two complimentary
halves, voltage and current". Decades
later we end up with books which are
teaching kids something like this: "the
two forms of electricity are static
electricity and current electricity."
Wrong. Yet we can see where the crazy
stuff originally came from.
Seeing the Invisible Voltage
Magnetic fields are invisible, and so is
voltage. Both can be made visible. Iron
filings let us see magnetic fields. To
see voltage, suspend some metal or
plastic fibers in oil, or sprinkle grass
seeds on a pool of glycerine. If we then
expose the oil to the strong
voltage-field surrounding a charged
object, the fibers or grass seeds will
line up and
show the shape of the field. Rub a
balloon on your head, hold it near the
suspended fibers, and you'll "see" the
lines of e-field flux. Measuring Voltage
To measure current, we allow the
magnetism around a coil of wire to
deflect a compass needle. To measure
voltage, we allow the "electricism"
between a pair of delicately suspended
metal plates to deflect one of those
plates. The simplest voltmeter is called
a "foil-leaf electroscope." We find such
things in books about "static
electricity", when they really should be
in all electronics books. A more
complicated version of the foil-leaf
electroscope is called a "quadrant
electrometer." These two devices can
measure voltage directly, without
creating any electric current at all.
Besides the moving capacitor plates,
there are a few
other ways to measure voltage too. The Voltage of Light
Here's a strange idea: Flowing
Electromagnetic energy always has
voltage. For example, if you touch the
antenna of a powerful radio transmitter,
you can receive an electric shock
because of the high voltage at the
antenna. Radio waves are
electromagnetism, and the intense waves
surrounding a radio transmitter's
antenna will have a high voltage-field.
Radio waves can be measured in terms of
voltage. Even the brightness of the
light from the sun can be measured in
terms of volts per meter. So can the
energy which comes from the electric
generators and flows along wires to a
120v table lamp. All of these involve
electric fields (and voltage), and
magnetic fields (and current.) Expose All Students to High Voltage!
:)
"High voltage." Might you already know
what that is? It's not just the
dangerous devices behind the electric
company fence. High voltage is also
balloons rubbed upon your hair, and
"static electric generators" and their
very long sparks. You might be
interested to know that ALL voltage does
the same things as "High Voltage." The
effects are just weaker. Understand
"high voltage," and you'll understand
voltage itself. High voltage devices are
not just toys, they are educational:
they let us experience voltage directly.
If you want to understand magnetism,
then play with electromagnet coils and
bar magnets. If you want to understand
voltage, then get yourself a VandeGraaff
generator. Voltage has wrongly been hidden within "static electricity" and
declared to be an obsolete and useless
science, important only for historical
reasons. But in a certain sense, "static
electricity" *IS* voltage. Static
electricity is a high-voltage phenomena.
If we stop teaching about "static
electricity," and regard it as ancient
and useless "Ben-Franklinish" stuff,
then we also stop teaching about
voltage. Can you see why voltage has
become such a mystery? We've nearly
eliminated "static electricity" from
high school science classes, and so
we've also throw away our basic voltage
concepts.
MISC.
Imagine a waterwheel being turned by a
stream of water pouring from above. If
the water is like the flowing electric
charge, and the waterwheel is like an
electric motor, then what is voltage?
Voltage is like the height of the stream
above the wheel, or like its slope from
the top of the wheel to the pool below.
Without a height difference, there can
be no water current and no work done by
the waterwheel. Without a voltage
difference across an electric motor,
there can be no electric current and no
work done by the motor. voltage is like an electrical pressure or push, it can cause electric
charges to flow. Or, if flowing charge
is suddenly blocked, this can cause a
voltage to appear. But current can exist
without voltage, and voltage can exist
without current.
voltage exists in space, not just on
surfaces. Rub an inflated balloon on
your arm hairs, then wave the balloon
around so it makes the hairs stand up.
You are seeing and feeling voltage in
the space between the balloon and your
arm. Think about a 9v battery. The 9
volts aren't on the surface of the
battery terminal, they are in the space
between the terminals, like the magnetic
field between a north and a south pole.
A 9v battery is like an "electret", the
electric version of a bar magnet.
An inductor (an electromagnet coil)
is an electric current device. A
capacitor is an electric voltage device.
If energy is stored in a shorted coil,
the energy is in the surrounding
magnetic field, and there must be an
electric current circulating in the
coil. If energy is stored in a
non-shorted capacitor, the energy is in
the voltage field between the plates. If
the short is suddenly removed from the
inductor, there is a loud bang, and a
huge voltage briefly appears. If a short
is suddenly connected to a capacitor,
there is a loud bang and a huge current
briefly appears. Capacitor, coil.
Electro, magnetism. "EM" energy.
voltage is the stuff that connects
the protons and electrons of atoms to
each other, and it connects atoms
together to form objects. Pull on your
finger, and you are feeling the
microscopic voltage between the atoms.
Without voltage, there would be no
solids or liquids in the universe, just
gas. When you break a solid object, you
are defeating the attractive microscopic
voltages which were binding it's atoms
together.
The bonds between atoms are often
associated with a constant voltage. If
one atom is positive and the other
negative, then there is a voltage
between them. If billions of atoms could
be line up in parallel, the voltage of
the atoms could be easily measured. What
would happen if we could align billions
of atoms in parallel? We've just
re-invented the battery. A battery is a
couple of metal plates immersed in
liquid. At the surface of the liquid
where it touches each plate, all the
atoms line up in parallel, and a voltage
appears between the liquid and the
metal. That's what causes the voltage of
any battery: the micro-thin layer of
atoms at the surface of the metal plates
inside the battery. Everything else in
the battery is just there to provide the
electrical connections and the chemical
fuel supply. Ideally, a flashlight
battery could be three atoms thick (a
thin film of liquid sandwiched between
two thin metal films,) and it would
still put out 1.5 volts.
Everyday electric motors operate by
magnetic forces surrounding a coil, with
electric current in the windings of the
coil. Let's call this sort of device by
the name "current motor". Electric
motors in everyday life are invariably
"current motors", but "voltage motors"
exist too. They operate because of
voltage-forces between charged objects.
The microscopic motors used in
cutting-edge nanotechnology are voltage
motors. The linear chemical-motors
inside your muscles are voltage motors.
The spinning cilia on the tail ends of
bacteria are little voltage motors. The
mechanical enzymes which assemble ATP
molecules (the 'energy molecules' of the
cell) are voltage motors. The tiny
microscopic parts inside a living cell
are like little robots. They all rely on
voltage motors, none use magnetic
motors.
Potential energy involves stretching,
squeezing, pressure and forces. Voltage
is associated with electric charge which
has been "stretched" or "pressurized."
Spin a flywheel, that's an analogy for
electric current and magnetism. Stretch
a rubber band, that's an analogy for
voltage and charge separation.
Is magnetism like a warping of space?
Then so is voltage. Voltage and
magnetism can be combined to become a
traveling wave of warped space. We call
these waves "light," or "radio," or
"electrical energy." When the Electric
Utility Companies sell you some
"electricity", they really are selling
you pulses of "space warp" which are
guided to you by a pair of copper wires.
They are selling you a combination of
voltage and current. When voltage and
current are there, electromagnetic
energy is flowing down the wires. |
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