HOW ARE WATTS, OHMS, AMPS, AND VOLTS RELATED?
(c)1999
William
J. Beaty Here's the extremely short answer. Conductors are always full of
movable electric charges. Voltage pushes charges through
the conductive object which has a certain amount of
electrical resistance or "friction," and this heats up
the resistive object. The flow rate of the moving
charges is measured in amperes, while the transfer of
electrical energy as well as the rate of heat output is
measured in watts. The electrical resistance is measured
in ohms.
First the watts and amperes. Watts and amps are
somewhat confusing because these are the names of flows,
yet we rarely talk about the STUFF that flows. (Could we
ever understand water-flow without first grasping the
"water" concept?!) Electric current isn't a stuff.
Electric current is the flow of a stuff. What's
the name of the stuff that flows during an electric
current? The flowing stuff is called "Charge."
AMPERES What flows in wires?
- Charges of electricity
- Electrons
- Electric charge
- Electrical substance
- "Charge-stuff"
A quantity of charge is measured in units called
COULOMBS, and the word "ampere" means the same thing as
"one coulomb of charge flowing per second." If we were
talking about water, then Coulombs would be like
gallons, and amperage would be like gallons-per-second
of water flow.
Why do I think amperes are confusing? Simple: textbooks almost always
teach us about amperes and current without first clearly
explaining coulombs and charge! Suppose that you had no
name for water, yet your teachers wanted you to learn
all about "flow". Suppose you had to understand
"gallons-per-second," but you had to do this without
knowing anything about water or about gallons. If you'd
never learned the word "gallon", and if you had no idea
that water even existed, how could you hope to
understand "flow?" You might decide that "flow" was an
abstract concept. Or you might decide that invisible
wetness was moving along. Or you might just give up
on trying to understand the ideas. Concentrate on the
math and get the right answers on the tests, but end up
with no gut-level understanding. That's the problem with
electricity and amperes.
We can only understand the electrical flow in wires
(the amperes) if we first understand the stuff that
flows in wires: the charge, the particle-sea, the
Coulombs.
CHARGE
"Charge" is the stuff inside wires, but usually nobody
tells you that all metals are always full of
movable charge. Always. A hunk of metal is like a tank
full of water, where the "water" is the movable electric
charge inside the metal. In physics classes we call this
"the electron sea" or even "electric fluid." This
movable charge is part of all metals. In copper, the
electric fluid is actually the outer electrons of all
the copper atoms. In metals, the outer electrons of all
the atoms do not orbit the individual atoms as diagrams
usually show. Instead they drift around inside the metal
as a whole.
The movable charge-stuff within a metal give the metal its silvery
color. We could even say that charge-stuff is like a
silver liquid (at least it appears silver when it's in
metals. When i some other materials the movable charges
don't look silvery, so it's not a hard and fast rule.)
Note that this charge-stuff is "uncharged", it is
neutral. It's uncharged charge. Is this impossible? No.
On average, the charge inside a metal is neutral because
each movable electron has a corresponding proton nearby,
and the electric force fields from the opposite charges
cancel each other out. The overall charge is zero
because equal quantities of opposite polarity are both
present. For every positive there is a negative. But
this doesn't mean that the charge-stuff is gone! Even
though the charge inside a metal is cancelled out, we
can still force one polarity of charge to move along
while the other polarity remains still. Metal is made of
negative electrons and positive protons, it's like a
positive sponge soaked with negative liquid. We can make
the "negative liquid" flow along.
ELECTRIC CURRENT Whenever the charge-stuff within metals is forced to
flow, we say that "electric currents" are created. The
word "current" simply means "charge flow." We normally
measure the flowing charges in terms of amperes. The
faster the charge-stuff moves, the higher the amperage.
Watch out though, since amperes are not just the speed
of the charges. The MORE charge-stuff that flows
(through a bigger wire for example,) the higher the
amperage. A fast flow of charge through a narrow wire
can have the same amperes as a slow flow of
charge through a bigger wire. Double the speed of
charges in a wire and you double the current. But if you
keep the speed constant, then increase the size of the
wire, you also increase the amperes.
Here's a way to visualize it. Bend a metal rod to form a ring, then
weld the ends together. Remember that all metals are
full of "liquid" charge, so the metal ring acts like a
water-filled loop of tubing. If you push a magnet's pole
into this ring, the magnetic forces will cause the
electron-stuff within the whole ring to turn like a
wheel (as if the ring contained a movable drive-belt).
By moving the magnet in and out of the metal donut, we
pump the donut's movable charges, and the charges flow.
That's essentially how electric generators work.
Electric generators are magnet-driven charge pumps.
The moving magnetic fields push the wire's movable sea
of charges, creating the amperes of charge flow, but
this can only occur when a closed ring or "complete
circuit" exists. Break the ring and you create a
blockage, since the charges can't easily escape the
metal to jump across the break in the ring. A complete
ring is a "closed electric circuit," while a broken ring
is an "open circuit."
Cut the ring and install a battery in the cut. This
lets the battery pump the ring's charge-stuff in a
circle. Batteries and generators are similar in that
both can pump charge through themselves and back out
again. With a battery installed in our metal ring, the
battery draws charge into one end and forces it out the
other, and this makes the entire contents of the metal
ring start moving. Make another cut in the metal ring,
install a light bulb in the cut, and then the "friction"
of the narrow light bulb filament against the flowing
charge-stuff creates high temperatures, and the wire
filament inside the bulb glows white-hot. The battery
drives the ring of charge into motion, the charge moves
along like a drive belt, and the light bulb "rubs"
against the moving charge, which makes the filament grow
hot.
Important note: usually the charge-stuff flows
extremely slowly through the wires, slower than
centimeters per minute. Amperes are an extremely slow,
circular flow. See
SPEED
OF ELECTRICITY for info.
WATTS
"Watts" have the same trouble as amperes. Watts are the
name of an electrical flow... but what stuff does the
flowing? Energy! A "watt" is just a fancy way of saying
"quantity of electrical energy flowing per second." But
what is a quantity of electrical energy? I'll get to
that in a sec. Any sort of energy is measured in terms
of Joules. A joule of electrical energy can move from
place to place along the wires. When you transport one
joule of energy through a channel every second, the
flow-rate of energy is 1 Joule/Sec, and "one Joule per
second" means "one watt."
What is power? The word "power" means "energy flow." In order to
understand this stuff, it might help if you avoid using
the word "power" at the start. The word "power" means
"energy flow", so if you first practice thinking in
terms of energy flow instead of in terms of power, and
also think in terms of joules per second rather than
watts, eventually you'll gain a good understanding of
the ideas behind them. Then, once you know what you're
talking about, you can start speaking in shorthand. To
use the shorthand, don't say "energy flow", say "power."
And say "watts" instead of "joules per second." But if
you start out by saying "power" and "watts", you might
never really learn what these things are, because you
never really learned about energy flow and joules.
FLOWING ELECTRICAL ENERGY
OK, what then is electrical energy? It has another name:
electromagnetism. Electrical energy is the same stuff as
radio waves and light. It's composed of magnetic fields
and electrostatic fields. A joule of radio waves is the
same as a joule of electrical energy. But what does this
have to do with understanding electric circuits? Quite a
bit! I'll delve deeper into this. But first...
How is electric current different than energy flow? Let's take
our copper ring again, the one with the battery and the
light bulb. The battery speeds up the ring of charge and
makes it flow, while the light bulb keeps it from
speeding up too much. The battery also injects joules of
electrical energy into the ring, and the light bulb
takes them out again. Joules of energy flow continuously
between the battery and the bulb. They flow at nearly
the speed of light, and if we stretch our ring until
it's thousands of miles long, the light bulb will still
turn off immediately when the battery is removed. (Well,
not really immediately. There will still be some
joules left briefly moving along the wires, so the bulb
will stay on for a tiny instant , until all the energy
arrives at the bulb.) Remove the battery, and the light
bulb goes dark ALMOST instantly.
AMPERES ARE NOT A FLOW OF ENERGY Note that with the battery and bulb, the joules of
energy flowed ONE WAY, down BOTH wires. The battery
created the electrical energy, and the light bulb
consumed it. This was not a circular flow. The energy
went from battery to bulb, and none returned. At the
same time, the charge-stuff flowed slowly in a circle
within the entire ring. TWO THINGS WERE FLOWING AT THE
SAME TIME THROUGH THE ONE CIRCIUT. There you have the
difference between amperes and watts. The coulombs flow
slowly in a circle, while the joules flow rapidly from
an "energy source" to an "energy sink". Charge is like a
rubber drive belt, and electrical energy is like the
'horsepower' sent between the distant parts of the belt.
Amperes are slow and circular, while watts are fast and
one-way. Amperes are a flow of copper charges, while
watts are a nearly-instant flow of electrical energy
created by a battery or generator.
But WHAT ARE JOULES? That's where the electromagnetism comes in. When
joules of energy are flying between the battery and the
bulb, they are made of invisible fields. The energy is
partly made up of magnetic fields surrounding the wires.
It is also made from the electric fields which extend
between the two wires. Electrical-magnetic.
Electromagnetic fields. The joules of electrical energy
are the same "stuff" as radio waves. But in this case
they're attached to the wires, and they flow along the
columns of movable electrons inside the wires. The
joules of electrical energy are a bit like sound waves
which can flow along an air hose. Yet at the same time,
electrical energy is very different than sound waves.
The electrical ENERGY flows in the space around the
wires, while the electric CHARGE flows inside the wires.
VOLTS
There is a relationship between amperes and watts. They
are not totally separate. To understand this, we need to
add "voltage" to the mix. You've probably heard that
voltage is like electrical pressure. What's usually not
taught is that voltage is a major part of static
electricity, so whenever we deal with voltage, we're
dealing with static electricity. If I grab electrons
away from a wire, that wire will have excess protons
left behind. If I place those electrons into another
wire, then my two wires have oppositely-imbalanced
charge. They have a voltage between them too, and a
static-electric field extends across the space between
them. THIS FIELD *IS* THE VOLTAGE. Electrostatic fields
are measured in terms of volts per distance, and if you
have an electric field, you always have a voltage. To
create voltage, take charges out of one object and stick
them in another. You always do this when you scuff your
shoes across the carpet in the wintertime. Batteries and
generators do this all the time too. It's part of their
"pumping" action. Voltage is an electrostatic concept,
and a battery is a "static electric" device.
Remember the battery in the copper ring from above? The battery acted
as a charge pump. It pulled charge-stuff out of one side
of the ring, and pushed it into the other side. Not only
did this force the circle of charges to begin moving, it
also caused a voltage-difference to appear between the
two sides of the ring. It also caused an electrostatic
field to appear in the space surrounding the ring. The
charges within the copper ring began moving because they
responded to the forces created by the voltage
surrounding the ring. In this way the voltage is like
pressure. By pushing the charges from one wire to the
other, a voltage causes the two wires to become positive
and negative... and the positive and negative wires
produce a voltage. (In hydraulics we would use a
pressure to drive water into a pipe, and because we
drove water into a pipe the pressure in that pipe would
rise.)
So, the battery "charged up" the two halves of the
copper ring. The light bulb provided a path to discharge
them again, and this created the flow of charge in the
light bulb filament. The battery pushes charge through
itself, and this also forces a pressure-imbalance in the
ring, and forces charges to flow through the light bulb
filament. But where does energy fit into this? To
understand that, we also have to know about electrical
friction or "resistance."
OHMS
Imagine a pressurized water tank. Connect a narrow hose
to it and open the valve. You'll get a certain flow of
water because the hose is a certain size and length. Now
the interesting part: make the hose twice as long, and
the flow of water decreases by exactly two times. Makes
sense? If we imagine the hose to have "friction", then
by doubling its length, we double its friction. (The
friction always doubles whether the water is flowing or
not.) Make the hose longer and the water flows slower
(fewer gallons per second,) make the hose shorter and
the reduced friction lets the water flow faster (more
gallons per second.) Now suppose we connect a very thin
wire between the ends of a battery. The battery will
supply its pumping pressure (its "voltage"), and this
will cause the charge-stuff inside the thin wire and the
charge-stuff within the battery to start moving. The
charge flows in a complete circle. Double the length of
the wire, and you double the friction. The extra
friction cuts the charge flow (the amperes) in half. THE
FRICTION IS THE "OHMS", IT IS THE ELECTRICAL RESISTANCE.
To alter the charge-flow in a circle of wire, we can
change the resistance of our piece of wire by changing
its length. Connect a long thin wire to a battery and
the charge flow will be slow (low amps.) Connect a
shorter wire to the battery and the charge will be
faster (high amps.) But we can also change the flow by
changing the pressure. Add another battery in series.
This gives twice the pressure-difference applied to the
ends of the wire circle... which doubles the flow. We've
just discovered "Ohm's Law:" Ohm's law simply says that
the rate of charge flow is directly proportional to the
pressure difference, and if the pressure goes up, the
flow goes up in proportion. It also says that the
resistance affects the charge flow. If the resistance
goes up while the pressure-difference stays the same,
the flow gets LESS by an "inverse" proportional amount.
The harder you push, the faster it flows. The bigger the
resistance, the smaller the flow (if the push is kept
the same.) That's Ohm's law.
Whew. NOW we can get back to energy flow.
VOLTS, AMPS, OHMS, ENERGY FLOW
Lets go back to the copper ring with the battery and
bulb. Suppose the battery grabs charge-stuff out of one
side of the ring and pushes it into the other. This
makes charge start flowing around the whole circle, and
also sends energy instantly from the battery to the
light bulb. It takes a certain voltage to force the
charges to flow at a certain rate, and the light bulb
offers "friction" or resistance to the flow. All these
things are related, but how? (Try
bicycle wheel analogy.)
Here's the simplest electrical relation: THE HARDER THE PUSH, THE
FASTER THE FLOW. "Ohm's Law", can be written like this:
VOLTS/OHMS = COULOMBS/SEC The harder the push, the faster
flows the charge
Note that coulombs per second is the same as "amperes."
It says that a large voltage causes coulombs of charge
to flow faster through a particular wire. But we usually
think of current in terms of amps, not in terms of
flowing charge. Here's the more common way to write
Ohm's law:
VOLTS/OHMS = AMPERES Voltage across resistance causes current
Voltage divided by resistance equals current. Make the
voltage twice as large, then the charges flow faster,
and you get twice as much current. Make the voltage
less, and the current becomes less.
Ohm's law has another feature: THE MORE FRICTION YOU HAVE, THE SLOWER
THE FLOW. If you keep the voltage the same (in other
words, you keep using the same battery to power your
light bulb), and if you double the resistance, then the
charges flow slower, and you get half as much current.
Increasing the resistance is easy: just hook more than
one light bulb in a series chain. The more light bulbs,
the more friction, which means that current is less and
each bulb glows more dimly. In the bicycle wheel analogy
mentioned above, a chain of light bulbs is like several
thumbs all rubbing on the same spinning tire. The more
thumbs, the slower the tire moves.
Here's a third way of looking at Ohm's law: WHEN A
CONSTANT CURRENT ENCOUNTERS FRICTION, A VOLTAGE APPEARS.
We can rewrite Ohm's law to show this:
AMPERES x OHMS = VOLTS A flow of charge produces a voltage if
it encounters resistance
If resistance stays the same, then the more current, the
more volts you get. Or, if the current is forced to stay
the same and you increase the friction, then more volts
appear. Since most power supplies provide a constant
voltage rather than a constant current, the above
equation is used less often. Usually we already know the
voltage applied to a device, and we want to find the
amperage. However, a current in a thin extension cord
causes loss of final voltage, and also transistor
circuits involve constant currents with changing
voltages, so the above ideas are still very useful.
But what about joules and watts? Whenever a certain amount of charge is
pushed through an electrical resistance, some electrical
energy is lost from the circuit and heat is created. A
certain amount of energy flows into the "frictional"
resistor every second, and a certain amount of heat
energy flows back out again. If we increase the voltage,
then for the same hunk of charge being pushed through,
more energy flows into the resistor and gets converted
to heat. If we increase the hunk of charge, same thing:
more heat flows out per second. Here's how to write
this:
VOLTS x COULOMBS = JOULES It takes energy to push some charge
against the voltage pressure
Charge flows slowly through the resistor and back out
again. For every coulomb of charge that's pulled slowly
through the resistor, a certain number of joules of
electrical energy race into the resistor and get
converted to heat.
The above equation isn't used very often. Instead, we usually think in
terms of charge flow and energy flow, not in terms of
hunks of charge or hunks of energy which move. However,
thinking in terms of charge hunks or energy hunks makes
the concepts sensible. Once you grasp the "hunks"
concepts, once you know that energy is needed to push
each hunk of charge against a voltage force, afterwards
we can rewrite things in terms of amps and watts.
Afterwards we can say that it takes a FLOW of energy (in
watts) to push a FLOW of charge (in amps) against a
voltage. Yet first it's important to understand the
stuff that flows. Think in terms of coulombs of charge
and joules of energy.
The charge-flow and the energy-flow are usually
written as amps and watts. This conceals the fact that
some quantities of "stuff" are flowing. But once we
understand what's really going on inside a circuit, it's
simpler to write amperes of charge-flow and watts of
energy-flow:
VOLTS x COULOMBS/SEC = JOULES/SEC It takes a flow of energy to make
charge flow forward against pressure
Don't forget that "Amps" is shorthand for the charge
inside wires flowing per second. And "watts" is
shorthand for flowing energy. We can rewrite the
equation to make it look simpler. It's not really
simpler. We've just hidden the complexity of the above
equation. It's shorthand. But before using the
shorthand, you'd better understand the full-blown
concept!
VOLTS x AMPERES = WATTS Pushing a current through a voltage
requires energy flow or "power."
We can get the Ohms into the act too. Just combine this
equation with Ohm's law. Charge flow is caused by volts
pushing against ohms, so let's get rid of amps in the
above equation and replace it with voltage and ohms.
This forms the equation below. Notice: increasing the
voltage will increase the energy flow that's required,
but it also increases the charge flow... which increases
the energy flow too! If voltage doubles, current
doubles, and wattage doesn't just double, instead the
doubling doubles too (wattage goes up by four times.)
Tripling the voltage makes the wattage go up by NINE
times. Write it like this:
VOLTS x (VOLTS/OHMS) = WATTS Voltage applied across ohms uses up a
constant flow of electrical energy
So, if you double the voltage, energy flow increases by
four, but if you cut the friction in half while keeping
voltage the same, energy flow goes up by two, not four.
(The amperes also change, but they're hidden.)
Here's one final equation. It's almost the same as the one above, but
voltage is hidden rather than ampereage:
(AMPERESxOHMS) x AMPERES = WATTS When charge is flowing against ohms,
electrical energy is being used up
So, the watts of energy flow will go up by four if you
double the current. But if you can somehow force the
current to stay the same, then when you double the
friction in the circuit, the energy flow will only
double (and the voltage will change, but that part's
hidden.)
And finally, here are a couple of things which can mess you up. Think
about flowing power. Try to visualize it. I hope you
fail! Remember... POWER DOESN'T FLOW! The word "power"
means "flow of energy." It's OK to imagine that
invisible hunks of electrical energy are flowing across
a circuit. That's sensible. Electrical energy is like a
stuff; it can flow along, but "energy flow" cannot flow.
Power is just flowing energy, so "power" itself never
flows. Beware, since many people (and even textbooks)
will talk about "flows of power." They are wrong. They should be
talking about flows of electrical energy. "Flow of power" is a wrong
(and fundamentally stupid) concept. |
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