MadSci Network: Physics

Re: how in physic terms do u describe overloading of a circiut

Date: Fri May 19 23:14:05 2000
Posted By: Vernon Nemitz, , NONE, NONE
Area of science: Physics
ID: 957408829.Ph

Greetings, Mehdi:

You are encountering phenomena associated with "electrical resistance".
Almost all substances that conduct electricity do so imperfectly.  (Some
of them, under very special conditions, can conduct electricity perfectly,
but so far it costs too much to create those special conditions everywhere
that we might want to conduct electricity.)

Whenever electricity is conducted imperfectly, the amount of imperfection
is usually described as an amount of "electrical resistance".  Different
substances have different amounts of resistance, and whenever electric
current flows through an imperfect conductor, some heat will be produced.
The more electrical resistance a substance has, and the more current
flowing through it, the hotter it gets.  Electric ovens are deliberately
constructed with high-resistance conductors, to create the heat needed to
cook food.  Ordinary "incandescent" light bulbs are aptly named; their
electrical resistance causes them to glow white-hot.

Steel wool is not a good conductor of electricity.  Steel itself is not an
especially good conductor, and for ANY conductive substance, the finer the
wires that you make from it, the poorer a conductor it becomes.  Since
steel wool consists entirely of rather fine strands of steel, it is easy
to see that when electricity flows through it, a fair amount of heat is
quite likely to be produced.

Before getting to that, however, there is one more thing to describe.  The
"voltage" that you applied to your steel wool is the pure force that can
cause an electric current to flow.  There is a fairly simple equation that
describes the relationship between voltage, current, and resistance:

 voltage (Volts) equals current (Amperes) multiplied by resistance (Ohms)

What that means with respect to the two, six, or twelve Volts that you
applied to the steel wool is this:  Regardless of how much current flowed
when two Volts was applied, we can call it a "base amount of current".
Thus, when applying six Volts, three times the base amount of current
flowed, and applying twelve Volts caused six times the base amount of
current to flow.  (I should mention a modification that frequently must be
noted:  The hotter an electrical conductor, the more resistance it has.
Applying twelve Volts may indeed cause six times the base current to flow,
but as the wool heats up, its increased resistance causes the flowing
current to diminish a bit.  An equilibrium will be reached between the
flowing current and the heat-caused increased electrical resistance; the
mathematical description of this is somewhat complicated, and we need not
worry about it here.  Crude events such as this can be described well
enough without the fancy modifications -- but one should remember that
there will be times when such modifiers are critically important.)

As you observed, the more Volts, the hotter the fine wires of steel wool
became, simply because more current was being forced to flow through them
-- being poor electrical conductors.

The next relevant fact concerns the "surface area" of steel wool.  Think
about a single spherical ball of steel, weighing 25 grams (a bit less than
one ounce).  The amount of steel exposed to the air is quite minimal; most
of the metal, after all, is locked away inside the body of the ball.  But
if we chopped the ball in half, then suddenly two more (circular) areas
of steel are exposed to the air, in addition to the original exterior
sperical surface.  The more we chop the ball up, the more and more total
amount of air is exposed to the air....

The preceding is important because air contains oxygen, a major ingedient
of most fires.  As you know, steel tends to rust, as the oxygen in the air
combines with the iron in the steel.  THIS CAN ONLY HAPPEN TO THE PARTS OF
THE STEEL THAT IS EXPOSED TO THE AIR.  Only as pieces of rust flake off of
a solid piece of steel, do the innards of the metal become exposed to air,
and become able to rust.

Well, if we have helped the oxygen in the air, by taking a solid piece
of steel and making a lot of very fine wires out of it (steel wool), it
follows that all that steel can rust rather quickly, simply because fine
wires have a lot of surface area exposed to the air.

Now note that another word for the verb "rust" is "oxidize" -- and note
that the process of oxidation usually generates heat.  A fire consists of
material in the process of oxidizing quite rapidly; the heat that a fire
generates makes it easier for the next bits of material to burn.

If you put the above pieces together, you should see that by itself, the
steel wool would rust/oxidize reasonably quickly.
When electricity is made to flow through it (pushed by two Volts), the
steel wool heats up and this makes it easier to oxidize even faster.  This
generates enough extra heat that it begins to smoke.
When six Volts push current through the steel wool, it heats up even more,
oxidizes faster still, and begins to burn outright, as you observed.
And when twelve Volts is applied, oxidation of the steel wool occurs at a
pretty fair clip, indeed!  Steel can burn quite truly, with actual flames,
when it is finely divided into wool, and heated enough.

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