I only continue this on an Imperial list becuase I feel it is important to
understand the characteristics of electricity to properly maintain our cars.
I find it unfortunate when someone has been deceived by misconceptions.
"Often, the story can be more complex that it appears"
Often a concept is made to appear more complex than it is. This is one case
in point. This is not a complex issue. Current flow causes heat.
Resistance does not cause heat. That's it.
"more heat generated due to higher R"
Wrong. Resistance REDUCES current and therefore REDUCES heat. Current
causes heat, NOT resistance. If you are seeing more heat somewhere it is
because there is either simply more current flowing or the current is
flowing through a smaller conductor. EVERY conductor heats up when current
flows through it. You normally don't notice it because of the skills of our
engineers. The engineers design conductors to be of sufficient size to
disipate the heat caused by the current flow. If you use a smaller
conductor for the same amount of current, that conductor could heat up.
"small local increase in the resistance will leave the current flow largely
unaffected"
Wrong. Don't forget Kirchov's Law. "The total resistance of any circuit is
a result of ALL resistance in the circuit" <---simplified to make it
readable
That means, EVERY resistance in any circuit WILL affect current flow be it
large or small. I'm not sure how you can change the resistance of a circuit
and not change the current flow all other factors remaining constant. If
you doubt this, calculate the effects of adding a resistance in PARALLEL to
another resistance in a circuit. You may be surprised at the effect on
current.
If heat increases in an electric circuit, it will be caused by one or both
of two factors. Current has increased (possibly by increasing the voltage
or reducing the resistance) or the size of the conductor has decreased (such
as a corroded connector or switch)
The problem in this whole discussion is the concept of a "resistor".
Consider for a moment that there is no such thing as a resistor, only a
conductor. That will assist anyone is understanding why "resistance" does
not cause heat and why CURRENT does. Given the same voltage a 1 Ohm
resistor will always heat up more than a 1000 Ohm resistor, given that the
resistors are of the same composition and wattage value. WHY? Well, the 1
Ohm resistor is carrying more current because it has a smaller "resistive"
value.
Another example, a very exagerated one to really get the idea. Take the
battery out of your car. Connect a heavy wire such as a booster cable or
starter cable to each of the two terminals. Hold the other ends of the
wires in the air about 6 inches apart. What you have there is a very poor
conductor (6 inches of air). One might call air a very good resistor. How
much heat is produced? None. Why? Because there is no measurable current
flow. OK, now, hook those two cables together (DON'T DO THIS!). You might
then say that you have a very GOOD conductor or a very LOW Ohm resistor.
How much heat do you have? LOTS!! You have enough heat to blow the cables
right out of your hands! Why? CURRENT FLOW! The current flowing through
that cable was so great that it heated the cable enough to melt it.
We all know that switches and connectors heat up when they are in poor shape
or corroded. Why is this. Well, if there is corrosion between the
connection, and only a part of that connection is making contact then you
have essentially reduced the size of the conductor. THAT causes heat. Now,
what you have is the same amount of current flowing through a much smaller
conductor and you have heat, NOT because you have resistance because as you
can see, resistance DOES NOT cause heat, current flow does.
----- Original Message -----
From: <dardal@xxxxxxxxxxxxxxx>
To: <mailing-list@xxxxxxxxxxxxxxxx>
Sent: Tuesday, September 17, 2002 9:41 AM
Subject: Re: IML: What causes heat in conductors?
Quoting chrysler1978@xxxxxxxxxxx:
> Simply said, current flow causes heat, end of story.
>
> Brad Hogg <----also a qualified electronics engineer.
> Winnipeg, Manitoba
Often, the story can be more complex that it appears. What DB is saying
(and
it makes sense to me) is that a localized increase in resistance can be
small
enough to not reduce the current by much (you need to remember, the current
depends on the TOTAL resistance of the WHOLE circuit, if the total
resistance
is significant, a small local increase in the resistance will leave the
current
flow largely unaffected). Then, if you focus on that small portion of the
circuit with increased resistance, it sees virtualy the same current as
before,
and more heat generated due to higher R. If the wire is insulated, this
heat
may have nowhere to go, so it will keep on heating it up. Let's push it a
bit
further. This heat could increase R further, making things worse, till the
insulation goes, and the short occurs, and there is your increase in
current!
Those simple discussions can be helpful to understand simple things like
that,
which can help us diagnose problems. And the more "wrong" we happen to be,
the
more we can benefit from them...
D^2
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