why does a car battery spark

When did you last get your eyes tested ?:-)

...Jim Thompson

You'd probably get a faint spark from a 9-volt transistor battery if you tried it in the dark (which it's not now, so I can't try it now.)

When metals contact, there's no "elecric field" type breakdown... the current flows through metal, not air. The point of contact is generally very small compared to the conductors, so the current density there is very high, so metal melts and maybe vaporizes at the point of contact and bits of vaporized metal fly around and maybe oxidize (burn) to add to the fun. The more current the source can pump into the contact zone, the bigger the spark.

John

Please don't feed the gmail trolls.

don

A bit like this:

"A spark is triggered when the electric field strength exceeds approximately 4 - 30kV/cm[2] ? the dielectric field strength of air."

I can't tell if they mean 4V - 30kV or 4kV - 30kV, if they mean the latter... then batteries are no where near that

-----ah, but have you considered inductance? Energy equals inductance times the square of current. When you break the circuit, that energy causes a spark. You could get thousands of volts from an AA battery and a coil. How do you think an automotive ignition coil creates thousands of volts to fire the spark plugs, even though you only have 12 volts available from the battery? You'll get a spark on breaking a circuit mechanically even if it only goes through a few inches of wire. John Larkin mentioned another effect, heat created as a result of high resistance at the point of contact.

so... the difference between the car battery and the table top battery is how much current the thing can source, right? so is that the parameter... can a 2V battery that can source 10A then cause a spark (if you short the leads) while a 10V battery that can source 100mA can't

I'm only talking about visible sparks, and I'm not talkin about static electricity, just shorts... no esd

any thoughts? thanks

he

thanks for the response,

i did think about inductance, as far as the spark plug side of things... but didn't think that isolating just a battery and some wire had much inductance going on... at least not enough to get that thousand volt voltage spike

things... but didn't think that isolating just a battery and some wire had much inductance going on... at least not enough to get that thousand volt voltage spike

You can use the hydraulic analogy. In such case the voltage will be infinite (hydraulic hammer). Or the electron gas analogy. In this case the voltage will be higher then supply.. Sparks are always. Use a magnifying glass. S*

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hmmmm.... so, 12V battery, but through wire... let's say 0.01ohm wire..... there's 12V/0.01 current =3D 1200A... but not infinite voltage... there's essentially 0V across a wire

sparks are always, that's why i mentioned visible... didn't want to go off on that tangent

Look carefully; ye olde standard 9V battery can give a spark. On one extreme, one can have a very low voltage (say 0.5V) and a goodly amount of current available, and at the other extreme one can have high voltages (thousands of volts) and need a very small amount of current. The criteria, it seems, is the energy available (a hyperbolic power curve in the I-V plane. Looking in the Fire Protection Handbook, spark energy curve bottoms out at about 0.4mJ for ignitability of Methane. Now if one wants to get nasty, Acetylene or Hydrogen needs only

Check.

No doubt. When I was a kid I realized I could set a relay into a mode where it would buzz. I'd feed this out to a step up transformer and you got a nice pulsed current coming out the other end. Never had a chanced to measure it but I was using a 9V battery to drive the whole thing and I think it was 4:1 step up, so maybe 36V.

Come to think of it, it was my first spark gap transmitter.

my terminology might be wrong, english isn't my first language but.

spark: happends when the electric field exceeds the dielectric field strength of e.g. air

arc: current flowing through ionized air, plasma

the air can be ionized either from a spark in the high voltage case or from vaporised metal in the low voltage case

e.g. scratch starting a tig welder vs. high voltage starting with a spark

-Lasse

A spark is not the same as an arc. My office opens onto a machine/weld shop; grinders make GREAT sparks, and there's no voltage involved at all.

John Larkin gave the best answer - because of the immense current, a small amount of copper actually vaporizes, shooting molten copper, which is so hot that it's incandescent, and that's what the sparks are.

Yes, you could probably get teeny tiny sparks from a 9V battery, but you'd need a magnifying glass in a dark room to see them.

Hope This Helps! Rich

off

Thanks John and Rich, yes that does make sense and does help. Would you say then that the wikipedia entry for spark is wrong, here it is:

"Sparks Main article: Corona discharge

A spark is triggered when the electric field strength exceeds approximately 4 - 30kV/cm[2] =E2=80=94 the dielectric field strength of air= . This may cause a very rapid increase in the number of free electrons and ions in the air, temporarily causing the air to abruptly become an electrical conductor in a process called dielectric breakdown. Lightning over Ryma=C5=84. Northern Poland.

Perhaps the best known example of a natural spark is a lightning strike. In this case the potential difference between a cloud and ground, or between two clouds, is typically hundreds of millions of volts. The resulting current that flows through the ionized air causes an explosive release of energy. On a much smaller scale, sparks can form in air during electrostatic discharges from charged objects that are charged to as little as 380 volts (Paschen's law)." - wikipedia (9/16/09)

You actually can't strike an arc without being able to ionize air, which takes about 30 eV per ion pair. So you won't get any photons from a 9V battery unless you make something incandescent or get an inductive kick big enough.

Car batteries spark because (a) there are a bunch of inductive things connected to them; and (b) when the contact is broken, the last little bit of metal that remains in contact gets really hot.

Cheers

Phil Hobbs

I just took a RatShack 9 volt battery and shorted the tabs with a small screwdriver. In office light, there's a distinct spark. Sometimes you can even see a tiny comet-like thing shoot out a couple tenths of an inch. I can't imagine that there's enough inductance here to make a difference.

I suppose nobody else here has the advanced equipment necessary to try such an experiment.

John

Hey, if you're careful about the gap, you can get a tiny, hissing, sustained arc, mostly white but with flashes of yellow and blue. A slightly rusty steel screwdriver is more dramatic than a shiny chrome one.

Under the Mantis, no illumination, the initial contact does splatter out a bunch of tiny comet sparks. This looks just like a miniature version of a big arc. Things scale.

John

My thoughts? Despite the fact that you are obviously in the very early stages of electronics or electrical educational endeavors, I would say that you lack the intuition for the discipline. Seek success in a different industry. Sorry, but that is the way the bones fell.

Is there ever any opportunity to post this most retarded request in Usenet that you pass up?

Is that what makes your pathetic world go 'round, little boy?

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