Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: sparks



At 06:55 PM 2/17/01 -0800, Ben Crowell wrote:

When you unplug a high-power electrical applicance like a
halogen lamp or a space heater, you reliably get a big blue
spark between the plug and the light socket.

Right.

When you plug it
back in, it seems like you sometimes -- but not always -- get
another spark.

I would be very surprised to see a comparably-big spark.
I would expect typically little or no spark.

I was interpreting this as the release of some of the energy stored
in the magnetic field in the space around the coil of the
step-down transformer where the power line feeds into my
house.

Not a chance. It's a transformer, not a simple inductor. That means that
its mutual inductance essentially nullifies its self-inductance under
anything approaching normal conditions. So you get not the inductor law V
= L dI/dt but rather the transformer law V1 = N V2 and I1 = I2 / N.

The other interpretation is that you simply create a strong
electric field in the air when the plug is close to, but not
quite touching, the socket.

That's true as stated, but there's a lot more to the story.

The real story has to do with ions. Once the air is ionized, the ions stay
in the vicinity for many milliseconds before they recombine. If there is a
field, the ions carry a current. The moving ions undergo violent
collisions with air molecules, which produces more ions, so the arc can
sustain itself under a wide range of conditions. Under a fairly-wide
subset of these conditions, if you manage to extinguish the arc, it will
stay extinguished.

These phenomena have very great practical importance, because every switch
and every circuit breaker must deal with this. It does not suffice to move
the contacts apart; the hard part is to extinguish the arc. Using AC
makes this a lot easier. Imagine how hard it is to shut off high-power DC
circuits!

I am fascinated by the tricks they use to switch the really high-power
circuits at power plants and substations. Typically three-phase power is
involved, and you want to trip all three phases. So there is a place where
the current is carried by three parallel movable bars. In the space
between the bars there is a synchronous motor, synchronized to the phase of
the CURRENT (which may not be identical to the phase of the voltage). It
has a big wheel that just sits there and spins all the time. When it is
desired to trip the circuit, a dog pops out of the wheel and strikes the
bars, in order, in phase, bang-bang-bang, hitting each bar in phase with
the zero-crossing of the current in that phase. If you knock out the bar
when there's zero current, there's never an arc to extinguish!


On a related note, does anyone know how strong a field is
required to ionize air, and how strongly this depends on
humidity?

It's about 3 megavolts per meter. You can guestimate this answer based on
the previously-described model: an ion needs to pick up enough energy
between collisions to have a fair chance of ionizing whatever it
hits. That requires a field strength on the order of a fraction of a volt
per mean-free-path.

The breakdown field depends strongly on density, and not very strongly on
humidity.