Taken from the internet www
When your fingers are close enough to the door handle, a spark jumps. Or in other words, the intense electric field in the space between your fingers and the metal handle will tear the air molecules apart. First the field stretches the molecules by attracting their alike charges while repelling the unlike ones. Then finally an electron pulls loose from one molecule. This electron takes off at extremely high speed, driven by the e-field. It quickly strikes another air molecule, which liberates more electrons, which then repeat the process. It resembles a landslide, where one pebble strikes another, freeing it to strike others. This "electron avalanche" glows violet, since some electrons are recaptured by air molecules, and they emit violet light typical of ionized nitrogen/oxygen mixtures. Some of the light is ultraviolet, and this light knocks electrons off neighboring air molecules. Also, the region of space that's filled with electrons and positive ions is a conductor, a plasma, and so it distorts the flux lines of the electric field. Plasmas typically take the shape of a long filament, a "lightning leader," since the tip of the plasma filament is somewhat sharp, and it causes the e-field to concentrate there (which promotes faster creation of plasma.)
The electron avalanche and the plasma filament can start out on the car door, then reach outwards toward your fingers. Or it can start out on your fingers and leap towards the door. Or it can be triggered by dust motes in the space between the two, and then leap in both directions. Charge polarity doesn't make too much difference, and the visible "leaping" of sparks is NOT a motion of charges, it's not a visible current. Instead it's an outbreak of glowing plasma, and this outbreak can go in either direction.
Finally the plasma filament touches your finger and the car door. It's a conductor with a typical resistance of a few tens of ohms. This conductor EXPLODES. It has shorted out the capacitor plates formed by your body and the metal car. The e-field in the space between you and the car then collapses inwards towards the spark. Electrical energy that was in the space near your hand is flowing inwards towards the spark. (Energy doesn't flow across the spark, instead the energy behaves like a cylinder shape that surrounds the spark and shrinks inwards.)
A huge electric current appears in the spark, and temperatures in the air (and in the dead skin surrounding your salty conductive flesh) rise to immense values. The air emits sound and bright light, while your dead skin is cooked or even vaporized by the electrical energy pouring into the spark. The pain you experience is not necessarily electrical, it's similar to having your finger poked by a white-hot needle. If you grasp a metal coin or some keys, and let the spark jump to the metal, you'll feel almost nothing. The metal prevents the burn while doing little to stop the current.
Painful finger-sparks can measure a few amperes, but they only last for a hundredth of a microsecond. The worst ones can range up to many tens of amperes, with peak energy flow up in the megawatts. But these sparks last for incredibly brief times. Your nervous system only responds on time scales of a tenth or a hundredth of a second. Your nervous system "blurs" the energy and charge flows, and it "thinks" that the wattage and current of the spark is roughly a million times weaker than it actually is. Hurts though.
For more info about the typical "human finger sparks" used by manufacturers for stress-testing new appliances, search for HBM or "Human Body Model": Google: ESD, HBM, CDM, MM
