A colleague just came into my office to ask me, a middle-school science teacher, why she got shocked. Here is the scenario:
She was holding a damp paper towel in one hand. She used the other hand to pick up the charger to her computer by the prongs. The charger had been unplugged for at least two minutes. She got shocked. I am trying to explain why but I have never actually taught an electricity unit so I am just not sure. Anyone?
Thank you!!! Valerie
This sounds like good old static electricity, the same as if she had shuffled across a carpet and touched a doorknob. The wet paper towel and laptop charger had really nothing to do with it. Physical movement can cause a charge imbalance, which gets redistributed suddenly when touching a conductive object. It would have been the same had she not been holding the wet paper towel, or if she had touched any other metal object. Cold, dry air is a good electrical insulator, which is why static shocks tend to be more common and painful during the winter.
Were the 'prongs' those that plug into the mains socket? I suspect that there may be a capacitor connected directly or indirectly across them as part of the circuitry inside the charger. The capacitor could be left with a high voltage across it when unplugged, but it would probably have a low capacitance, so the charge that flowed through your colleague's hand would be small, though nonetheless unpleasant.
I doubt if the damp paper towel played any part, though if your colleague felt the shock in the hand that held it, as well as in the other hand, we need a different explanation!
There's an Electrical Engineering stack exchange too ya know ;)
What has happened is the charger has a capacitor across the power leads. This is used for EMI reduction (keeps the charger from turning your household wiring into a transmitting antenna and blowing away your neighbors radio)
Such a cap is called an "X Cap" as it sits across the power lines. "Y caps" go from line to ground (but since you have a 2-prong plug, there is no ground reference, ergo you have only an X cap here). They are special kinds of caps which 'self heal' if they experience a shorting event - If you open up a charger, you'll see the cap has VDE, UL and other safety agency ratings printed right on it. You would likely not be able to find a modern wall-adaptor/charger that did NOT have such a capacitor as it would surely fail FCC/EU EMI regulations.
Usually these caps don't deliver a noticeable ZAP if you touch both prongs, and indeed there are regulations preventing a large enough cap being installed that would allow this to happen. But that's assuming the person touching it has some level of electrical resistance and the charger is operating correctly. THE CAPACITOR WILL STILL DISCHARGE THROUGH YOU, but do it slowly.. slow enough you can't feel it.
However, since your friend was holding a wet paper towel, it's also quite likely her OTHER hand was WET, which causes that resistance to drop dramatically. The result is the discharge could occur MUCH MUCH faster and you'll feel it.
If there is any doubt, plug in the charger, unplug it, and then measure the voltage on the prongs with a DC voltmeter. You'll find something non-zero there.
Note the cap sits across AC lines, so (in the United States) 60 times / second the voltage on the cap goes from 0 to 170V to 0 and then -170, finally back to 0 again. In Europe, those voltages would be MUCH higher.
The amount of voltage left on the cap will depend on the exact moment you happen to disconnect the power, so you may have to try that experiment a couple times before you get a reading. Also possible the voltmeter will drain the cap before you get a reading (if the meter is kinda 'slow'). There's also (usually) a high-value resistor connected across this cap to perform this draining, so you gotta move fast if you wanna measure it.
I just did this experiment with a DELL charger and easily was able to read 10+ volts without even trying hard. I'm sure if I kept at it, I could get much higher readings.
