When I touch a laptop's corner, USB or VGA port ground with finger, the resulting spark seems not to affect anything.

However I've just tried touching the VGA port's outer ground with a pencil instead of directly with a finger, and laptop's LCD screen "blinked" for a moment at the same time the spark ran between pencil's tip and the ground.

Why can this happen? I expected the pencil will make static discharge more gradual and "soft"...


1 Answer 1


In my opinion it is a "spike" effect due to the peak of the pencil, the conductance of graphite and the capacity of the pencil wood/plastic. Similar to the effect of a lightning rod but in a small scale, based upon the principle that the electric field strength is great around a pointed object. The difference between your finger and the pencil (when you bring it near to the USB or VGA) is, that the pencil creates a higher E-potential field than your finger, it causes that the current during the spark is higher too (density of chargers is higher than using your finger). Well, during the spark the USB or VGA connection has a so higher voltage that your power source (Leptop batteries have more less 12V) blackout for this short time due to the back-current, and then the LCD blinks.

  • \$\begingroup\$ So is it only because of the pencil is pointy and the finger is blunt? Or it is because of pencil+hand = capacitor? \$\endgroup\$
    – Vi0
    Apr 25, 2014 at 21:25
  • \$\begingroup\$ Yes it is because pencil is pointy and conductive. The spike of the pencil is necessary to create a strength field and the plastic or wood to act like a ground. The amount of charges is very low, and the capacitance of the pencil with the plastic or wood will be enough high to accumulate them. Yes, the pencil act also like a capacitor. \$\endgroup\$
    – Alf
    Apr 25, 2014 at 21:39

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