it's because of the current that is leaked by the class y capacitor connecting primary and secondary of the power supply to suppress EMI. this will mean there's

it's because of the current that is leaked by the class y capacitor connecting primary and secondary of the power supply to suppress EMI. this will mean there's something like 50 - 100v potential to earth on your macbook, but at a very low (non dangerous) current. that's why you get zapped or feel a tingle. if you have the earth connection, that power will be sunk through the earth lead, but otherwise a metal case will generally be connected to the DC ground (and also earth if present on the plug), which will have this low current AC voltage present.