An electric field greater than about 3 * 106 Vm causes air to break down (electrons are removed from the atoms and then recombine, emitting light). See Section 17-2 and Table 17-3. If you shuffle along a carpet and then reach for a doorknob, a spark flies across a gap you estimate to be 1 mm between your finger and the doorknob. Estimate the voltage between your finger and the doorknob. Why is no harm done?