February 2007 Here is a common dorm prank: On a day with low humidity, you walk quietly up behind a friend and reach a finger out toward his ear lobe. When your finger is close to the lobe, a spark jumps between the two surfaces and your friend jumps up into the air with a shriek of surprise. Obviously you became charged as you walked toward him, but why?
Unless you live in continuously humid conditions, you have probably been shocked by a spark after walking over certain types of floors and then reaching for a computer, door knob, faucet, or some other conducting object. In those sparking circumstances, walking over the floor charges you up and then the charge can be neutralized with a spark. The process can also be described in terms of electric potential (or voltage). As you walk, the electric potential of your body increases. When you reach for a conducting object, the electric field between you and the object becomes large enough to ionize some of the air molecules (tear electrons out of the molecules). Then the air becomes conducting and electrons can move along a path between you and the object. Those moving electrons collide with the air molecules along the way. The molecules emit light, and the energy dumped into the air along the spark’s path causes the air to rapidly expand, which sends out the sound of the spark.
This much has been known for a long time, but the lingering question has been, “Why does your body become charged and the electric potential increase?” Here is a recently published explanation by T. Ficker of the Technical University of Brno in the Czech Republic, based on his measurements on someone walking over a nonconducting floor (electrons cannot easily move through the floor). Suppose you wear shoes with rubber soles and walk on such a floor. Electrons can move from the sole of the shoe onto the floor, leaving the sole positively charged and the floor negatively charged.
The culprits. If both the sole and the floor do not conduct well (charge cannot move through them easily), the charges stay put. But your body and the floor support are both conducting. So electrons come down from your body to your foot, to be near the positive charge on the sole. And electrons in the floor support move away from the floor surface. You get two double layers (closely lying layers of positive and negative charge): (1) the sole and your foot, (2) the floor surface and just below the surface.
Walking. As you walk, you leave the double layer on the floor in your trail but your shoe continuously touches fresh points on the floor and loses even more electrons. The result is that the double layer of charge of the sole and your foot grows stronger. Moreover, each time you raise your foot, the increased distance from the floor increases the electric potential (the voltage) on your body. So, you become more and more charged and each step increases the electric potential. If the air humidity is high, the water moisture quickly neutralizes the charge, and then putting your finger near the ear lobe of a friend is just weird. But if the air humidity is low, you eventually become charged enough to send a spark between finger and ear lobe.
Walking in place. If you walked in place, touching the same points on the floor, you would not lose so many electrons and your electric potential would not increase very much. If you walked barefooted across the floor, you wouldn’t become charged at all because, with the foot conducting, you cannot build up a double layer of charge.
Cats. Oh, by the way, you might enjoy sparking to a friend but never ever spark to a cat because its claws will extend outward by 20 cm (like Wolverine in X-Men) and then it will slice the clothes from your body, which is very embarrassing to explain to the emergency medical crew (and then to the animal protection agency).
Ficker, T., “Charging by walking,” Journal of Physics D: Applied Physics, 39, 410-417 (2006)
Ficker, T., “Electrification of human body by walking,” Journal of Electrostatics, 64, 10-16 (2006)