Desco - Questions And Answers

A few minutes ago I attended an ESD training session given by Mr. Jim Mckeon from DESCO INDUSTRIES, and I asked him about a question that I had on my Physics Ph.D. qualifying exam 23 years ago. He said he did not know the answer and that I should ask you about it. The question was this: "On a cold winter day how many electrons do you collect on your body when you walk across a carpeted room?" My solution to the problem was to estimate a voltage based on the length of the spark discharge from your finger to a metal door knob (taking into account the breakdown voltage of dry air), assume that the average human body can be approximated by a sphere (in which case the formula for the capacitance is simple), calculate the charge from the equation Q = CV, and then divide Q by 1.610e-19 coulombs. But I have always wondered if my answer was correct. Basically, all I want is an estimate of the capacitance of the average human body. Hope you can answer my question! - Anonymous, Oceanport, NJ

Fun question. Of course there are a myriad of variables including the material of the soles of the shoes and the type of carpet (regarding the triboelectric series), RH, temperature, size of body, area of feet (foot print), dielectric of air, geometry of finger, resistance of skin, etc. The ESD Association uses 100 pF as an average capacitance for a typical human body model. The way you approached your answer was very good, simplifying the variables. Typical charges found on a body from a cold (low RH) day can approach 30 kV (about a 3 inch spark gap). Assuming 100 pF capacitance of the body and using Q=CV, then the # electrons would be well over 18x10^12.

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