Last Week's Question

The absolute capacitance of a person is approximately,

  1. 100 pF
  2. 1,000 pF
  3. 0.01 μF
  4. 1 μF

Answer

ESD test being performed on a laptop computer

The correct answer is "a". The exact value for the absolute capacitance of a person will depend on the size of the person and the position they are in, but it will always be on the order of 100 pF.

The absolute capacitance of a sphere is given by the formula, C = 4πε0a, where a is the radius of the sphere and ε0 is the permittivity of free space. That works out to about a picofarad per centimeter of radius. In other words, a golf ball has about 2 pF of absolute capacitance, while a basketball has about 12 pF of absolute capacitance. The 150 pF capacitance in an ESD simulator is designed to represent the worst-case absolute capacitance of a person (or perhaps a person sitting in a chair at their desk).

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