EMC Question of the Week: October 11, 2021
A person wearing leather shoes shuffles across a polyester carpet on a dry day collecting a significant charge. The polarity of the charge
- is positive
- is negative
- depends on the season
- depends on the hemisphere
Answer
The best answer is “a.” Leather is a material that is higher in the Triboelectric Series than polyester. Its electrons are more loosely bound than the electrons in polyester. When the two materials are in close contact then quickly separated, the leather accumulates a positive charge and the polyester accumulates a negative charge.
People are generally conductive enough that any concentrated charge on the bottom of a shoe quickly spreads to the rest of the body. This allows the shoe to continue to pick up positive charge until the person attains a voltage high enough to prevent further charge accumulation (perhaps as high as 25,000 volts). If a charged person comes into contact with an uncharged person or object, an electrostatic discharge (ESD) event can occur.
Note that a person wearing rubber-soled shoes walking on a nylon carpet would accumulate a negative charge. ESD standards generally require testing with both positive and negative discharges.
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