EMC Question of the Week: September 2, 2019
A person charged to 15,000 volts catches an uncharged glass sphere as it falls through the air. The amount of charge transferred to the glass sphere depends on the sphere's
- resistance
- capacitance
- inductance
- all of the above
Answer
The correct answer is "b". The larger the sphere, the greater its absolute capacitance, and the more charge it will hold at a given voltage. Assuming the sphere is small relative to the person, charge would flow from the person to the sphere until they both reached the same potential (slightly less than 15,000 volts).
The resistance of the sphere would affect the rate at which the charge flowed, but not the final amount of charge. Glass is a very poor conductor, but even poor conductors allow charge to flow on them (albeit more slowly). At standard temperature and humidity, a glass sphere would obtain the same amount of charge as a metal sphere within seconds.
Inductance is a property of current loops. It is not meaningful to refer to the inductance of a sphere in this context. Even if we were to calculate an equivalent inductance associated with the moving charge, it would have no effect on the final value of the charge on the sphere.
Circuit boards and electronic components can also become charged when they are handled by someone who is not at zero potential. Generally, the larger the object the more charge it will hold at a given voltage.
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