Comment: Voltage on insulators IS meaningful!

 

I disagree with the notes presented by Niels Jonassen in the latest issue of ?Threshold?.  It is not ?meaningless? to talk of the voltage on an insulator.  The voltage is there ? but needs to be considered and measured with care. 

If I approach a surface with a fieldmeter and adjust the voltage to get zero electric field (voltage follower probe) then, if one is sufficiently close that there are no significant influences from anything else nearby, one will have a measure of the local potential of the surface.  This is true whether the surface is a conductor or an insulator.  This is an entirely ?meaningful? concept.

If I approach a surface with an earthed fieldmeter then as the fieldmeter gets closer the added capacitance will affect the actual potential of the surface (as Jonnasen rightly points out).  The variation of the surface potential however depends on the relative capacitance experienced by the surface charge from the nearby fieldmeter and from all other sources.  To keep the influence of the fieldmeter low it is obviously sensible to keep the separation distance fairly large ? but the larger the separation the more readings are influenced by the size of the area charged and by any other sources of charge around. 

It is perfectly sound to set up a fieldmeter to read surface voltage at a defined separation distance so long as one remembers that the presence of the fieldmeter may influence the observations. 

Many of the problems from ?'static? arise not from the charge on surfaces per se but from the influence such charge has on things nearby via the electric fields that the charge creates.  These effects include induction charging of items, the initiation of electrostatic spark breakdown, and the attraction of dust, dirt and thin films.  The effects are the same as if the dielectric surface had a potential - so why the distinction? 

It is worth noting that there are problems in using a fieldmeter to measure surface charge density ? even if the fieldmeter is properly guarded to ensure field uniformity.  On many materials surface charge may experience a high capacitance from within the material itself [1,2].  In this situation the simple relation between charge density and local electric field is not directly applicable.

 

John Chubb ? John Chubb Instrumentation

(jchubb@jci.co.uk)

 

[1] J. N. Chubb "The assessment of materials by tribo and corona charging and charge decay measurements" 'Electrostatics 1999' Univ Cambridge, March 1999 Inst Phys Confr Series 163 p329

[32] J. N. Chubb ?Measurement of tribo and corona charging features of materials for assessment of risks from static electricity? Trans IEEE Ind Appl 36 (6) Nov/Dec 2000 p1515-1522