Where do the charges come from?

  • #1
Once I was doing an experiment on capacitors, I discharged a capacitor totally(I mean 99.999% coz theoretically we can't totally discharge a capacitor) and checked by a microammeter, the reading was zero. Then I put the capacitor on a bench (insulator), after a while, I connected the capacitor with a microammeter and the pointer of the microammeter jumped to 0.5mA! I repeated it several times and same result could be obtained.
Now I am wondering where do the charges come from, from the bench? Could someone explain it please.
 

Answers and Replies

  • #2
chroot
Staff Emeritus
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It may be a property of the ammeter itself. Many ammeters of microamp sensitivity take on the order of seconds to stablize to correct readings. When you connect even an "empty" capacitance to the leads of the meter, you are suddenly dramatically changing the capacitance of the input circuit. This sudden change in capacitance can cause charges to move in the transistors in the input amplifiers, causing a little "blip" that the meter shows as a current spike. In reality, the ammeter is probably not designed to deal with suddenly and drastically changing reactance across its leads; also, as I've said, most of those meters take at least a second to stabilize.

Remember, no test/measurement equipment is really ideal!

- Warren
 
  • #3
398
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Nope, it's not the ammeter

It's a property of the dielectric in the capacitor, called "retentivity" and the makers of capacitors will generaly list it in their data sheets. If a voltage is applied for a considerable time to the capacitor some of the displacement charge will become "frozen" into the dielectric. This is also the princple of the electrostat in electrostatic microphones.

It's also the reason large and potentially (no pun intended) dangerous capacitors are shipped with shorted leads.
 

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