question about capacitors

[Solaris]

hi, i have a silly question

ok, say for instance we have a 10 volt 1000uf capacitor
but we only put say 5 volts into it... would it now be equivalent to a 5 volt 1000uf capacitor?

that is to say, would the energy stored be equal to a 5 volt 1000uf capacitor charged to 5 volts?

this confuses me because i see PSU's with 20 volt capacitors on the 12 volt rail
it makes me wonder if the capacitance is the same at almost half the voltage.

thanks

[ElektroQuark]

Yes.
The marked voltage is the maximun voltage the capacitor is reliable for.
At same charged voltage, equal capacity capacitors store the same energy.

[Simon]

The capacitance is always the same. Electrolytic capacitors are like high speed batteries and have liquid acid in them so you NEVER run them full voltage or temperature that is why your 12V PSU wisely uses 20V capacitors. The behavior will be the same regardless of voltage but over-voltage permanently damages them quickly and no good designer uses them at their rated voltage (yes i has taken some stuff apart designed by clods).

[Solaris]

thanks this explains the mystery.

[Jeroen3]

A recent EEVblog episode went into some details of ceramic capacitors and a possible trap for young players regarding this issue.
Might be related.

[Simon]

That was a different problem and regarded specific type of capacitors in tiny surface mount packages