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capacitor COE question

Started by John Larkin October 26, 2012

If a capacitor has a largish temperature coefficient (hundreds of PPM
capacitance change per degree C maybe), what happens to a charged cap
when the temperature changes? Presumably C*V stays constant, so the
stored energy 0.5*C*V^2 changes. 

So, would the specific heat of the capacitor be different, as a
function of whether it is charged or not? 


-- 

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators
On Oct 26, 11:32=A0am, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> If a capacitor has a largish temperature coefficient (hundreds of PPM > capacitance change per degree C maybe), what happens to a charged cap > when the temperature changes? Presumably C*V stays constant, so the > stored energy 0.5*C*V^2 changes. > > So, would the specific heat of the capacitor be different, as a > function of whether it is charged or not?
Fun! Sure why not. But could you measure it? Assume the heat capacity of the cap is about 1 J/(gram-K) What's the Q^2/C energy change? (for 0.1uF and 10 V looks like only ~10uJ's of total energy) George H.
> > -- > > John Larkin =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0Highland Technology Incwww=
.highlandtechnology.com=A0 jlarkin at highlandtechnology dot com
> > Precision electronic instrumentation > Picosecond-resolution Digital Delay and Pulse generators > Custom timing and laser controllers > Photonics and fiberoptic TTL data links > VME =A0analog, thermocouple, LVDT, synchro, tachometer > Multichannel arbitrary waveform generators
On Fri, 26 Oct 2012 08:32:18 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

> > >If a capacitor has a largish temperature coefficient (hundreds of PPM >capacitance change per degree C maybe), what happens to a charged cap >when the temperature changes? Presumably C*V stays constant, so the >stored energy 0.5*C*V^2 changes. > >So, would the specific heat of the capacitor be different, as a >function of whether it is charged or not?
Yes, the heat capacity is going to depend on the charge. Can't be otherwise.
On Fri, 26 Oct 2012 08:32:18 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

> > >If a capacitor has a largish temperature coefficient (hundreds of PPM >capacitance change per degree C maybe), what happens to a charged cap >when the temperature changes? Presumably C*V stays constant, so the >stored energy 0.5*C*V^2 changes.
Why should C*V be constant? w.
> >So, would the specific heat of the capacitor be different, as a >function of whether it is charged or not?
On Fri, 26 Oct 2012 08:32:18 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

> > >If a capacitor has a largish temperature coefficient (hundreds of PPM >capacitance change per degree C maybe), what happens to a charged cap >when the temperature changes? Presumably C*V stays constant, so the >stored energy 0.5*C*V^2 changes.
That does not compute. If the capacitance changes, and the charge does not, the voltage must change inversely, keeping the stored electrical energy constant.
>So, would the specific heat of the capacitor be different, as a >function of whether it is charged or not?
-- John
On Fri, 26 Oct 2012 12:01:40 -0400, Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> wrote:

>On Fri, 26 Oct 2012 08:32:18 -0700, John Larkin ><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: > >> >> >>If a capacitor has a largish temperature coefficient (hundreds of PPM >>capacitance change per degree C maybe), what happens to a charged cap >>when the temperature changes? Presumably C*V stays constant, so the >>stored energy 0.5*C*V^2 changes. >> >>So, would the specific heat of the capacitor be different, as a >>function of whether it is charged or not? > >Yes, the heat capacity is going to depend on the charge. Can't be >otherwise. >
Yeah, I thought so. But as George says, it wouldn't be easily measurable for a real capacitor. Thinking about this, for no good reason, I tripped across this site: http://www.electronics-cooling.com/2012/09/thermal-facts-fairy-tales-the-temperature-dependence-of-the-specific-heat/ -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
On 10/26/2012 11:09 AM, JOF wrote:
> On Fri, 26 Oct 2012 08:32:18 -0700, John Larkin > <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: > >> >> >> If a capacitor has a largish temperature coefficient (hundreds of PPM >> capacitance change per degree C maybe), what happens to a charged cap >> when the temperature changes? Presumably C*V stays constant, so the >> stored energy 0.5*C*V^2 changes. > > That does not compute. If the capacitance changes, and the charge does > not, the voltage must change inversely, keeping the stored electrical > energy constant.
No, OP was right, and it's a cute question. Stored energy is proportional to C and V^2, so even if V is inversely proportional to C, then the energy will change. A simpler way of putting it is that CV=Q will be constant (if the capacitor is isolated) and the energy is proportional to Q^2/C, which obviously changes.
> >> So, would the specific heat of the capacitor be different, as a >> function of whether it is charged or not? >
On Fri, 26 Oct 2012 11:09:04 -0500, JOF <quias@yahoo.com> wrote:

>On Fri, 26 Oct 2012 08:32:18 -0700, John Larkin ><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: > >> >> >>If a capacitor has a largish temperature coefficient (hundreds of PPM >>capacitance change per degree C maybe), what happens to a charged cap >>when the temperature changes? Presumably C*V stays constant, so the >>stored energy 0.5*C*V^2 changes. > >That does not compute. If the capacitance changes, and the charge does >not, the voltage must change inversely, keeping the stored electrical >energy constant. >
1 farad, 1 volt, 0.5 joules 0.5 farad, 2 volts, 1 joule. Same charge, different energies. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
On Fri, 26 Oct 2012 18:05:24 +0200, Helmut Wabnig <hwabnig@.- ---
-.dotat> wrote:

>On Fri, 26 Oct 2012 08:32:18 -0700, John Larkin ><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: > >> >> >>If a capacitor has a largish temperature coefficient (hundreds of PPM >>capacitance change per degree C maybe), what happens to a charged cap >>when the temperature changes? Presumably C*V stays constant, so the >>stored energy 0.5*C*V^2 changes. > > >Why should C*V be constant? >w.
Assuming no leakage, Q should be constant.
On Fri, 26 Oct 2012 09:24:16 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

>On Fri, 26 Oct 2012 11:09:04 -0500, JOF <quias@yahoo.com> wrote: > >>On Fri, 26 Oct 2012 08:32:18 -0700, John Larkin >><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: >> >>> >>> >>>If a capacitor has a largish temperature coefficient (hundreds of PPM >>>capacitance change per degree C maybe), what happens to a charged cap >>>when the temperature changes? Presumably C*V stays constant, so the >>>stored energy 0.5*C*V^2 changes. >> >>That does not compute. If the capacitance changes, and the charge does >>not, the voltage must change inversely, keeping the stored electrical >>energy constant. >> > > >1 farad, 1 volt, 0.5 joules > >0.5 farad, 2 volts, 1 joule. > >Same charge, different energies.
Ok, I see it now. -- John