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

# capacitor COE question

Started by ●October 26, 2012

Reply by ●October 26, 20122012-10-26

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

Reply by ●October 26, 20122012-10-26

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.

Reply by ●October 26, 20122012-10-26

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?

Reply by ●October 26, 20122012-10-26

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

Reply by ●October 26, 20122012-10-26

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

Reply by ●October 26, 20122012-10-26

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? >

Reply by ●October 26, 20122012-10-26

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

Reply by ●October 26, 20122012-10-26

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.

Reply by ●October 26, 20122012-10-26

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