Before reporting spectacular results from a student project let me present
its methodology. Do you see anything objectionable?
Suppose a black box with two electric terminals (for example, a supercap,
C=10 F) is connected to a constant voltage source (for example, Vs=2.5 V)
through a resistor (for example, 15 ohms). How much energy is received by
the box during a finite time interval (for example, 500 seconds)?
A voltage probe from Vernier was used to measure the d.o.p. on C, Vc,
every two seconds. There were no need to measure Vr, the d.o.p. on R,
because Vs+Vr=Vb. For any known Vc one has I=(Vs-Vc)/R. The total amount
of energy entering the box during a given time t is the sum of partial
energies, dE, delivered during consecutive time sub-intervals, dt.
where I1 and Vc1 refer to the beginning of each short time sub-interval
while I2 and Vc2 refer to the end of that sub-interval.
And how much energy flows out from the box when it is disconnected from
the source and discharged through R? The same approach (measurements of
many instant values of Vc, calculations of dE and a summation) can be
used to answer the second question. Do you agree?
By the way, the input impedance of the Vc voltmeter was enormous (many
giga-ohms, as verified by discharging a very good capacitor through
the probe over a time interval of several weeks).
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: Gedanken-ing is not enough; physics is an experimental science! :
: Inspired by thinking about phys-L messages on capacitors :
: Ludwik Kowalski :
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