On Jan 29, 2012, at 2:02 PM, jim wrote:

> If there is a current of 10ma charging a 1uF capacitor, what is the volta=
ge
> across the capacitor at 100ms?

v(T) =3D 1/C (integrate(i(t), dt, t0, T)
     =3D 1e6 * 0.01 * .1
     =3D 1e3 V

Is that what you got?  Sanity check: if we has a 1e3 V power supply, then t=
o limit current to 10mA we would use a 100k ohm resistor, and the RC time c=
onstant would be
1e-6*1e5 =3D 0.1s.  Yep.  (This would not result in a constant current of 1=
0mA, and the capacitor voltage would not be 1000V after 1 time constant, bu=
t that it is the correct order of magnitude is reassuring.)

It does seem surprising, since 1uF and 10mA are values you'd expect to find=
 in a real circuit, and 1e5 V isn't.  However, pumping constant current int=
o a cap is not trivial to actually accomplish.  Most real circuits are clos=
er to voltage sources with resistance, and current into caps quickly falls =
to small amounts.

BillW

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