Calculating inductance values and other questions on capacitors

Thread Starter


Joined Jan 13, 2008
Hi guys, I've only just signed up but have been reading the forum for a few days and it looks good!

I've got a couple of questions I need a bit of help with, due to not working with electronics for years! And I'm having problems finding the correct formulae to find the answers I need.

First one:

Second one is this: A conventional ignition circuit with the contact breakers open and a 3 microfarad capacitor receives a voltage of 12v, state its charge in coulombs.

If the primary coil has a resistance of 5.2 ohms is included in series with the capacitor, what would the state of charge now be? (ignoring any inductance qualities)
Also state the circuits time constance and the initial charge current.

Any help would be much appreciated!

Many Thanks


Thread Starter


Joined Jan 13, 2008
I've just read the rules for posting in this section and have realised this thread might be breaking some of the rules stated.

Just to clarify, I'm not after the answers to the posted questions, just some sort of guide towards which formulae required to answer them myself. I've tried searching online and offline for them and I'm not having much luck, hence my posting here.

I can imagine this might be very basic electronics principle but if anyone has any pointers that would be superb.

The only progress I've managed to make is to find the charge in coulombs for part of the 2nd question,
using the forumula C=Q/V ----> Q=CV
Q=0.000003 x 12?

I think once I have the forumula for the first question, it should be a case of dropping the correct figures into it to find the correct answer?

I hope this covers my back so to speak!

Ron H

Joined Apr 14, 2005
Do you know Ohm's law? It applies for inductors (and capacitors) as well as resistors. The general equation is Z=V/I, where Z is impedance. For a pure inductor, Z=2*pi*F*L, where L is the inductance, and F is the frequency. This is all the information you should need.
Your charge calculation is correct. I don't understand the third question.