Page 38 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

Conductors and ~nsMlators 7 9

The maximum current IMAX occurs at time To when there is the greatest
difference between Vpos and VCAP; from Ohm’s Law, I,,, = Vpos/R. The
capacitor is considered to be completely charged by time TFuLL, at which
point the flow of current falls to zero.
The time T,, equals R x C and is known as the RC time constant. With R
in ohms and C in farads, the resulting T,, is in units of seconds. The RC time
constant is approximately equal to the time taken for V,,, to achieve 63% of
its final value and for the current to fall to 37% of its initial value.”

I nd srctance

This is the tricky one. The author has yet to see a water-based analogy
for inductance that didn’t leak like a sieve <grin>. Consider two electric fans
facing each other on a desk. If you turn one of the fans on, the other will start
to spin in sympathy. That is, the first fan induces an effect in the second.
Well, electrical inductance is just like this, but different.
What, you want more? Oh well, how about this then. . . a difference in
electrical potential between two ends of a conducting wire causes current to
flow, and current flowing through a wire causes an electromagnetic field to be
generated around that wire (Figure 3-1 1).

/-
I

Figure 3-1 1. Current flowing through a wire
generates an electromagnetic field

12 During each successive T,, time constant, the capacitor will charge 63% of the remaining distance
to the maximum v’oltage level. A capacitor is generally considered to be fully charged after five
time constants.