Experiment 28: Making a Coil React
248 Chapter 5
Here’s one last variation on this experiment to test your understanding of elec-
trical fundamentals. Remove the 220Ω resistor, and substitute a 1K resistor (to
protect your LED from sustained current). Remove the coil, and substitute a
very large capacitor—ideally, about 4,700 μF. (Be careful to get its polarity the
right way around.) What will you see when you press the button? Note that
you will have to hold it down for a couple of seconds to get a result. And what
will you see when you release the button? Remember: the behavior of capaci-
tance is opposite to the behavior of self-inductance.theory
Alternating current concepts
Here’s a simple thought experiment. Suppose you set up a 555 timer to send a
stream of pulses through a coil. This is a primitive form of alternating current.
We might imagine that the self-inductance of the coil will interfere with the
stream of pulses, depending how long each pulse is, and how much inductance
the coil has. If the pulses are too short, the self-inductance of the coil will tend
to block them. Maybe if we can time the pulses exactly right, they’ll synchronize
with the time constant of the coil. In this way, we can “tune” a coil to allow a
“frequency” to pass through it.
What happens if we substitute a capacitor? If the pulses are too long, compared
with the time constant of the capacitor, it will tend to block them, because it
will have enough time to become fully charged. But if the pulses are shorter, the
capacitor can charge and discharge in rhythm with the pulses, and will seem to
allow them through.
I don’t have space in this book to get deeply into alternating current. It’s a vast
and complicated field where electricity behaves in strange and wonderful ways,
and the mathematics that describe it can become quite challenging, involving
differential equations and imaginary numbers. However, we can easily demon-
strate the audio filtering effects of a loudspeaker and a coil.