To understand tuned circuits, we first have to understand the phenomenon of self-induction. And to understand this, we need to know about induction.

**Need for tuned circuits:**

To understand tuned circuits, we first have to
understand the phenomenon of self-induction. And to understand this, we need to
know about induction. The first discovery about the interaction between
electric current and magnetism was the realization that an electric current
created a magnetic field around the conductor. It was then discovered that this
effect could be enhanced greatly by winding the conductor into a coil. The
effect proved to be two-way: If a conductor, maybe in the form of a coil was
placed in a changing magnetic field, a current could be made to flow in it;
this is called induction.

So imagine a coil, and imagine that we apply a
voltage to it. As current starts to flow, a magnetic field is created. But this
means that our coil is in a changing magnetic field, and this induces a current
in the coil. The induced current runs contrary to the applied current,
effectively diminishing it. We have discovered self-induction. What happens is
that the self-induction delays the build-up of current in the coil, but
eventually the current will reach its maximum and stabilize at a value only
determined by the ohmic resistance in the coil and the voltage applied. We now
have a steady current and a steady magnetic field. During the buildup of the
field, energy was supplied to the coil, where did that energy go? It went into
the magnetic field, and as long as the magnetic field exists, it will be stored
there.

Now imagine that we remove the current source.
Without a steady current to uphold it, the magnetic field starts to disappear,
but this means our coil is again in a variable field which induces a current
into it. This time the current is in the direction of the applied current,
delaying the decay of the current and the magnetic field till the stored energy
is spent. This can give a funny effect: Since the coil **must **get rid of the stored energy, the voltage over it rises
indefinitely until a current can run somewhere!** **This means you can get a surprising amount of sparks and arching
when coils are involved. If the coil is large enough, you can actually get an
electric shock from a low-voltage source like an ohmmeter.

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