36 June 2014
BUILD IT YOURSELF
By Richard Panosh
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On a warm summer day, there were a few
kids playing on a swing set in the park.
Farthest from me was a boy swinging with
another boy pushing him from behind.
Closer to me was a girl swinging all by
herself. Her swing went as high as the
boy's and the arc of her swing continued
without decaying. It was easy to
understand the motion of the boy being
pushed as a reaction to an equal and
opposite force, but the girl had nothing to
push against. What magic was she using?
The swing can be thought of as a mechanical resonate oscillator that alternately stores energy in the form of both kinetic and potential energy. The kinetic energy is maximum at the bottom of the swing where
the velocity is maximum. The potential energy is
maximum at the extremes of the swing where it is at
the highest points and the kinetic energy is zero. The
boy pushing the swing adds kinetic energy to
maintain the motion, while the girl adjusts her center
of mass alternately up and down to add potential
energy to maintain her motion.
The concept applies to both mechanical and
electronic oscillators. An electronic oscillator
alternately stores kinetic energy in the inductor as the
magnetic field, and potential energy in the capacitor
as the electric field.
This explanation is very much simplified but is
sufficient to describe the concept of a parametric
amplifier. Additional information can always be found
on an Internet search.
■ FIGURE 1.