The ABC's of Resonance
Resonance causes an object to move back and forth or
up and down.
This motion is generally called oscillation. Sometimes the oscillation
is easy to see such as the motion of a swing on a playground or the
vibration in a guitar string. In other cases the oscillation is impossible to see
without measuring instruments. For example, electrons in an
electrical circuit can oscillate but it happens on a molecular
level.
In resonance the oscillation occurs at a specific
frequency. These oscillations build up rapidly to very
high levels. Ultimately some of the energy in the oscillations has to be removed
from the object or the size of the oscillations get so large that the object
breaks.
Resonance requires 3 basic conditions:
A) An Object With a Natural Frequency: The object can
be a mechanical device or an electronic circuit. An object's natural frequency
is the frequency it tends to oscillate at when disturbed. The oscillation can be
a mechanical vibration as is the case when the string of a guitar is strummed.
In an electronic circuit the oscillation is a variable voltage or current. An
object can have more than one natural frequency. These are called harmonics. A
guitar string sounds musical because it vibrates with several harmonics when it
is strummed.
B) A Forcing Function at the Same Frequency as the Natural Frequency:
In mechanical systems the forcing function is a variable force. In electronic
circuits it arises from a variable electric field. In either case the forcing
function does work on the object it is applied to. Since work is a form of
energy transfer it causes energy to build up in the object.
C) A Lack of Damping or Energy Loss: For an object to
resonate, mechanical or
electrical energy has to build up in the object. Anything which removes these
forms of energy tends to interfere with resonance. Damping is a means of
removing electrical or mechanical energy by converting it to heat. The term
damping should not be confused with the term dampening which means to make
something slightly wet. Friction,
air resistance, and viscous drag can all provide damping in mechanical systems.
Electrical resistance performs the same function in electronic circuits. Other
forms of energy loss can include sound (musical instruments) or light emissions
(lasers).
When the forcing function's frequency matches the natural frequency of an object it will
begin to resonate. The forcing function adds energy at
just the right moment during the oscillation cycle so that the oscillation is
reinforced. This makes the oscillation's amplitude grow larger and larger. These
oscillations would eventually become infinitely large. However, as mentioned
earlier, long before the oscillations reach infinity one of three things happens: 1) the object's dynamics change
so that the resonant frequency and forcing functions no longer match, 2) the energy lost as heat, sound, or light becomes equal to the energy input. or 3)
the object breaks (see The Dark Side of Resonance).
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