|
Resonance occurs widely in nature, and is exploited in many man-made devices. Tuning two tones to unison will present a peculiar effect: When the two tones gradually approach unison, the beating slows down and disappears, giving way to full-bodied unison resonance.
Resonance is a common thread which runs through almost every branch of physics; it causes an object to move back and forth or up and down. This motion is generally called oscillation. Resonance accounts for the tendency of a system to oscillate with larger amplitude at some frequencies than at others. These are known as the system's resonant frequencies. At these frequencies, even small periodic driving forces can produce large amplitude oscillations, because the system stores vibrational energy.
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.
Imagine a father pushing his child on a swing. Small pushes can cause the child to swing fairly high, if Dad pushes at the right time. If Dad pushes as the swing returns to its highest point, gentle pushes are magnified, and the child keeps swinging higher. This case is an example of resonance. If Dad pushes randomly, the pushes cancel out and swing does not go high. Resonance does not occur.
| Whenever an oscillatory motion receives a periodic force at the same frequency as the natural frequency (Fig 1), resonance occurs. Resonance magnifies the effect of the force or oscillation. If the two frequencies are different resonance does not occur. Forces applied randomly cancel each other out.
In physics resonance can have the effect of amplifying relatively small forces or effects so that they become much larger. This occurs when a system is able to store and easily transfer energy between two or more different storage modes (such as kinetic energy and potential energy in the case of a pendulum). However, there are some losses from cycle to cycle, called damping. When damping is small, the resonant frequency is approximately equal to a natural frequency of the system, which is a frequency of unforced vibrations. Some systems have multiple, distinct, resonant frequencies.
Resonance phenomena occur with all types of vibrations or waves: there is mechanical resonance, acoustic resonance, electromagnetic resonance, nuclear magnetic resonance (NMR), electron spin resonance (ESR) and resonance of quantum wave functions.
Resonant systems can be used to generate vibrations of a specific frequency (e.g. musical instruments), or pick out specific frequencies from a complex vibration containing many frequencies.
|
 |
Fig 1
|
|
Without resonance we wouldn't have radio, television, music, or swings on playgrounds, not to mention scalar coils. Yet, it is one of the most striking and unexpected phenomenon in all of physics.
|
