esonance on the nuclear scale is somewhat analogous to mechanical resonance. It is a response of an object or system that vibrates in step or phase.
A glass can be broken by the voice of a singer or a bridge can oscillate and collapse when the marching rhythm of a column of soldiers or the undulation caused by strong winds correspond to its own structural resonance frequency, such as the Tacoma Narrows Bridge across the Pudget Sound (Figure 02-08).
This correspondence of frequencies allows energy to be transferred from the external world (the soldiers' legs) to a given physical system (the bridge).
Similarly, a resonance phenomenon will occur when an electromagnetic wave of appropriate frequency (equal to the Larmor frequency) reaches the nuclei; then, nuclei located in the state of lower energy will be transferred to the state of higher energy.
Figure 02-08:
The collapse of the Tacoma Narrows Bridge across the Pudget Sound in Washington state (U.S.A.) in November 1940 was caused by high winds making the structure oscillate. Resonance can contribute to such accidents.
Watch a newsreel movie of this incident.
Newsreel (video in mp4):
The Tacoma Bridge Disaster. The 1.9 kilometers long Tacoma Narrows Bridge (called Galloping Gertie) was the first suspension bridge across the Pudget Sound, connecting the Olympic Peninsula with the mainland of Washington state. Four months after its opening, on the morning of 7 November 1940, in a wind between 65 and 75 kilometers per hour the bridge went into a series of torsional oscillations which passed into the bridge's natural resonance frequency. The amplitude steadily increased until the bridge span broke up.
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Chapter 2 — Page 5 |
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