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Application to the SHO: choice of ``natural'' units

Rather than working in units where mass in measured in gm, the most natural unit of mass for the problem of the SHO (2) is to take the mass of the oscillating particle to be the unit mass for the problem. Mathematically, , or equivalently so that is just the numerical value of the mass of the particle when measured in gm. Perhaps the most intuitive unit of time for the problem would be the period of the oscillator. However, this choice leads to a proliferation of factors of in formulae. A more natural choice is the amount of time it takes for the oscillator to move through of phase, this will set the value of the angular frequency within our unit system to be . From the general conversions in TABLE I, we see that , and our choice thus sets the value so that the inverse of the natural unit of time is the numerical value of the frequency of the oscillator when measured in the traditional unit of .

Having set um and ut, there remains one basic unit to fix, the unit distance. Their also remains one final experimental constant remains in the TISE for the SHO, . It is a general feature of dimensional analysis that choosing units for three independent dimensions completely determines the new system of units. In our case, the most convenient choice for the unit distance is one where the numerical value of is . This fixes so that , which sets .

With the units for the basic dimensions determined, we may now generate the basic units for all physical quantities by inserting the choices we have made for the basic units, , and in the table above. The results of doing this we summarize in the table below. Note that the results are always just the only combination of powers of m, and that have the appropriate dimensions.

In our new system of units , and and the TISE, in the position and momentum representations respectively, reads

 

Here we adopt the notation that quantities measured in our new system of units always appear as either capital or calligraphic letters.

The conversion from quantities expressed in the natural units for this problem and the standard cgs units is straight forward using Table II. For instance, the traditional value of energies E will just be the values we compute in our new units times the new unit of energy . Thus an energy of corresponds to an energy of . It is common practice to drop writing our specialized units and write simply . Similarly, a distance or position X=1 as measured in the new system corresponds to in the traditional system.

With the relation between basic quantities defined, the relationship between functions may then be determined by physical reasoning. For instance, the probability of finding the particle with a position in the range with of traditional positions corresponds to finding the position as measured in the new system in the range , thus Hence, given in the new system we can always convert it to a traditional through



next up previous
Next: Self-trasnform property of Up: Dimensional Analysis Previous: Motivation and General



Prof. Tomas Alberto Arias
Thu Oct 12 22:00:51 EDT 1995