Sunday, July 28, 2013

Science Clock Series: Part VI

Today's number, like the previous one, comes from chemistry and is given by:

$\approx\rho\text{ of Zn }(\times10^{-8}\,\Omega\cdot\text{m at }20^\circ\text{C})$ The letters Zn are the chemical symbol for the metal zinc, and at first glance you might think that the rho ($$\rho$$) in this equation is the same as the rho in the equation for the number one, standing for density. This is not the case. Confusingly, rho can also represent resistivity, as it does here.

Resistivity is the property of a substance to resist the flow of electricity. The maguscule omega ($$\Omega$$) stands for ohms, the standard unit of measure for resistance, which is a slightly different property than resistivity. Resistance depends on circumstances such as how a substance is shaped, while the resistivity of a substance is independent of the shape it takes. (For example, a short, fat copper wire has a lower resistance than a long, thin copper wire, but the resistivity of the copper making up the wire is the same in both cases.)

In one respect, resistivity and density are similar: they are both temperature dependent, which is why the resistivity is specified at $$20^\circ$$C. If we look up the resistivity of zinc at $$20^\circ$$C, we find it to be $$5.90\times10^8\,\Omega\cdot\text{m}$$.

Tune in next time for a number from geology! Click here to jump directly to it.