Problem of the Week

Chapter 22: Current Electricity

Problem of the Week

Incandescence
Incandescence is the process of heating up an object so hot that it gives off electromagnetic radiation in the visible region, in other words, you heat it till it glows.

All warm bodies emit electromagnetic radiation, from lightbulbs to index fingers.

Lightbulbs —basically, tungsten filamented, electricity-to-light/heat converters

<a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=gif::::/sites/dl/free/0078458137/193800/ch22_light.gif','popWin', 'width=NaN,height=NaN,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif"> (0.0K)</a> All lightbulbs have power ratings, for instance, 60 W. You have learned that power depends upon current and voltage in the relation P = IV. Because the wall voltage is a constant 120 volts, the power must depend upon the current flowing through the bulb. Current, in turn, depends upon the resistance of the lightbulb, that is, the resistance of the tungsten wire. What might influence the resistance of the tungsten wire?

<a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=gif::::/sites/dl/free/0078458137/193800/ch22_resistence.gif','popWin', 'width=NaN,height=NaN,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif"> (0.0K)</a> The longer the wire, L, the more resistance. The larger the wire, A, the less the resistance. Think of a pipe carrying water, this relation R

<a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=gif::::/sites/dl/free/0078458137/193800/alpha.gif','popWin', 'width=NaN,height=NaN,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif"> (0.0K)</a> L/A makes sense. Rho is a property of the metal, in this case,

<a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=gif::::/sites/dl/free/0078458137/193800/rho.gif','popWin', 'width=NaN,height=NaN,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif"> (0.0K)</a> _tungsten= 5.6 x 10^-8

<a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=gif::::/sites/dl/free/0078458137/193800/omega.gif','popWin', 'width=NaN,height=NaN,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif"> (0.0K)</a>•m.

In 150-W bulb, how long is the filament? Tungsten filament is drawn to a radius of 45 microns (µm).

How might you pack a semi-truck long filament into a fist-sized glass bulb? The solution, make a coil of the wire. If that coil is still too long, coil the coil; and that is what is done, a filament is a coiled coil.

To closely pack the filament wire, the coils are wound at 250 turns per inch. In order to for the current to flow through the entire wire, none of the coils can touch its nearest neighbor. What is the space gap between each coil?