Calculate the number of turns in the coil wire needed to light blub. Would this

Question

Calculate the number of turns in the coil wire needed to light blub.

Would this high wind warning feature be useful?

Anemometers (pictured) are used to measure wind speeds. Wind striking the cups causes the assembly to rotate; a gauge measures the frequency of this rotation and translates the data into wind speed. The length of each arm, as measured from cup to cup, is 42.5 cm. The friction in the bearinas is nealigible so the tangential speed of the cups matches the wind speed fairly closely bulb Your company makes anemometers and plans to install light bulbs on them for high wind warnings.The idea is to attach a coil of wire of radius 13.2 cm to one of the arms and use the EMF induced by the magnetic field of the Earth to light the bulb. The bulbs possess 234 of internal resistance but need 11.5 W of rms power dissipation to be visible at night. (The resistance of the wire is negligible.) Although company management wants this new feature, you have doubts about its practicality. Assuming the magnetic field of the Earth is _ 0.500 gauss and is oriented roughly horizontal with the Earth, and that R 90.5 mph is considered the threshold of dangerous wind speed calculate the number of turns in the coil wire needed to light the bulb Vwind turns Would this high wind warning feature be useful? O O No, the coil would be too large to fit on the anemometer Yes, this device is practical from a design standpoint.

Explanation / Answer

Flux = BAcos(wt)
induced emf = nBAwsin(wt)
emf^2/234 = 11.5, so emf = 51.87 V = n*(0.5*10^-4)(3.14*0.132^2)(40.45/0.425)sin(40.45t/0.425) = n*2.6*10^-4sin(95.17t)
n*2.6*10^-4 = 51.87 * sqroot(2) => rms value
n = 282135 turns
the device is not practical as it will be too heavy

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