What length of a certain metal wire of diameter 0.15 mm is needed for the wire to have a resistance of 15 ω? the resistivity of this metal is 1.68 × 10-8 ω ∙ m?

The resistance of the cylindrical wire is .

Here is the resistance, is the length of the wire and is the area of cross section. Since the wire is cylindrical . Rearranging the above equation,

Here .

Substituting numerical values,

The length of the wire is

as we know our atmosphere or air have different levels. every level has other gasses, and every gas has another permeability. when the light comes from lower permeability to higher permeability, it changes its direction. so that means that when the light comes from the moon to the earth, it changes it's direction many more times. it basically means that the shape of the light part of the moon is changed on the earth.

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