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

when insulating materials rub against each other, they may become electrically charged . electrons , which are negatively charged, may be 'rubbed off' one material and on to the other. the material that gains electrons becomes negatively charged. the material that loses electrons is left with a positive charge.