The magnitude j(r) of the current density in a certain cylindrical wire is given as a function of radial distance from the center of the wire's cross section as j(r) = br, where r is in meters, j is in amperes per square meter, and b = 1.95 ✕ 105 a/m3. this function applies out to the wire's radius of 2.00 mm. how much current is contained within the width of a thin ring concentric with the wire if the ring has a radial width of 14.0 μm and is at a radial distance of 1.20 mm?

a skateboard rolling across the street - apex

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.

conservation of mass

mass before something happens = mass afterwards

365.25 earth days are in the year of a light year