Two concentric circular loops of wire lie on a tabletop, one inside the other. The inner wire has a diameter of 20.0 cm and carries a clockwise current of 12.0 A, as viewed from above, and the outer wire has a diameter of 30.0 cm. What must be the magnitude and direction (as viewed from above) of the current in the outer wire so that the net magnetic field due to this combination of wires is zero at the common center of the wires?

Explanation:

Magnetic field due to circular wire at the center = μ₀ I / 2 r

I is current and r is radius . μ₀ = 4π x 10⁻⁷.

field B₁ due to inner loop

B₁ = 4π x 10⁻⁷ x 12 / 2 x .20

= 376.8 x 10⁻⁷

Field due to outer loop

B₂ = 4π x 10⁻⁷ x I / 2 x .30

For equilibrium

B₁ = B₂

376.8 x 10⁻⁷ =  4π x 10⁻⁷ x I / 2 x .30

I = 18 A.

The direction should be opposite to that in the inner wire . It should be anti-clockwise.

no sign of dia of fuel cell

scattering of light by tiny particles in air to produce the colors we see on sky, is dependent on the following factors:

(i) wavelength of light

(ii) size of the scattering particle  

(iii) distance traveled by light through the atmosphere.

according to rayleigh's law of scattering, the amount of scattering is inversely proportional to the fourth power of its wavelength. shorter the wavelength, greater is its scattering. hence violet/blue light is scattered more than red.

sun's light is a composite light ( visible light) with wavelengths in the range 400 nm (violet) to 700 nm (red). as sunlight passes through our atmosphere, the molecules of air scatter light according to their wavelengths. during sunrise and sunset, the sun's position is below the horizon. hence light from the sun travels a greater distance in the atmosphere when compared to its position a noon.

as sunlight enters the earth's atmosphere, blue light is scattered away and at the end of its journey in the sky, only red and orange wavelengths remain in the beam.

hence at sunrise and sunset, the sky appears reddish orange.