A 20W light bulb emits a continuous stream of light in all directions. At a part

Question

A 20W light bulb emits a continuous stream of light in all directions. At a particular distance away from bulb, the electric field has strength of 12.5 mu N/C? the magnetic field direction at this point is in the magnetic field direction at this point u in the What is the magnetic field strength at this point? What is the total energy density at this point? What is the intensity of the light at this point? How far away from the Iight bulb is our point of interest? How far back in time would a person standing at the point of interest see the light bulb? What is the direction of the Pyonting vector?

Explanation / Answer

Given, electric field E = 12.5 N/C

We know speed of the light in air c =3×108 m/s

(a) E/B = c, so magnetic field B = (12.5×10-6)/(3×108) = 4.2×10-14 Tesla

(b) Energy density = energy per unit volume u = 0E2 = 8.854×10-12×(12.5×10-6)2 = 1.383×10-21 J/m3

(c) Intensity I = ½ c0E02 = 2.07×10-13 W/m2

(d) Given, power P = 20 W

Intensity I = P/A, So the area A = P/I = 20/(2.07×10-13) = 9.66×1013 m2

A = 4r2, So the distance r = 2.77×106 m

(e) Time t = r/c = 9.2×10-3 s

(f) Pyonting vector S = (E×B)/0

The direction of the electric field is x direction

The direction of the magnetic field is –z direction

So, the direction of the Pyonting vector is y direction

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