Question

Find the electric field amplitude in an electromagnetic wave radiated by a $100$ W bulb at a distance $3$m from it, assuming an efficiency of a bulb is $2.5$% and it behaves like a point source.

• A. $29$
• B. $4.07$
• C. $2.9$
• D. $40.7$

Answer 1

Answer: (B)

$4.07$

The bulb, as a point source, radiates light in all directions uniformly.

At a distance of 3m the surface area of the surrounding sphere is

$A=4{\pi }^2=113m^2$

The intensity at this distance is

$I=\frac{power}{area}=0.022\frac{W}{m^2}$

Half of this intensity is provided by the electric field and half by the magnetic field

we get

$E_{rms}=2.9\frac{v}{m}$

Since the electric field is sinusoidal, the peak electric field $E_0$ is

$E_0=\surd 2E_{rms}$

$E_0=4.07\frac{v}{m}$