Question 225: EM waves

Question

An electromagnetic wave with poynting vector $6 \mathrm{Wm}^{-2}$ is absorbed by a surface area $12 \mathrm{~m}^2$. The force on the surface is

Accepted Answer

To determine the force on the surface due to the absorbed electromagnetic wave, we use the following reasoning:
The poynting vector (also called intensity) is given by $$S = 6 \ \mathrm{Wm}^{-2}.$$
The area of the surface is $$A = 12 \ \mathrm{m}^2.$$
So, the total power incident on the surface is:
$$P = S 	imes A = 6 	imes 12 = 72 \ \mathrm{W}.$$
For a surface that completely absorbs the electromagnetic wave, the momentum delivered per unit time (which equals the force) is given by:
$$F = \frac{P}{c},$$
where $$c = 3 	imes 10^8 \ \mathrm{ms}^{-1}$$ is the speed of light.
Substituting the values:
$$F = \frac{72}{3 	imes 10^8} = \frac{72}{300,000,000} = 2.4 	imes 10^{-7} \ \mathrm{N}.$$
Notice that Option A is written as $$24 	imes 10^{-8} \ \mathrm{N},$$ which is equivalent to:
$$24 	imes 10^{-8} \ \mathrm{N} = 2.4 	imes 10^{-7} \ \mathrm{N}.$$
Thus, the correct answer is Option A.

Answer

$24 	imes 10^{-8} \mathrm{~N}$

Answer

$30 	imes 10^{-5} \mathrm{M}$

Answer

$2.4 	imes 10^{-12} \mathrm{~N}$

Answer

$3.6 	imes 10^8 \mathrm{M}$

EM waves

An electromagnetic wave with poynting vector 6 \mathrm{Wm}^{-2} is absorbed by a surface area 12 \mathrm{~m}^2. The force on the surface is

Choose the correct answer: