Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

From what I know, In electromagnetic waves, the components electric field and magnetic field generate each other i.e. magnetic field generated from moving electric charge generates another electric field and which also generates another magnetic field and so on thus, creating an electromagnetic field. So each of them gets weaker with distance which results in weaker EMF. Am I correct here?

share|improve this question

3 Answers 3

up vote 10 down vote accepted

The weakening or otherwise of EM radiation is purely a matter of geometry.

If you have an isolated point charge the field lines travel radially outwards from the charge. At some distance $r$ from the charge the field lines are spread out over a sphere of radius $4 \pi r^2$ so the field line density, i.e the field strength, is proportional to 1/$r^2$. Hence the inverse square law.

On the other hand, if you had a perfectly collimated laser its brightness would be unchanged no matter how far the beam travelled. A perfectly collimated laser is impossible for various reasons, but if you've been to any rock festival you'll know that lasers stay bright for a large distance (though they may be attenuated by the herbal smoke they travel through).

share|improve this answer

The EMW weakening is due to decreasing the photon flux, just like flux of particles decrease in vacuum after explosion. Kinetic energy of a given particle is the same, but the number of particles per square meter per second decreases with distance. Photons are still capable of exiting atoms at large distances - photon energy (frequency) does not change with distance.

share|improve this answer
I like the reminder of the particle nature of light. Really helps to visualize in the context of the question. –  John Jan 17 '12 at 22:57

I will assume you are discussing a situation of war with an (EMP) ElectroMagnetic Pulse originating by a nuclear detonation in the stratosphere.

A HEMP( high altitude nuclear EMP) could only be intentional and it means war.

Nuclear reactors are heavily shielded so that radiation cannot escape to the environment. Thus they are also shielded from incoming radiation by the tons of earth and cement enclosing them, certainly from keV electromagnetic pulses. I expect that in a war situation a metal shield properly designed will be in place to shield from low energy electromagnetic waves.

The effects of surges in the general power lines and electronics etc will be great and unavoidable depending on various factors analyzed in this link but nuclear power reactors should be safe as long as they have cut offs for surges of external power.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.