Radio Waves and Concrete
Name: Rebecca M.
How does the cell phone signal pass through a concrete wall?
The frequencies used in cell phones are too low for the electrons in
concrete to oscillate in resonance with them, so they are approximately
transparent to those frequencies. The steel reinforcing bars are another
matter. These can act as absorbers over a wide range of frequencies and
reinforced concrete will interfere with the transmission of radio waves over
a wide range of frequencies.
Electric and magnetic fields can pass through insulators without
trouble. A conducting wall, such as one made from copper, on the
other hand, stops these fields quite efficiently. That is because the
electric field moves charges which continue moving until there is zero
electric field (and therefore zero force) acting on them. Since this
reduces the electric field to zero, the electric field does not
penetrate the copper wall. Similarly, changing magnetic fields
generate currents in conductors which, by Lenz's Law, produce magnetic
fields which oppose the change that produced them, thereby cancelling
the incident magnetic field.
In insulators, such as concrete, the electrons are not free to move
about and so do not cancel the rapidly changing electric and magnetic
fields in the cell phone signal.
You can check this if you have a strong magnet and a thin concrete
wall. Compare the force on a piece of magnetic material or another
magnet at the same distance with and without a concrete wall between
Best, Dick Plano
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Update: June 2012