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Name: Rebecca M.
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Date: 1/13/2003

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.

Vince Calder

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 them.

Best, Dick Plano

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