The electrons in a receiving antenna oscillate, can we establish if they respond to an electric or a magnetic field?

How can we know if there is an electric field apart from the one caused by the magnetic field that makes the charges oscillate?

  • $\begingroup$ a magnetic field is an electric field surely? someone please correct me if im wrong $\endgroup$ – Alex Robinson Apr 12 '17 at 8:21
  • $\begingroup$ From Coulomb law $ \vec{E}(x,y,z) = \frac{q \vec{r}}{4 \pi \epsilon_0 r^3}, $ knowing that $ c = \frac{1}{\sqrt{\epsilon \mu}}$ and using and using Biot-Savart law we get $ \vec{B}(x,y,z) = \frac{\mu_0 e (\vec{v} \times \vec{r}) }{4 \pi r^3} = \frac{(\vec{v} \times \vec{r}) q}{c^2 (4 \pi \epsilon_0 r^3)} = \frac{\vec{v} \times \vec{E}}{c^2} $, ($r = \sqrt{x^2 + y^2 +z^2}$). From this follows that the magnetic field is an "electric" field from a moving charge. That's why in special relativity we put B & E into a symmetric tensor $F^{\mu\nu}$.And it transforms depending on who is observing $\endgroup$ – Mihai B. Apr 12 '17 at 8:59
  • $\begingroup$ But only B component will for example induce an electric field in a coil. You could make a device such that only one component gets to activate the device. A device will respond differently to a B field than to an E field. It's all in Maxwell's equations. $\endgroup$ – Mihai B. Apr 12 '17 at 9:02
  • $\begingroup$ A changing magnetic field may be inducing an electric field (and vice versa) but the Lorentz force on an electron is $$\vec{F} = q\vec{E} + {q\over c}(\vec{v} \times \vec{B}),$$ so the magnetic part of the force is suppressed by a factor of $v/c$ relative to the electric part - that's at least there orders of magnitude smaller for conduction electrons. To prevent misunderstanding, this is the Lorentz force in CGS units. Using SI units shifts the $c$ from the force law to the magnetic field, but of course the ratio of the forces does not change. $\endgroup$ – NickD Apr 15 '17 at 18:27

A stronge signal one get if one use an receiving antenna rod that is directed in the same direction as the emitting antenna rod. Since the electric field component of the radio wave is directed parallel to the emitting antenna this component is responsible for the acceleration of the electrons in the receiving antenna rod.

Otherwise a strong signal one get too if one use a Loop antenna:

The small loop antenna is also known as a magnetic loop since it behaves electrically as a coil (inductor) with a limited but non-negligible radiation resistance due to its small size compared to one wavelength. It can be analyzed as coupling directly to the magnetic field in the region near the antenna – the opposite of the principle of a Hertzian dipole which couples directly to the electric field.


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