0
$\begingroup$

At this point in this documentary about the history of electricity: https://www.youtube.com/watch?v=oPnS2WO2_0k&t=4m40s the guy says the Maxwell calculations predicted the generation of certain types of waves, when there is a change in the direction of the current.

I did not have time to study the derivation of the Maxwell equations, so I don't know exactly how they work, but could somebody show me what calculations predicts this phonomena?

Further, in this video, the guy shows a wireles telegraph, simply made with this theory. What is the minimum knowledge I need to build one? (i'm gonna build it inside my mind)

I need to know if there's a paper or book that talks about this subject: how to generate this waves with an experiment, and how to construct a simple wireless telegraph.

$\endgroup$
4
  • 1
    $\begingroup$ How well versed are you in the field of multi-variable calculus? do you know these terms: gradient, divergence, curl? $\endgroup$ Jul 8, 2014 at 7:17
  • $\begingroup$ There is that old TV show, The Mechanical Universe that explained these subjects wonderfully, if you can find it. $\endgroup$
    – rodrigo
    Jul 8, 2014 at 8:00
  • 1
    $\begingroup$ The calculation to derive the EM wave equation is described in detail in the Wikipedia article Electromagnetic wave equation. $\endgroup$ Jul 8, 2014 at 10:05
  • $\begingroup$ Basically, you want a radio :) $\endgroup$
    – sailx
    Jul 8, 2014 at 11:06

1 Answer 1

2
$\begingroup$

First of all, I'm not an expert, but that can be an advantage in trying to explain the equations in lay terms...

Maxwell's equations are these, in differential form:

  1. $$ \nabla \cdot \mathbf{E} = \frac {\rho} {\varepsilon_0}$$
  2. $$ \nabla \cdot \mathbf{B} = 0 $$
  3. $$ \nabla \times \mathbf{E} = -\frac{\partial \mathbf{B}} {\partial t} $$
  4. $$ \nabla \times \mathbf{B} = \mu_0\left(\mathbf{J} + \varepsilon_0 \frac{\partial \mathbf{E}} {\partial t} \right) $$

Eq. 1 means that an electric charge creates a proportional electric field .

Eq. 2 means that there is no magnetic charges (monopoles).

Eq. 3 means that a variation in the magnetic field creates an electric field.

Eq. 4 means that a variation in the electric field plus currents (moving chages) creates a magnetic field.

Now, with that in mind think of what all this means: an electric charge creates an electric field, the charge moves, the electric field changes, that change creates a mangnetic field... All the known electric and magnetic phenomena.

But, hidden in these equations there is another interesting possibility. Without the need of any electric charge, a sinusoidal electric field could create a (cosinusoidal?) magnetic field, that in turn will create another electric field, and so on. That is, a standing electro-magnetic wave, without the need of any charge at all!

Those are what they call solutions to the Maxwell's equations.


Now, you want to create a radio transmitter. The simplest form is just an antenna, that is a long piece of wire, in which you inject an electric current that oscillates at the same frequency you want to transmit (from a few kHz to hundreds of MHz). The variable electric field in the wire will create an electro-magnetic wave.

The funny part is to build the receiver. For that you will need another antenna. There, the coming electro-magnetic wave will induce a current (fractions of a mA), that you will amplify with an electronic circuit (a valve or transistor will do) and send to whatever device you use to generate the final output.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

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