Hi Guys.

Here is a pic of an EM wave.

In real life, do the waves really go up and down like a slithering snake while travelling or is the picture trying to tell is that the voltage of the wave reaches a certain maximum value and then goes to zero volts in a certain direction .

 

No. This is just a graphical representation of amplitudes.
Wave motion such as waves on water is actual physical displacement of f(x,y,z,t).

With EM waves there is no physical displacement but changes in electric and magnetic field intensity.

 

No. This is just a graphical representation of amplitudes.
Wave motion such as waves on water is actual physical displacement of f(x,y,z,t).

With EM waves there is no physical displacement but changes in electric and magnetic field intensity.

Wow! I am so glad I asked this question. Ty very much!

 

No. This is just a graphical representation of amplitudes.
Wave motion such as waves on water is actual physical displacement of f(x,y,z,t).

With EM waves there is no physical displacement but changes in electric and magnetic field intensity.

Hi MrChips:

If EM waves are about changes in Electric and Magnetic fields, then why does a Yagi antenna call for a certain length for its 'Driven Element'? Isn't length a physical property?

( I used the yagi as an example; I meant any antenna which asks for a certain length of the driven element)

 

If EM waves are about changes in Electric and Magnetic fields, then why does a Yagi antenna call for a certain length for its 'Driven Element'? Isn't length a physical property?

You seem to be confusing the EM amplitude with it's wavelength.
There is no relation between the two.
The antenna dimensions are determined by the wavelength of the EM wave, which is indeed physical (the speed of light divided by the EM wave frequency).

 

My dude- these are good questions to ask. As was previously stated, the diagram you posted initially shows electric field values in space and time. The length of the Yagi elements is chosen such that the element resonates with maximum induced current at that wavelength, like a tuning fork does for sound.

 

What I meant by no physical displacement is that there is no material mass that moves up and down or left and right.
What is changing are intensities of electric and magnetic fields at a spacial point (x, y, z).

If we take any point in space (x, y, z) there is an electric field and a magnetic field function which we can write as E(x, y, z, t) and B(x, y, z, t).

At any given time t0, the E and B fields at two different points (x1, y1, z1) and (x2, y2, z2) are different, E(x1, y1, z1, t0) and B(x2, y2, z2, t0). That is on account of the phase difference as a result of the propagation of the E and B waves.

This is what happens along the length of an antenna. The phase of the field changes along the length of the antenna.

 

A Yagi antenna or any resonant antenna in general operates on the principle of electrical length.
https://en.wikipedia.org/wiki/Electrical_length

 

You seem to be confusing the EM amplitude with it's wavelength.
There is no relation between the two.
The antenna dimensions are determined by the wavelength of the EM wave, which is indeed physical (the speed of light divided by the EM wave frequency).

Ty. So going by what you said, when my local radio station transmits an FM song and the wave is propagated into the air to the front of my face , is there a 3 meter long wave that is shaped like an up and down slithering snake, in front of my face?

 

Ty. So going by what you said, when my local radio station transmits an FM song and the wave is propagated into the air to the front of my face , is there a 3 meter long wave that is shaped like an up and down slithering snake, in front of my face?

No.
Imagine that you have three lamp posts positioned at different distances ahead of you. Each lamp is illuminated by the travelling 100MHz EM wave.
One is 3m away, the next is 4.5m away and the third is 6m away.
If you were able to capture the light emitted by each lamp at a very high frequency, say 500MHz, you will observe that the 3m and 6m lamps are always in phase while the 4.5m lamp is out of phase.

(This gets tricky if the lamps illuminate equally on positive and negative halves of the cycle. In this case, we would position the lamps at 3, 3.75 and 4.5m distances, i.e. quarter wavelength apart.)