# Electromagnetic waves and phase difference

Discussion in 'Physics' started by logearav, Oct 14, 2011.

1. ### logearav Thread Starter Member

Aug 19, 2011
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Electromagnetic waves contain time varying electric and magnetic field perpendicular to each other and also to path of progression. The phase difference between electric field vector and magnetic field vector is zero

How the phase difference is zero? Because when E is at ∏/2, B is at -∏/2. When phase difference is zero, Both E and B are in phase with each other, which means when E is at zero, B should also be at zero and when E is at ∏, B should also be at ∏. I am confused. Revered members, please help

2. ### nsaspook AAC Fanatic!

Aug 27, 2009
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If the phase difference is zero then the wave is propagating in far-field resistive free space so all power is real, near the antenna source the phase relationship changes up to 90 degrees as the power can be mainly reactive with small antennas.

http://en.wikipedia.org/wiki/Near-field_electromagnetic_ranging

Last edited: Oct 14, 2011
3. ### logearav Thread Starter Member

Aug 19, 2011
248
0
Thanks nsaspook.
Let me quote the following images for my doubt. First image is EM wave and the second image is similar to EM wave where the Red wave is out of phase with Blue wave. So how can we say EM wave is in phase with both the components involved.? Aren' t the E and B vectors out of phase?

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4. ### nsaspook AAC Fanatic!

Aug 27, 2009
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Think of the phase of the two fields in relationship to time and space moving together but one has been rotated 90 degrees to the other. http://www.youtube.com/watch?v=4CtnUETLIFs The phase is the polarity relation of the fields to each other not the fixed 90 degree rotation.

If they were out of phase then we would have an imaginary (negative) power component. Like (in)phase fields always results in real (positive) power just like it does when you multiply numbers.

Last edited: Oct 15, 2011
5. ### logearav Thread Starter Member

Aug 19, 2011
248
0
Thanks a lot for your reply. I dont understand nsaspook. What do you mean by polarity relation of fields?.

6. ### Wendy Moderator

Mar 24, 2008
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Each field is big on one axis and small (nonexistent) in the other. For the electric and magnetic fields to form a electromagnetic field they must be in perfect sync and 90° from each other. This means you can have horizontal or vertical electromagnetic fields. This is usually based of the orientation of the antenna, and the antenna itself is polarized.

The term polarized is not a coincidence, since light itself is also electromagnetic radiation. If you use a polarized lens with something like a LED or LASER diode you will find the light will pass and one angle and be blocked at another. It is a practical demonstration of polarization and how it words.

7. ### steveb Senior Member

Jul 3, 2008
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Bill is correct, but to help clarify further, you can think of the orientation of the electric field as a type of "marker" to identify the polarization. If the electric field points in the y direction, then you can say the wave is y-polarized. If the electric field points in the z-direction, you can say that the field is z-polarized. If it points in another direction and the wave is still going in the x-direction, you can use vector addition and resolve the wave into a y-component and a z-component.

Basically, light is a transverse wave capable of having different polarization states. If you have ever studied Lissajous patterns (http://en.wikipedia.org/wiki/Lissajous_curve) on an oscilloscope, you can make a direct analogy to wave polarization. You can have linear patterns, circular patterns and even elliptical patterns of the electric field vector in the yz-plane. You think of having two separate electromagnetic waves, one with y-polarization and one with z-polarization, and adding them together. You allow one wave to be shifted along the x-axis to make a phase shift. Once you understand the basics about your good question, you can explore this concept and really solidify your understanding of polarization states of propagating radio waves and light waves.

Last edited: Oct 16, 2011

Feb 5, 2010
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