Electromagnetic waves and phase difference

Thread Starter

logearav

Joined Aug 19, 2011
243
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
 

nsaspook

Joined Aug 27, 2009
6,481
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
http://q-track.com/phocadownload/IEEE-APS-2005.pdf

The current and voltage are in step with each other because they vary identically with time.
Because the current and the voltage reach their maximum values at the same time, they are said to be in phase. Notice that the average value of the current over one cycle is zero. This is because the current is maintained in one direction (the positive direction) for the same amount of time and at the same magnitude as it is in the opposite direction (the negative direction). However, the direction of the current has no effect on the behavior of the resistor in the circuit: the collisions between electrons and the fixed atoms of the resistor result in an increase in the resistor’s temperature regardless of the direction of the current.
 
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Thread Starter

logearav

Joined Aug 19, 2011
243
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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nsaspook

Joined Aug 27, 2009
6,481
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?
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.
 
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Wendy

Joined Mar 24, 2008
21,906
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.
 

steveb

Joined Jul 3, 2008
2,436
http://www.youtube.com/watch?v=4CtnUETLIFs
Thanks a lot for your reply. I dont understand nsaspook. What do you mean by polarity relation of fields?.
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.
 
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