# Energy at zero axis in an Electromagnetic field?

• mgkii
In summary, the energy of an EM wave is a potential energy that is dependent on the amplitude of the field at a particular point in time.
mgkii
Can someone correct my understanding on electromagnetic waves please; I've clearly got the wrong end of a stick somewhere, but can't figure out where my misunderstanding is! Thanks.

If I draw a classic EM wave with the electric field on the up and down axis and the magnetic field at right angles, then both of the sine waves cross the time (or distance) axis at the same points. What I really don't get is "where is the energy" at this point? I can comprehend that a moving electric field generates a magnetic field and vice-versa and that this is how the wave propagates over a distance, but if the heights above and below the axis represent field strength and therefore (possible mistake) a level of energy, then what's happening at the point they both cross the axis, and how does the next phase of the wave get "kick started"?

Don't shoot me... I know I've missed something, I just can't see what!

Thanks
Matt

It's not a 'moving' electric field that generates a magnetic field, it's a 'changing' electric field. Same for the magnetic field. How this causes the wave to propagate, I don't know.

Thanks Drakkith. I'm perfectly happy to accept that there is a causal relationship between the two components that causes propagation, the thing I'm struggling to get my head around is what's happening at the point both the two components cross the zero point on the time/distance axis?

Does the height of the wave in any way relate to energy? If it does, then I'm confused in one way; if it doesn't then how do you get a larger electric and magnetic field at one location compared on a line to another if the total energy is the same... or not?! Brain just exploded :-(

mgkii said:
the thing I'm struggling to get my head around is what's happening at the point both the two components cross the zero point on the time/distance axis?

I've yet to take an E&M course, but I believe that when the graphs of the fields are passing the X axis, the change in the fields is also the highest, so you should get a continuous transfer of energy over one period of the wave.

mgkii said:
Does the height of the wave in any way relate to energy?

Sure. The higher the amplitude of the wave, the more energy it has. Remember though that you're looking at a graph. A real EM wave has no height. Its components only have a magnitude and direction.

<Slaps head hard> Thanks Drakkith, you've just spotted where my thinking fell down. Ironic really... I've just been posting on a thread (you were on) about the dangers of confusing the math with the reality, and that's exactly what I've done here! Much appreciated

Drakkith
Hi mgkii:

A very, very important point to understand the Field Energy of a propagation EM wave is that, this energy is a kind of 'potential energy'. The actual power of a EM-wave on a given time is not only dependent on the exact field amplitude at this time, but also dependent on 'the potential that the amplitude can be how much higher'. As an analogy, if a ball is falling from a table and bounce from the floor back and forth, we can not state that at the time the ball hit the floor the ball has zero energy. As the matter of fact, if the collision from the ball to the floor is perfect elastic(without any energy loss), the ball will bounce forever, and the total energy (kinetic+potential) will be conservative. And much similar to this, the power of a EM wave in a loss-less medium is uniform versus time.

The value of the intensity of the field can be estimated by 0.5*absolute(E)^2 which the E-field is a complex number, the imaginary part (represent for the oscillation phase of the wave) must not be ignored.

That's a really superb analogy. Many thanks.

## 1. What is the zero axis in an electromagnetic field?

The zero axis in an electromagnetic field refers to the point where the electric and magnetic fields are equal in magnitude and opposite in direction, resulting in a net energy of zero. This point is also known as the center or origin of the field.

## 2. Why is the zero axis important in an electromagnetic field?

The zero axis is important because it represents a point of equilibrium where the electric and magnetic fields cancel each other out. This allows for the conservation of energy within the field and plays a crucial role in various applications of electromagnetism.

## 3. How is the zero axis determined in an electromagnetic field?

The zero axis is determined by the placement and strength of the electric and magnetic sources in the electromagnetic field. These sources can be controlled by adjusting the position and magnitude of charged particles or electric/magnetic currents.

## 4. What happens to energy at the zero axis in an electromagnetic field?

At the zero axis, the energy in the electromagnetic field is at its lowest point and is essentially zero. This is due to the opposing forces of the electric and magnetic fields canceling each other out. However, energy can still be transferred and conserved within the field.

## 5. How does the zero axis affect the behavior of electromagnetic waves?

The zero axis plays a crucial role in the behavior of electromagnetic waves. As the wave propagates through space, the zero axis acts as a point of reflection, causing the wave to change direction. This phenomenon is known as refraction and is used in many practical applications such as lenses and prisms.

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