# Naked Science Forum

## Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: hamdani yusuf on 28/02/2018 12:25:25

Title: What is the ratio between electric and magnetic field in electromagnetic waves?
Post by: hamdani yusuf on 28/02/2018 12:25:25
What is the ratio between amplitude of electric and magnetic field in electromagnetic waves?
Is it a constant? does it depend on the medium? does it depend on the frequency?
Is it possible to generate pure electric wave or magnetic wave?
Title: Re: What is the ratio between electric and magnetic field in electromagnetic waves?
Post by: opportunity on 28/02/2018 13:06:22
What is the ratio between amplitude of electric and magnetic field in electromagnetic waves?
Is it a constant? does it depend on the medium? does it depend on the frequency?
Is it possible to generate pure electric wave or magnetic wave?

It's a great question. Can you offer some background ideas to your question, as for instance why you would ask this, as of course we can't assume anything in physics? You're essentially asking what divines a magnetic field in proportion with an electric field, why it is so. Some argue a field has no source. Granted. Complicated post that was. So what divines an e/m field as it is? A resolved feature of the big bang? Why the question.......don't you know better not to ask these questions (just kidding).....?
Title: Re: What is the ratio between electric and magnetic field in electromagnetic waves?
Post by: Colin2B on 28/02/2018 18:38:03
B=E/c
the  magnetic  field  is   numerically much  smaller  than  the  electric  field,  although they both carry the same energy

You can have a time varying electric field or magnetic field but remember each produces the other so if you vary them the electric field will create an EM wave.
Title: Re: What is the ratio between electric and magnetic field in electromagnetic waves?
Post by: geordief on 28/02/2018 19:45:08
B=E/c
the  magnetic  field  is   numerically much  smaller  than  the  electric  field,  although they both carry the same energy

You can have a time varying electric field or magnetic field but remember each produces the other so if you vary them the electric field will create an EM wave.
Are the electric and magnetic components of the wave so produced of equal amplitude and frequency? (no matter what the strengths of the respective fields)
Title: Re: What is the ratio between electric and magnetic field in electromagnetic waves?
Post by: Colin2B on 28/02/2018 23:02:27
B=E/c
the  magnetic  field  is   numerically much  smaller  than  the  electric  field,  although they both carry the same energy

You can have a time varying electric field or magnetic field but remember each produces the other so if you vary them the electric field will create an EM wave.
Are the electric and magnetic components of the wave so produced of equal amplitude and frequency? (no matter what the strengths of the respective fields)
Sorry, i don’t really understand the question.
E and B are the electric field strength and magnetic field strength components of the wave - remember what we said in the other thread about a field being a set of measurements at points in space and time. No, they are not of equal ‘amplitude’ they are related as B=E/c.
Title: Re: What is the ratio between electric and magnetic field in electromagnetic waves?
Post by: Colin2B on 28/02/2018 23:22:24
does it depend on the medium? does it depend on the frequency?
Sorry, missed this bit.
Yes depends on medium because speed in the medium v=1/√(με)
Doesn’t depend on frequency.
Title: Re: What is the ratio between electric and magnetic field in electromagnetic waves?
Post by: geordief on 28/02/2018 23:48:28

Sorry, i don’t really understand the question.
E and B are the electric field strength and magnetic field strength components of the wave - remember what we said in the other thread about a field being a set of measurements at points in space and time. No, they are not of equal ‘amplitude’ they are related as B=E/c.
My fundamental understanding must be very flawed.

This is very tricky .

Still B=E/c seems worth knowing .

Do I have it right that the em wave has no need of the E or B fields to propagate once it has been created?

Title: Re: What is the ratio between electric and magnetic field in electromagnetic waves?
Post by: syhprum on 01/03/2018 00:03:51
If you pursue this line don't you get to the velocity of propagation c or the impedance of free space 377 ohms ?
Title: Re: What is the ratio between electric and magnetic field in electromagnetic waves?
Post by: Colin2B on 01/03/2018 00:27:07
Do I have it right that the em wave has no need of the E or B fields to propagate once it has been created?
As @syhprum points out, you will have real problems with this idea.

The E and B fields are the wave components at any point in space/time, they are inseperable from the wave.
Title: Re: What is the ratio between electric and magnetic field in electromagnetic waves?
Post by: geordief on 01/03/2018 00:42:39

As @syhprum points out, you will have real problems with this idea.

The E and B fields are the wave components at any point in space/time, they are inseperable from the wave.
Do the E and B fields weaken over space and time? (But the em wave seems not to...)

EDIT: Does the em wave create its own pair of (E and M) fields with it as it moves through spacetime ?

So the original field does not weaken(it disappears once the wave is created?)

Title: Re: What is the ratio between electric and magnetic field in electromagnetic waves?
Post by: lightarrow on 01/03/2018 00:55:04
What is the ratio between amplitude of electric and magnetic field in electromagnetic waves?
In which unit of measure? In SI is c, as already said: ||E|| = c||B||; in other systems, the rate may not be c; in CGS - GAUSS the rate is 1: ||E|| = ||B||.
And this in the "far field" of the wave, that is far enough from the sources, or things become very complicated
Quote
Is it a constant? does it depend on the medium? does it depend on the frequency?
Is it possible to generate pure electric wave or magnetic wave?
You want to know too many things at once  :)
In electrodynamics, things can become very complicated if you want to analyze all the possible situations. In most of the cases you will consider em waves in the void and far from the sources (the distance is many times the wavelenght), then the rate is constant and doesn't depend on frequency. It's not possible to generate a pure electric or a pure magnetic wave: in a wave you necessarily have a time variation of a field and electrodynamics' laws (see Maxwell's equations for example) tells us that every time variation of an electric field generates a magnetic field and every time variation of a magnetic field generates an electric field.

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lightarrow
Title: Re: What is the ratio between electric and magnetic field in electromagnetic waves?
Post by: hamdani yusuf on 01/03/2018 02:16:21
What is the ratio between amplitude of electric and magnetic field in electromagnetic waves?
Is it a constant? does it depend on the medium? does it depend on the frequency?
Is it possible to generate pure electric wave or magnetic wave?

It's a great question. Can you offer some background ideas to your question, as for instance why you would ask this, as of course we can't assume anything in physics? You're essentially asking what divines a magnetic field in proportion with an electric field, why it is so. Some argue a field has no source. Granted. Complicated post that was. So what divines an e/m field as it is? A resolved feature of the big bang? Why the question.......don't you know better not to ask these questions (just kidding).....?
I asked because I found several sources, including discussions in internet forums, are making contradicting statements. I want to know what is the currently accepted explanation by mainstream scientists.
I imagined an experiment where a permanent bar magnet is rotated at very slow rate (say one rotation per day). The magnetic field can be very high (compared to earth magnetic field that effect compasses), but the induced electric field could be negligible. The situation is reversed when an electret is used instead of magnet.
Title: Re: What is the ratio between electric and magnetic field in electromagnetic waves?
Post by: hamdani yusuf on 01/03/2018 02:23:25
B=E/c
the  magnetic  field  is   numerically much  smaller  than  the  electric  field,  although they both carry the same energy

You can have a time varying electric field or magnetic field but remember each produces the other so if you vary them the electric field will create an EM wave.
Your formula seems to imply that the ratio is a constant. Can you provide the source of the formula above?
How to relate it to the Maxwell–Faraday equation here?
https://en.wikipedia.org/wiki/Maxwell%27s_equations#Macroscopic_formulation
Title: Re: What is the ratio between electric and magnetic field in electromagnetic waves?
Post by: syhprum on 01/03/2018 08:17:44
As Hamdani Yusuf has pointed out any rotating magnet emits electro magnetic waves, an interesting case arises in the case of the rotation of the Earth as the magnet pole does not coincide with the axis of rotation the earth must be radiating on a frequency of 0.00001157 Hz (2.592*10^13 M) I wonder what the ERP is ?
Title: Re: What is the ratio between electric and magnetic field in electromagnetic waves?
Post by: Colin2B on 01/03/2018 09:05:11
Your formula seems to imply that the ratio is a constant. Can you provide the source of the formula above?
It’s in the article you linked to. Go down to “Formulation in Gaussian units convention”.
As @lightarrow pointed out this is for SI units and I should have shown modulus, but I didn’t want to overcomplicate for you.
Derivation is by using the equations shown in section “Macroscopic formulation” and any good physics textbook should take you through it. I’ll see if i have any references to online teaching material, @jeffreyH  might have some to hand.
As @lightarrow also points out, this is farfield, not nearfield.

Title: Re: What is the ratio between electric and magnetic field in electromagnetic waves?
Post by: evan_au on 01/03/2018 10:09:55
Quote from: geordief
So the original field does not weaken(it disappears once the wave is created?)
Power from a radio transmitter is fed into an antenna, where the varying voltage and current causes acceleration of electric charges, which produces an electromagnetic wave, which propagates away "to infinity".

The strength of this wave decreases with distance, weakening according to the inverse-square law. This merely reflects the fact that if you measure the power passing through the surface of a sphere around the transmitter:
- the power through the surface of a small sphere is the same as the power passing through the surface of a larger sphere
- But the surface area of the bigger sphere is much greater than the surface area of a small sphere
- So the power density becomes much lower, the farther you are from the transmitter

Returning to the transmitter:
- If you disconnect power from the transmitter, the charges in the antenna are no longer accelerated, and so no more EM radiation is emitted.
- In practice, transmitters have a filter constructed with inductors and capacitors which are able to store some energy at the transmission frequency (and block signals at other frequencies). So in practice, the electromagnetic signal will continue for a few cycles after the transmitter power is disconnected - exactly how long depends on the "Q" or "Quality Factor" of these inductors and capacitors.
See: https://en.wikipedia.org/wiki/Q_factor#Individual_reactive_components

Quote from: syphrum
rotation of the Earth as the magnet pole does not coincide with the axis of rotation
Another interesting case is the Sun, with it's magnetic field reversing about every 11 years.
This will produce an electromagnetic wave with a wavelength of 22 light-years, or a frequency around 1 nanoHertz.
Events like the Maunder Minimum mean that it is an amplitude-modulated signal.
You would need to travel 100 light-years before you could take far-field measurements of this signal!
See: https://en.wikipedia.org/wiki/Solar_cycle
Title: Re: What is the ratio between electric and magnetic field in electromagnetic waves?
Post by: hamdani yusuf on 01/03/2018 11:37:25
What is the ratio between amplitude of electric and magnetic field in electromagnetic waves?
In which unit of measure? In SI is c, as already said: ||E|| = c||B||; in other systems, the rate may not be c; in CGS - GAUSS the rate is 1: ||E|| = ||B||.
And this in the "far field" of the wave, that is far enough from the sources, or things become very complicated

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lightarrow

How can merely change the unit from SI to cgs also change the physical dimension of the fields?

In the case of slowly rotating magnet, we can get amplitude of B much larger than amplitude of E when measured nearby. But if we measure from adequately far distance, amplitude of B diminish quicker than E until their ratio approach a constant?
Title: Re: What is the ratio between electric and magnetic field in electromagnetic waves?
Post by: lightarrow on 01/03/2018 12:56:03
What is the ratio between amplitude of electric and magnetic field in electromagnetic waves?
In which unit of measure? In SI is c, as already said: ||E|| = c||B||; in other systems, the rate may not be c; in CGS - GAUSS the rate is 1: ||E|| = ||B||.
And this in the "far field" of the wave, that is far enough from the sources, or things become very complicated

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lightarrow
How can merely change the unit from SI to cgs
Things are different in electrodynamics because the magnetic field B is defined differently, it's not always a mere conversion from a value in a unit of measure to the other value in the other unit, sometimes even the equations are different. But your is a good question. Let's see the effect of this, that is the magnetic force on a moving charge q at velocity v in a field B which, for simplicity, is orthogonal to v.

1) In SI: F = q*v x B where "x" here stays for vectorial product. So, for an em wave in the void in the far field: ||F|| =  q||v||*||B|| = q||v||*||E||/c.

2) In CGS:  F = q*(v/c) x B. So, for an em wave in the far field:
||F|| =  q(||v||/c)*||B|| = q(||v||/c)*||E|| = q||v||*||E||/c

and the two are the same  :)
Quote

In the case of slowly rotating magnet, we can get amplitude of B much larger than amplitude of E when measured nearby.
Yes.
Quote
But if we measure from adequately far distance, amplitude of B diminish quicker than E until their ratio approach a constant?
Yes. Do you remember what I wrote? Compute the wavelenght λ; how far from the source, let's say at distance r, you should evaluate the fields to be sure that r >> λ?
To compute the wavelenght remember that λ = c/f where f is the frequency. It's a trivial exercise.

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lightarrow