1
$\begingroup$

I had some questions (asked in the end ) regarding a paragraph of my textbook under a heading Physical Significance of electric Field. The paragraph of my textbook goes as follows :

" The true physical significance of the concept of electric field, however, emerges only when we deal with time dependent electromagnetic phenomena. Suppose we consider the force between two distant charges q1, q2 in accelerated motion . Now, the greatest speed with which a signal can go from one point to another is c, the speed of light. Thus , the effect of any motion of q1 on q2 cannot arise instantaneously. There will be some time delay between the effect(force on q2) and the cause (motion of q1) . It is precisely here that the notion of electric field ( strictly, EM field) is natural and very useful. The field picture is this : the accelerated motion of charge q1 produces electromagnetic waves, which then propagate with the speed c, reach q2 and cause a force on q2 . The notion of electric field elegantly accounts for the time delay. Thus, even though electric and magnetic fields are detected only by their effects(forces) on charges, they are regarded as physical entities, not merely mathematical constructs . They have an independent dynamics of their own. They can transport energy. Thus, a source of time-dependent electromagnetic fields, turned on briefly and switched off, leaves behind propagating electromagnetic field transporting energy. The concept of field was first introduced by Faraday and is now among the central concepts of physics. "

Here now, I list my questions :

1 - I know that nothing can travel faster than speed of light. But why an abstract idea like information is anyhow related to something more physical thing like waves? Or more precisely, I want to ask that why is transfer of voices and news ( and also other information online) between satellites and radios in our houses necessarily implies that all information must travels through EM waves. Overall, I want to know that if instantaneous effects can happen in our universe or not? What intuition am I missing?

2 - Why the accelerated motion of charge leaves electromagnetic waves? I have a slight idea that how electromagnetic waves are created (like in bohrs model , it was due to conservation of energy when there is a re-positioning of electrons in lower energy shells ) . Does it imply that charges which are accelerated will vanish with time due to loss of energy through EM waves?

3- As in 17th line, how can a wave cause a force? Wave is massless, so how can it exert any force ?

4- The paragraph says, the notion of electric field elegantly explains time delay. However, I am still unable to understand how field explains time delay i.e. I am failing to understand the main intention of the paragrap for which it is written in my textbook. I would be glad if it is explained.

5- I know that electric field is a region where an electric force can be felt by a charge. Then, as said in last 4-5 lines of paragraph, how does a region propagates to transport energy from one place to another?

(P.S. I do not know high level physics and currently studying basics of electrostatics so I would request intuitive and conceptual answers than more mathematical ones.But, I appreciate all answers . And I am sorry if the question's texts is very long or if I could not ask in a more brief text. Thank you)

Improve this question
$\endgroup$
10
  • 2
    $\begingroup$ First point, second point, fourth point, First point again, fifth point $\endgroup$
    – Brian
    Commented Apr 25, 2022 at 17:02
  • 3
    $\begingroup$ I suggest you ask that as a separate question quoting the same paragraph. It is a good question and needs a treatment separately. If you add it here, then it would be unfocused (as another user) has pointed out. To be clear, I do like your question and I have upvoted it. $\endgroup$
    – Brian
    Commented Apr 25, 2022 at 17:26
  • 1
    $\begingroup$ Maybe this question I asked would be helpful as well , and also this one on interpreation of flux $\endgroup$
    – Brian
    Commented Apr 25, 2022 at 17:28
  • 2
    $\begingroup$ @Buraian Please do not abuse your edit privileges to substantially change another user's question. Even if you think you have found duplicates for some of the questions you edited out, this is still a major transgression against the author's intent (there is a reason we usually close questions as duplicates and give authors the chance to edit their question if they are not satisfied with the duplicate!). I have rolled back your edits. $\endgroup$
    – ACuriousMind ♦
    Commented Apr 26, 2022 at 1:19
  • 3
    $\begingroup$ Shiva, it's nevertheless true that five different questions are not a good fit for our site - we want questions to be clear and contain a single, well-defined question so that they can receive focused answers. If you have several different questions, please just ask multiple questions. $\endgroup$
    – ACuriousMind ♦
    Commented Apr 26, 2022 at 1:22

5 Answers 5

Reset to default
0
$\begingroup$

Well E.M. Wave carries Energy and Momentum $$p=\frac{u}{c}$$ & Force is change of momentum wrt time$$F=\frac{\Delta p}{\Delta t}$$ where $u$ is energy density and $c$ is speed of Light.

Improve this answer
$\endgroup$
1
  • $\begingroup$ F = dP / dt was given by newton. Does it applies for wave also? $\endgroup$
    – user326901
    Commented Apr 26, 2022 at 6:37
0
$\begingroup$

An electromagnetic wave is itself, an osscilation in the electromagnetic field. As such, it has an electric and magnetic component.

$\vec{F} = q(\vec{E} + \vec{v}×\vec{B})$

Meaning a force is exerted.

Improve this answer
$\endgroup$
4
  • $\begingroup$ Does this mean that an EM wave is just a field, but an oscillating one? I learnt electric field is a region where an electric force can be felt? Suppose an electric charge and it's nearby region of the field in vaccum. Inside the field, means in the vaccum, from where does that energy come? I mean vaccum means emptiness, so how can vaccum contain energy because of a charge? Does this mean that our whole vaccum is energy? $\endgroup$
    – user326901
    Commented Apr 26, 2022 at 6:35
  • $\begingroup$ An EM wave is just a field yes. Energy is associated with a charge , as work can be done. It comes fromcthe charges interaction with the EM field. The vacuum of space is filled with Energy yes $\endgroup$
    – jensen paull
    Commented Apr 26, 2022 at 10:14
  • $\begingroup$ I have also heard that mass and energy are equivalent. Does that mean whole vaccum is full of energy or full of mass? $\endgroup$
    – user326901
    Commented Apr 26, 2022 at 15:09
  • $\begingroup$ No, mass an associated energy with it. Doesn't mean all energy is mass. However all energy has a gravitation pull. $\endgroup$
    – jensen paull
    Commented Apr 26, 2022 at 17:41
0
$\begingroup$

Electromagnetic waves, as the name suggests, are propagating waves of electric and magnetic fields. When this electric field acts on a charge, the charge experiences a force.

And waves in general can carry energy. An example is a water wave at the beach. If you are standing still in the water and the wave hits you, you experience a force.

I think the main misconception you had was the fact that mass is needed to exert force on something. Forces act on objects with mass, but the source of the force itself does not need to have any mass.

Improve this answer
$\endgroup$
5
  • $\begingroup$ Hello, waves in water are described by upward and downward motion of water on their places. So when the waves reach near my body at the shore, their last molecules of water when go upward, exert force on me. And they have mass . I know it was possible due to waves only but here the actual source of force was water, which had mass. $\endgroup$
    – user326901
    Commented Apr 26, 2022 at 6:28
  • $\begingroup$ So, if momentum don't necessarily requires mass, then what is real definition of momentum ? Thank you for the answer . $\endgroup$
    – user326901
    Commented Apr 26, 2022 at 6:38
  • $\begingroup$ $\frac{\partial p}{\partial t} = f,$ if the time derivative of a qauntity exerts a force, then I would say that it has momentum $\endgroup$
    – jensen paull
    Commented Apr 26, 2022 at 10:20
  • $\begingroup$ did you mean a push/pull by saying force? $\endgroup$
    – user326901
    Commented Apr 26, 2022 at 15:10
  • $\begingroup$ Isn't it cyclic because force itself is dp/dt? $\endgroup$
    – user326901
    Commented Apr 26, 2022 at 15:11
0
$\begingroup$

The reason electromagnetic waves can exert a force on a charged particle is simply that electromagnetic waves are waves of electric field and magnetic field. These fields are precisely what exerts the electromagnetic force on charges, e.g., this is precisely the statement of the Lorentz force law.

Regarding mass, whether or not a field configuration has mass depends on the field configuration. Some configurations of electromagnetic waves do have mass (e.g., two electromagnetic waves travelling in opposite directions have mass). However, that's an issue that is irrelevant to whether or not something can exert a force on other bodies. What it means for a system to exert a force on another body is for it to exchange momentum with the said body. And that's what's relevant, i.e., do electromagnetic fields have momentum or not? The answer is that they do. Thus, when they exert a force on a charged particle, they're exchanging momentum with the said charged particle and this is allowed because electromagnetic fields are allowed to carry momentum. A great discussion of the subject of the momentum of fields can be found in Feynman Lectures.

Improve this answer
$\endgroup$
3
  • $\begingroup$ In your first para as " these fields are precisely what exerts the electromagnetic force " , did it mean that waves which are generated through the fields, can carry their fields from one place to another? How ? $\endgroup$
    – user326901
    Commented Apr 26, 2022 at 6:24
  • $\begingroup$ @Shiva No, the waves themselves are waves of electromagnetic fields. $\endgroup$
    – user87745
    Commented Apr 26, 2022 at 9:10
  • $\begingroup$ @Shiva In other words, what we call electromagnetic waves are simply configurations of the electric and magnetic fields that change with time according to the wave-equation. $\endgroup$
    – user87745
    Commented Apr 26, 2022 at 14:20
0
$\begingroup$

I will address this:

How can a wave cause a force? Wave is massless, so how can it exert any force ?

What is a wave? The simplest wave is an observation on watter, periodic changes on the levels of water static, or travelling. It is a physical observation that has been studied with the appropriate wave equation that models the changes in space of the volume of water. Wave equations have solutions with sines and cosines that can fit the periodic nature observed in the waves.

Then sound observations showed that the physical effect of sound could also be described with a wave equation.

There is transport of energy in travelling waves, although there is no definition of "mass" for a sound wave, that can be given by the mathematical solutions of the differential equations describing the waves. The waves carry momentum and when they interact there is a dp/dt , the simple definition of force.

Before Maxwell thought out of the box, the mathematical connection of light with electricity and magnetism was not known until electricity and magnetism laws were gathered by Maxwell into his theory. Then Maxwell saw that electromagnetic waves could be produced from the theory and could model light with velocity c. The equations allow for the transport of energy, as described by special relativity, (inherent in the mathematics of the the EM waves), and the theory models perfectly the results of experiments.

Improve this answer
$\endgroup$
7
  • $\begingroup$ Actually, the wave theory of light prevailed in the early 19th century, well before Maxwell, from the work of physicists like Young, Fraunhofer, Fresnel, and Airy on diffraction. Faraday proposed that light was electromagnetic waves before Maxwell worked out a detailed theory. $\endgroup$
    – John Doty
    Commented Apr 25, 2022 at 18:54
  • $\begingroup$ @JohnDoty thanks, I edited. $\endgroup$
    – anna v
    Commented Apr 26, 2022 at 3:51
  • $\begingroup$ Sound Waves are caused by the oscillating particles of the medium. The dp / dt which you said, meant that the momentum of each particles changed after some oscillations and after some time? $\endgroup$
    – user326901
    Commented Apr 26, 2022 at 6:31
  • $\begingroup$ @Shiva there is a collective motion of individual particles. the dp/dt of the wave is the total sum of the individual motions, The wave model is emergent, fitting the sum of the individual energy transfers (where there is energy transferred there is momentum change) $\endgroup$
    – anna v
    Commented Apr 26, 2022 at 8:06
  • $\begingroup$ yes thats what I was looking for. I am glad to know that the dp/dt was related to momentum of particles and not of waves. $\endgroup$
    – user326901
    Commented Apr 26, 2022 at 15:05