Energy transfer with electromagnetic waves

Question

As we know that the electrical energy is transferred via electromagnetic waves from the source to the load. My question is that, even if there is transfer of energy through the electromagnetic waves, then how these waves get to know that where the load is actually connected, i.e. at which position or where they have to actually go?

And, if an electromagnetic wave involves only magnetic field and electric field, then why, if any load brought towards the closed circuit doesn't starts to work? For an example, if a bulb is connected to a circuit and it's glowing and then, if we bring an another bulb towards it, then it should also start glowing without any connections because electromagnetic waves are still present in the surroundings which are transferring the energy to the bulb which was already in the closed circuit, then why electromagnetic waves only target the bulb which is connected to the circuit?

maybe you should take a course in electricity and magnetism? There is confusion in the terms you use and the relations you imagine . — anna v, Jan 25 '22 at 15:05
@annav you are free to edit the question if I'm wrong somewhere it's okay and I'm just a high school student and I'm on my basics, I'll read more about this and dive more deep into this topic. If you find this question misleading so I'm ready to delete the post no issues if you say :) but I'll be very thankful to you if you suggest me what are the things that I should correct in here! — Tejas Dahake, Jan 25 '22 at 15:27
@TejasDahake you should have stated you are a high school student in your profile, as I always check in the case with confusions. There are two types of energy transfers in you question. One has to do with closed electric circuits and it the the current that carries the energy, and the other is radiation which happens with antennas. There are no "loads", in circuits there are resistances capacitors and inductors . Radiation spreads from an antenna and another antenna can catch the signal, etc.etc.etc. — anna v, Jan 25 '22 at 16:31
@annav There absolutely are "loads". A load is anything that drops the voltage, or consumes the energy stored in the circuit. If you imagine using a hand-crank motor to generate power, a "load" is anything that you can add to the system that makes it harder for you to turn the crank. It's a perfectly legitimate word used in the context of electricity — Jim, Jan 25 '22 at 17:26

Roger V. · Accepted Answer · 2022-03-02T13:00:58.927

When we say that electromagnetic waves transfer energy, we mean that the electromagnetic field has energy stored in it - just like the particles of waves on whater have kinetic and potential energy.

Classical view
Let us consider a classical radio emitter: oscillations of current in the antenna produce electromagnetic field,a nd some of the energy of these oscillations is lost to the field - so we need a constant power supply to sustain these oscillations. The electromagnetic field propagates in all directions. Suppose now we have a receiver at some point - electromagnetic field induces current in this receiver, which is transformed into a signal, e.g., by a loudspeaker. The electromagnetic field around the antenna loses some energy to the oscillations in the receiver, but it has no effect on the field elsewhere.

Quantum view
From the quantum viewpoint the intensity of the EM field is the number of photons emitted. Each photon has energy $\hbar\omega$. Emitter creates photons and receiver absorbs them. If emitter emits $n$ photons, it furnishes the field with energy $n\hbar\omega$, whereas the receievr absorbs $m$ photons, i.e., energy $m\hbar\omega$, leaving $(n-m)\hbar\omega$ in the field ($n-m$ photons).

Impedance matching
Just because a circuit is surrounded by an EM field, it does not mean that there will be much energy transferred between them. The efficient condition for coupling an emitter or receievr to the field is knwon as impedance matching. Antenna is par excellence the best-known device used to achieve this goal - a classical antenna has length equal to half-wavelength of the emitted radiation. If it were too short, the field would not really feel the variation of the current in space. If it were too long, the effect of the positive and negative current on the field would cancel out.

For an example, if a bulb is connected to a circuit and it's glowing and then, if we bring an another bulb towards it, then it should also start glowing without any connections because electromagnetic waves are still present in the surroundings which are transferring the energy to the bulb which was already in the closed circuit, then why electromagnetic waves only target the bulb which is connected to the circuit?

The bulb connected to a circuit is coupled to it efficiently, whereas the other is not. The situation is however somewhat different here than in the radio transmission, since the first bulb is directly driven by the current, rather than via EM waves propagating in space.

@Tejas Additional maybe also interesting What are photons, EM radiation and EM waves — HolgerFiedler, Jan 26 '22 at 05:08

score 1 · Answer 2 · answered Jan 25 '22 at 16:28

I assume that this question is not asking about radio transmission, but about some recent Youtube videos that explained that even in DC (or low frequencies like 50-60 Hz) circuits with wires connecting all the elements, the energy is actually carried in the fields around the wires rather than the wires themselves.

As we know that the electrical energy is transferred via electromagnetic waves from the source to the load.

As pointed out in the comments, this would be better worded as saying energy is transferred via the electromagnetic field, rather than by waves.

how these waves get to know that where the load is actually connected, i.e. at which position or where they have to actually go?

The wires guide the fields.

Mathematically the wires put boundary conditions on the equations from which we determine the fields. More simply, the arrangement of the wires controls where the fields go, and so they control where the energy is transferred from and to.

if any load brought towards the closed circuit doesn't starts to work? For an example, if a bulb is connected to a circuit and it's glowing and then, if we bring an another bulb towards it, then it should also start glowing without any connections because electromagnetic waves are still present in the surroundings

As pointed out in the comments, this can work. I've never tried it myself, but if you bring an old-school fluorescent tube near high voltage (10's or 100's of kV) power transmission lines, the tube can light up.

In the case of smaller bulbs near lower-voltage lines, the metal (and to a lesser extent, other materials) of the bulb also affects the fields around it. Metals in particular effectively collapse the electric fields around them, because the electrons in the metal rearrange to create their own field that opposes the external field. Again, mathematically we'd say the metal creates a boundary condition on the field.

Only if the bulb is very large (like the fluorescent tube) or the field is very strong (like around the high voltage transmission line) do we see significant amounts of power transferred to the bulb floating in space near the other electric circuit.

Claudio · Answer 3 · 2023-11-07T12:33:30.170

In order to better understand what happens in case of lighting a bulb I suggest that you ask yourself the question how does the water know where to flow out if there is a hole in the bottom of a vessel. Why does the water not leave the vessel through its walls at any place after it has started to leave the vessel through the hole? How does the water not near the hole in the vessel "know" where to flow/move in order to leave the vessel through the hole later on?

Just connect an empty vessel by a water pipe to the vessel full of water and then let the water flow. It will flow only within the pipe. It would not fill an empty vessel placed next to this one connected by the pipe to the vessel with water. Observe what happens ... think about it ... experience this interchange of what you see with how you think about it ... This would help you to develop some deeper intuitive "understanding" how it could be that water which can potentially leave the vessel in any possible direction of flowing down does only choose the hole and the pipe to leave the vessel.

The problem with understanding phenomena like electricity and electromagnetic waves is rooted in problems with grasping extremely high velocities of effect propagation. Comparison with water vessels connected by a pipe demonstrates this. The water needs some time to arrive through the pipe in the other vessel. Electricity "arrives" at the target through the wire almost instantly because the propagation of energy happens at the speed of light.

What water flow and electric energy have in common is that no matter where the water pipe is placed and how it is shaped the water will flow like electricity does through the wire. The difference is that in case of electricity the energy arrives at the target vessel as good as instantly.

Our brain seems to have trouble to grasp and explain to itself such instant effects where the distance does not matter and the effect of some cause arrives as good as immediately at any point in space ( which needs to be ready to receive the cause and show the effect ) surrounding the area the cause is generated.

Next to this above it could be very helpful in gaining better understanding of what actually is and the how and why of it, to become aware of the difference between what actually is and how we think about it.

Electromagnetic waves and electric and magnetic fields are not what actually is there. The electromagnetic fields are constructs of our brain we have developed in order to use them in thinking helpful in arriving at an explanation of what we observe. If the thinking delivers the ability to predict the effect, we make part of the thoughts we use in the process mistakenly considered to actually represent what actually exist in reality. Mistaking thoughts and what actually is can then lead to several kinds of confusion which will be sometimes very hard to dissolve without dropping the believe that what we think is what really, actually is.

Energy transfer with electromagnetic waves

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