# How/Why does voltage reflect?

I'm trying to understand antenna theory again, and I'm again stumped by the concept of VSW (voltage standing waves). I understand standing waves, I remember these from a physics clases, however I do not really understand how voltage can be a wave, and how it reflects.

From my understanding of voltage it is really hard to understand how it can reflect.

A voltage is the difference in electrical potential energy (per unit charge) between two places. This electrical potential energy (per unit charge) difference describes the force on that unit charge (due to the electric fields), and the distance this charge will travel.

For example, with an electrical potential energy (per unit charge) difference of $10\, \mathrm{V}$ or $10\, \mathrm{J}/\mathrm{Q}$, an electron at the higher potential will travel to the lower potential. If the distance between these two potentials is $2\, \mathrm{m}$, the force on the electron will be $5\, \mathrm{N}$, as it travels from one potential to the other. (Only if the electric field is constant between the two potentials. If it is not, the integral of the force across the $2\, \mathrm{m}$ will still equal $10$, however the force will vary at different distances)

Voltage is just a made up concept to help explain the forces that charged particles experience. It isn't a real thing that can be reflected. So what is being reflected? I'm guessing it is the electric field, as this has an actual propagation speed, and even though the electric field is also a made up concept to describe the force a charged particle will experience at that point in the field, it is easier to imagine this being reflected.

So can someone please explain to me how this all ties together? How does an electric field propagate through a conductor? Do the vectors in this vector field change direction when there is a bend in the wire, and why do they do this? Why wouldn't the electric field escape outside of the conductor? And how does an electric field (or voltage) reflect at a short?

• It is electromagnetic waves that propagate along a transmission line and reflect (at discontinuities). When there is both a forward and reflected wave propagating along the line, the result is a standing wave. – Alfred Centauri Nov 20 '13 at 3:21
• Perhaps this is best asked as a question but isn't a voltage induced by a signal a "pressure wave" of electrons much the same way sound is a pressure wave of air molecules? Bunch up the electrons on one end of a conductor and the higher voltage will push electrons nearby and cause a traveling pressure wave in the electric field as electrons being bunched up together. – Brandon Enright Nov 20 '13 at 5:07
• I wrote an answer on EE.SE that might partially answer your question. Especially read the last paragraph. electronics.stackexchange.com/questions/93232/… – The Photon Dec 20 '13 at 19:29
• @BrandonEnright, that's not a good description. An EM wave propagates with or without electrons present. For example radio waves or light from the sun can travel through vacuum, where no electrons are present to form a "pressure wave". – The Photon Dec 20 '13 at 19:32
• @ThePhoton my comment was obviously imprecise and limited to an electric potential propagating in a conductor only. – Brandon Enright Dec 20 '13 at 20:29

## 1 Answer

Your query is valid: How is voltage a 'wave' that reflects and creates standing waves? Well, the answer is quite simple when you stop and think about it. All signals travelling across transmission lines are merely electromagnetic waves. Now these lines are commonly driven by voltage sources, hence they are 'voltage waves'. This makes perfect sense: If a simple circuit is driven by a sinusoidal voltage source, you expect that the resultant voltage(amplitude) will vary like the sine wave.

If you extend your thinking a bit, you will see that the same question can apply to any circuit with a sinusoidal voltage source. We don't usually apply the concept of 'waves' to a simple circuit (the concept is only useful for the analysis of TLs since reflections can be a bit of nuisance and at radio frequencies are important), but its present. Ever heard of LC circuits? These circuits are basically resonators where the energy from the source (a voltage source) keeps on changing forms. But how can that happen, if the source is a 'voltage'? This analogy would help you understand that all signals are just electromagnetic waves. The reason we call them 'voltage waves' in RF engineering is simply because at those frequencies the wavelike behavior of the source helps us in the analysis/design of the circuit (through the use of matching circuits, tuning circuits etc). Hope this answers your query. Feel free to inquire more.

OK it seems you have a few more queries. Let's get to them.

1) Yes, an EM wave doesn't require any medium to travel (that's how antennas in space work). However, 'containing' an EM wave inside a conductor isn't a big deal. Its a simple matter of creating a potential difference and letting it do all the work. You could, ofcourse, ask the same question when you connect a simple wire to a voltage source. The explanation remains the same.

2) Now why does the wave reflect at an open circuit? The answer to this question you will encounter in your course, but I will give a simple version of it here. At the open circuit point the current in the line is zero (by the definition of an open circuit). Since charge continues to arrive at the end of the line through the incident current, but no current is leaving the line, then conservation of electric charge requires that there must be an equal and opposite current into the end of the line. Essentially, this is Kirchhoff's current law in operation. This equal and opposite current is the reflected current and by Ohm's law, it creates a reflected voltage wave.

3) What Brandon Enright posted about is the tried and tested, age-old analogy between electricity/electronics and hydraulics, which every engineer has used at some point. It is perfectly correct. Please refer this link (http://en.wikipedia.org/wiki/Hydraulic_analogy) to further understand it and relate it to your query.

• Thank you sam29! That answer was very helpful and is much appreciated. I just have a few questions about your answer, if you don't mind answering: – Blue7 Nov 20 '13 at 12:15
• 1)You say "all signals traveling across transmission lines are merely electromagnetic waves". I thought that EM waves do not need a medium to travel through, so how are they contained inside the transmission line? – Blue7 Nov 20 '13 at 12:17
• 2)Say you have a source that produces an alternating potential, when it is at the higher potential, a positive electric field propagates down the transmission line, and when it is at the lower potential a negative electric field propagates down the transmission line. These fields propagate at the speed of light. the transmission line is an open circuit, so the electric field will reflect, and superimpose with the incident wave. But why does this wave reflect, and what happens when this reflected wave reaches the source? – Blue7 Nov 20 '13 at 12:18
• 3) And finally, is Brandon Enright definitely correct when he says the signal traveling down the wire is a "pressure wave" of electrons, much like sound is a "pressure wave" of air molecules? I've heard someone say this before but I can't find any sources on the internet to verify this. How true is this description? And can you please explain how these propagating electric fields and "pressure waves" of electrons are related to each other? Thanks! – Blue7 Nov 20 '13 at 12:23
• Please check my updated answer. In the future, please don't expand your question through the comments: instead, add it to your existing question. If possible, close the thread (by selecting an appropriate answer as correct) if it answers your original question. – Sam29 Nov 20 '13 at 15:29