# Why'd we have reflected waves in transmission lines?

I've learn today that when you applie a potencial V in a transmission line, we get a V in positive way, but we have to a V in negativa way, being reflected. What does it means? Like, I can understand it if we talk about electromagnetics waves, it colides with a superficie and be reflected, but what happens in a electrical circuit? It is a potencial being applied at a extremity and not a real/physical wave, right?

Mathematicaly I saw that a potencial in negative way is a valid answer (as a answer for those differential equation), but it is phisycaly possible?

• The potential (voltage) is propagating in a conductor with a finite speed, which makes it a real physical wave. In fact it is not the voltage which is propagating, but an electrical field which is creating the potential. – Eugene Sh. May 7 at 21:23
• but we use a source that provide potencial to the circuit and it creates the electrical field, doesnt it? – Victor Coutinho May 7 at 21:37
• Potential and field are inseparable entities. A field is a gradient of a potential mathematically speaking. – Eugene Sh. May 7 at 21:40

When you say,

I can understand it if we talk about electromagnetics waves, it colides with a superficie and be reflected, but what happens in a electrical circuit?

You must understand that a signal propagating in a transmission line is propagating as an electromagnetic wave. Consider a coaxial transmission line for example. The signal propagates as a Transverse Electromagnetic wave (TEM wave) very much like a free space EM wave.

If you already understand that a free space EM wave will "bounce off" a surface, let me reframe the perspective so you can also understand why signals in a transmission line will reflect.

Any EM wave will reflect off of a discontinuity in the propogating structure's impedance. Whether that is an EM wave traveling through 377 Ohm free space reflecting off of a metal plate that presents a 0 Ohm (short circuit) impedance, OR that could be a TEM wave propagating down a coaxial cable with a characteristic impedance of 50 Ohms that then reflects off of a short circuit at the end of the cable.

Both cases demonstrate an EM wave reflecting due to what we call an impedance mismatch ie. an abrupt change or discontinuity in the transmission line's impedance.

There is much more to learn about reflections and impedance mismatches such as the polarity of the reflected pulse, the exact ratio of the amplitude of the incident and reflected pulses, etc. However for the scope of this question I will stop here.

It is a potential being applied at a extremity and not a real/physical wave, right?

Whether it is real or not is a matter for philosophers.

It acts exactly as if it were real. So whether you want to think of it as being actually real, or a figment of someone's mathematical imagination -- fine. It still acts exactly as if it were real.

What does it mean?

It is a least-math way of understanding in detail what's going on in the transmission line. A voltage pulse (or wave) travels down the line and hits a short circuit. The short circuit constrains the voltage to be zero. Whether or not the reflected wave is "real" by your definition, if you model the behavior by saying that the current is reflected without a sign change and the voltage is reflected with a sign change, then the result is correct -- the voltage at the short circuit is zero, and the effect of the short circuit propagates back up the line in a way that's exactly consistent with it being a separate wave from the wave travelling down the line.