Transmission Lines and Reference Potential 0V...

Thread Starter

Spacerat

Joined Aug 3, 2015
36
Hello,
I was reading/studying the transmission line chapter in the allaboutcircuits online book. Very interesting and good explanation!

DC circuits: In DC circuit, it was mentioned that we can arbitrarily choose ANY point in the circuit and assign to it the value 0V. The reference point belongs to a conductor and all other points on that same conductor will also have 0V since the conductor is equipotential.. So we pick a reference point but in reality the entire conductor becomes the reference....I think that is correct...

AC circuits: In low frequency AC circuit, this approach still works since a conductor can still be approximately taken to be equipotential (all points belonging to the conductor are at the same electric potential V). Transmission line theory is required when we are dealing with high frequency AC circuits and the circuits dimensions/lengths are comparable with the signal's wavelength. the simplest transmission line is the two parallel wire line.

Here my question: there is potential difference between any pair of points facing each other on the two different wires BUT there is also a potential difference between different points on the same wire! Even on the same wire, different points have different electric potential. Each of the wires is not equipotential...
That said, do we still pick an arbitrary point to be the 0V reference point? Or is there an optimal choice for the reference 0V point in that case?

Thank you,
 

BobTPH

Joined Jun 5, 2013
4,767
0V is wherever you cliip the black lead on your multimeter. When dealing with wavelengths that are not large with respect to the length of a wire, you cannot assume that any other point on the wire us 0V.

Bob
 

nsaspook

Joined Aug 27, 2009
9,465
That said, do we still pick an arbitrary point to be the 0V reference point? Or is there an optimal choice for the reference 0V point in that case?

Thank you,
You can't realistically under those conditions. A arbitrary point reference point is a simplification that fails when circuit theory/Telegrapher equations fails.


Transmission theory limitations.

1. The conductors are geometrically uniform, i.e. the transmission line is not curved or bent.
2. The distance between the conductors of the transmission line is small compared to the wavelength of the exciting electromagnetic field.
3. The thickness of the conductors of the transmission is small if compared to the wavelength of the exciting electromagnetic field.
4. The conductors are perfectly conducting.

Assumptions made in circuit theory

1 Electrical effects happen instantaneously throughout a system.
2 The net charge on every component in the system is always zero.
3 There is no magnetic coupling between the components in a system.
 

drjohsmith

Joined Dec 13, 2021
383
A rule of thumb I was taught last century

At low frequencies, power transfer is all about the conductor resistance
its a bulk effect

At higher frequencies,
the power travels between the conductor and the reference,
i.e on a pcb , it travels in the dielectric

As an aside, to get you thinking
If we had a single cable that runs from Europe to US ( no repeaters , not optical etc )
it takes years for an electron put in at one end to come out the other
 

nsaspook

Joined Aug 27, 2009
9,465
A rule of thumb I was taught last century

At low frequencies, power transfer is all about the conductor resistance
its a bulk effect

At higher frequencies,
the power travels between the conductor and the reference,
i.e on a pcb , it travels in the dielectric

As an aside, to get you thinking
If we had a single cable that runs from Europe to US ( no repeaters , not optical etc )
it takes years for an electron put in at one end to come out the other
Electrical energy always travels in the dielectric but at low frequencies we can use the simplifications of circuit theory to model it as a bulk effect.
 

drjohsmith

Joined Dec 13, 2021
383
Electrical energy always travels in the dielectric but at low frequencies we can use the simplifications of circuit theory to model it as a bulk effect.
@nsaspook

I did say its a rule of thumb,

I think we agree

at low frequencies,
a rule of thumb is power is in the conductor
at high frequency
a rule of thumb is power is in the dielectric

Thank you for the clarification
 

nsaspook

Joined Aug 27, 2009
9,465
@nsaspook

I did say its a rule of thumb,

I think we agree

at low frequencies,
a rule of thumb is power is in the conductor
at high frequency
a rule of thumb is power is in the dielectric

Thank you for the clarification
I, personally, would say, never use a rule of thumb power is in the conductor because first it's NOT implied by circuit theory and is incompatible the with basic electromagnetic theory needed to understand basic electromagnetism like motors and magnetic circuits or how capacitors store and release energy in a EM compatible context. Circuit theory says nothing about where the power is, it only allows for calculation of power values. We should use the bulk model as a calculation aid, not as a substitute for reality.
 

Thread Starter

Spacerat

Joined Aug 3, 2015
36
@nsaspook

I did say its a rule of thumb,

I think we agree

at low frequencies,
a rule of thumb is power is in the conductor
at high frequency
a rule of thumb is power is in the dielectric

Thank you for the clarification
Thank you everyone. All your replies are useful and make sense. The topic of ground and 0V reference seems broad with topics like "ground planes", "ground loops", etc.

Things get interesting when we interconnect two or more systems and they all need to refer to the same common 0V point/conductor. When that does not happen, unwanted potential differences show up and the overall circuit does not work as intended.

Some of you mention power transfer which is essentially energy/unit time. I have seen a lot of discussion on Youtube due to a recent video by Veritasium (
) on how energy travels in a circuit. Regardless of AC or DC, energy seems to flow in AND outside the conductors and around them, even in the case of a simple DC circuit, while current clearly only flows inside the wire. Interesting!
 

nsaspook

Joined Aug 27, 2009
9,465
Thank you everyone. All your replies are useful and make sense. The topic of ground and 0V reference seems broad with topics like "ground planes", "ground loops", etc.

Things get interesting when we interconnect two or more systems and they all need to refer to the same common 0V point/conductor. When that does not happen, unwanted potential differences show up and the overall circuit does not work as intended.

Some of you mention power transfer which is essentially energy/unit time. I have seen a lot of discussion on Youtube due to a recent video by Veritasium (
) on how energy travels in a circuit. Regardless of AC or DC, energy seems to flow in AND outside the conductors and around them, even in the case of a simple DC circuit, while current clearly only flows inside the wire. Interesting!
https://forum.allaboutcircuits.com/threads/the-big-misconception-about-electricity.183285/
 
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