# Transmission line voltage delay

The voltage value on a wire a distance away from the signal-port (where we apply the initial voltage) has a phase delay because the current takes time to travel down the wire.

But how do you visualize this? First we have a voltage at the beginning, and that causes current to flow. Then a distance later, the current causes a different voltage at whereever the current is? Voltage is energy per charge, so does that mean the voltage is being "carried" by the current, and the voltage propagation delay is caused by how long it takes for the current to move?

So in the context of a charge in a uniform electric field within a vacuum:
1) Charge moves with electric field
2) E-field is force per charge, so multiply by the distance moved to get energy per charge, AKA voltage
3) This voltage is now stored within the charge? So because the charge takes time to move a distance, this results in voltage propagation delay?

Mister T
Gold Member
Your post confuses me. In the first paragraph you talk about things happening in a wire, but in the last paragraph you talk about things happening in a vacuum. Is the last paragraph a multiple choice question?

If you have a signal traveling along a wire, with one ammeter placed near the signal-port and another placed a distance away, will the two meters always read the same value?

Sorry, I was trying to draw a parallel between what happens in a vacuum vs a wire since I thought a vacuum would be easier to analyze.

To your question, the answer should be no because there is a voltage propagation delay between the signal port and the one a distance away, and my book says that's because current takes time to travel. I'm wondering why this is. How does a current that needs time to travel result in a voltage delay?

anorlunda
Staff Emeritus
How does a current that needs time to travel result in a voltage delay?

When you're talking those speeds, you can't separate electric/magnetic/voltage/current any more. You have electromagnetic fields that propagate.

For a more scientifically accurate description of what really happens in a wire, see this Wiki article. I think it's difficult stuff. https://en.wikipedia.org/wiki/Free_electron_model

Mister T
Gold Member
It's the electric field that takes time to travel. In a vacuum it travels at the speed of light, in a wire almost as fast. That's the reason for the signal delay. In a wire the charge carriers (electrons) respond to this field and move. Think of it as a command to move. The command itself moves down the wire at, like I said, almost the speed of light. When the command arrives the electrons move, but they move at a much much slower speed than the command.

An analogy is a line of soldiers, in single file. From near the rear of the line the sergeant shouts the command "march". All the soldiers start moving at almost the same time, but there's a slight delay for soldiers near the front of the line because the command travels to them at the speed of sound. The soldiers march at a much slower speed than the speed at which the command travels.

Saying that "a current takes time to travel" doesn't make sense to me. Current is the rate at which charge moves, so I can't understand what it would mean for current to travel.

NTL2009 and tech99
.... An analogy is a line of soldiers, in single file. From near the rear of the line the sergeant shouts the command "march". All the soldiers start moving at almost the same time, but there's a slight delay for soldiers near the front of the line because the command travels to them at the speed of sound. The soldiers march at a much slower speed than the speed at which the command travels. ... .
I think that's a helpful way to visualize it.

To put that back in electrical terms, a transmission line is a continuous line of inductance, resistance and capacitance. If you picture those components along a line, you can see that there will be a delay as a step voltage propagates along the line. Like this:

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