# Voltage transmission

For the sake of understanding, I shall refer to voltage as a "pushing force that causes electrons to move and hence generate current."

Now, I know that this force is "built up" and does not flow(unlike current). But what do people mean when they voltage is "transmitted" in transmission cables. Are they talking about the movement of voltage from one point to another?

• Voltage is "transmitted" by propagation of electrical field. Apr 12, 2019 at 15:22
• Could you elucidate? I'm looking for a layman type answer. Apr 12, 2019 at 15:32
• Sure..See my answer.. Apr 12, 2019 at 15:43

When people say that voltage is transmitted along transmission cables, they mean that the conductors in the cable cause the voltage at the far end to become the same as the voltage at the transmitting end.

For most amateur (and many professional) situations, we can regard this as instantaneous, and as a pure voltage 'magically arriving' at the far end, with no current flow.

In greater detail, what happens when we connect a voltage source to one end of a transmission cable is that an electromagnetic wave sets off into the cable. It consists of both a voltage wave, and a current wave, travelling at the speed of light (or nearly so). It's not until these waves get to the far end of the cable, and are reflected by a load or an open circuit, and these reflections travel up and down the cable several times, that eventually steady state conditions are reached and the current settles down to zero for an open cable, or it settles down to the right value for any given load.

For times that are long compared to the time it takes light to travel the length of the cable, we can disregard these waves.

• We used to track down shorts in long coax (thick ethernet) with a square wave generator and an oscilloscope. 3 ns/foot from the leading edge to the reflection completing the round trip since the 50ohm coax propagation was about 2/3 C. You could also count taps along the way because they caused an impedance mismatch. Apr 12, 2019 at 20:01

Voltage is a difference in potentials between two points. A potential is given as a function of electrical field. So the "voltage" is propagating with the same speed as electrical field propagates, which is the speed of light for vacuum and somewhat lower speed in commonly used conductors. This is for electrostatics.

For electrodynamics, as used in the transmission lines, the field is usually alternating and so the voltage "transmitted" and you can get different voltages at different points of the line depending of the phase of the wave at these points. So you can say that the voltage is "moving" as the phase of the electromagnetic wave is "moving" along the line.

In transmission lines the field propogates at a finite speed, often considerably less than the speed of light (which is about 3ns per metre). So if you feed it a very high frequency the "wave" has not got to the other end before the beginning of the cable gets a reverse polarity. 3ns corresponds to a frequency of about 300MHz (all round numbers, not precise).

Voltage is just a potential force, the real force comes from current drawn by a load from this potential and is limited by the resistance of the load.

Current is the rate of charges flowing per second.

ANALOGY

Voltage is like Water Pressure or the potential to supply water, that may drop when too much current is demanded like water pressure drops from a larger valve to flow more current. THen when shut the pressure builds up again or the potential to supply voltage. This variation depends on the impedance or resistance of the voltage source or battery. Coin cells are very high R in the kilohms and LiPo packs are in the xx milliohm range.

like a bucket brigade of water and this potential which is generated by "something" may be alternating current, AC or direct current, DC flowing just in one direction and continuing to "return" in the return path.

In a car lead acid battery there are 6 2V cells in series with only 2 terminal posts +,- giving 12.5V at full charge.

The energy is converted from the chemical reactions into charges on the electrode plates. This is just a potential voltage of 2V per cell. If we had just a 2V cell Pb cell or an Alkaline 1.5V cell with a load resistance to wires across the battery, ....

... then the charges flow from the acid to the electrodes to the wires to the load and back thru return wire to V- on the battery and then to the -ve electrode thru the acid all around in a circle, until the load switch is opened anywhere in the loop of current. The heat or power generated by this current is greatest where the resistance is highest. So wires, connectors must be very low resistance.

The bigger the battery, the lower the inner electrode resistance ( in milliohms ) and the more current it can generate to a load following Ohm's Law.

Since "secondary cells" are rechargeable the charge flow can be reversed and put back into the acidic chemicals which can be measured by "specific gravity" of the acid relative to water.

Similar but different chemistry occurs in Lithium cells which have a different voltage potential per cell.

• After the current flows through our home appliances it goes out through the return/neutral wire, right? What happens to it after that? Where does it go? Apr 12, 2019 at 18:42
• The potential voltage pair supplies current in that loop thru the load until switched open. Then no energy is transferred it’s just a potential (to supply current). The E field has a force but only if closely coupled like capacitance between wires but very little uA current flows Apr 12, 2019 at 19:18
• So, it's like a cycle? Current enters through the live wire, goes through the appliance then goes out through the neutral, back to the substation and then to the generating station? Apr 12, 2019 at 19:20
• You got it..... AC is a cycle back to source as polarity alternates , DC is not a cycle , just direct current flow in one direction to and back to battery or ACDC converter Apr 12, 2019 at 19:21
• So does the same current flow over and over again? If yes, for how long does this cycle go on? Apr 12, 2019 at 19:23