# How is it possible for the voltage to change from high to low midway?

So they say that it's easier to supply very high voltage in power lines and then have a transformer turn that voltage down.

But isn't voltage the potential difference between the two sources/electrodes that electricity flows between them? In other words, no matter how many transformers you put in the way, you can just decrease the current by increasing the resistance, not the voltage, cause by definition that's how voltage is defined.

But then again, why do people say the voltage is decreased? Voltage should always be fixed no matter what, unless you change the way you produce electricity all the way in the battery/power plant.

Also, wouldn't putting a lot of resistance in the way waste a lot of energy in heat?

To take this further, a device later down the line should be unable to measure the true voltage of something, because it has no way of knowing what the potential difference in the source is and how much resistance was put in the way. Rather the only thing it should be able to detect is how much current passes through it, which is determined by ohms law, but again, it has two missing variable. It has no way of finding out the source voltage and no way of knowing the resistance put in the way (unless of course you supply the device with said data). If it only had one of them, it could find the other, but it doesn't have both.

And so what does it even mean when a device operates by a known voltage?

What am I misunderstanding here?

Edit:

Now I am confused as to what actually voltage is. Is it not a measure of the difference in the number of electrons. What about current, is it not a measure of how many electrons flow per second?

What does a "unit" of charge physically represent and how would that unit of charge have more energy (does it increase its velocity or what)?

Since I have thought of voltage and ampere like a number thing, if the difference in number electrons is higher, then they flow much more violently, in other words, more electrons would move per second (more ampere). But I am being told that the number stays the same, it's just those electrons have more energy each. This would also raise the question of why would more voltage then even have to do with more current (since it only represents the amount of energy each electron has, not the number of them, so you could have a lot of voltage but very little ampere, and I am saying without even factoring resistance in)?

If we go with the idea that voltage is how much energy each "unit" has, and current is how many "units" there are, then what's really stopping something from have like a very low number of "units", but them being really high energy. It's not like the "units" having more energy would change the number of "units", so why would a higher voltage even mean more ampere?

If we assume higher voltage pushes electrons more violently, then why is that? Is it not due to the bigger difference (in number) between the electrons (when compared to protons) on both sides? Causing it to flow more violently (in more numbers, thus higher current)?

You could have technically very little charge to travel, but them having a very high difference (in number compared to each other?), but that would just mean you would get a really high current, but for a really short time. So again, current would be enough to measure the power.

The only case in which current would not be enough to measure the power is if the number of "units"/electrons/whatever moving doesn't change, yet somehow their energy does (but again, Ohm's law says that any increase in voltage would cause an increase in ampere too, so like what?).

• Sounds like maybe you don't understand what a transformer actually is: en.wikipedia.org/wiki/Transformer TLDR: There is no electrical connection between the input and the output of a transformer. The connection is magnetic. As a very rough analogy, You can think of a transformer as an electrically powered electric generator, and the relationship of the voltage supplied to the transformer's input side to the voltage provided by the transformer's output side is an arbitrary parameter of the transformer design. Jan 26, 2022 at 23:46
• @SolomonSlow Mm, you got that right, thanks for the clarification. There are still some lingering questions though, like why VI showcases power, even though V just shows the potential for power (V determines I and I is the end result you get in your cable or whatever), and I is the actual physical number of the current passing through? Wouldn't I be enough to see how much power is passing through an object? Jan 27, 2022 at 0:24
• Voltage is how much kinetic energy a unit charge would gain if pushed along a certain path (aka the electric potential between those points, and current is the number of unit charges per second flowing down the path. So to find the total energy gained per second (i.e. Power), you need both of them. $P=IV$. The static spark when you touch a doorknob is very high voltage, but very little charge moving, so very little energy is released Jan 27, 2022 at 1:42
• Hmm, I have too many questions that wouldn't fit inside a single comment. I will probably just edit my post. Jan 27, 2022 at 2:25

The problem is that we are not trying to transmit voltage or current, we are trying to transmit power, which is the product of voltage and current:$$P=V*I$$

This means that, if you use a higher voltage, less current is needed to transmit the same power, and vice versa. If you insert a resistor, yes, the current will reduce, but only because the resistor will absorb power; hence less power will arrive at the destination.

Most of our equipment does indeed depend on a fixed voltage, but this is another reason why a transformer is used instead of a resistor. With a transformer, the voltage at the secondary is indeed constant (as long as there is no overload). On the other hand, with a resistor the voltage will vary depending on how much current is drawn. Remember that $$V=I*R$$, in other words, if you double the current the drop across the resistor doubles, and the output voltage decreases accordingly.

The reason for using a transformer (and hence AC rather than DC) is to reduce the current that flows through the transmission lines. That reduces the losses caused by the resistance of those lines, so that more power arrives at the destination. At that destination, another transformer is used to bring the voltage back to something reasonably safe (like 240V) for local, short distance distribution. Most equipment then has another internal transformer that converts the 240V into a voltage suitable for driving the circuitry (e.g. 5V). And, of course, it is then also rectified to produce DC.

what does it even mean when a device operates by a known voltage?

Most electric and electronic devices are designed to be powered by a constant voltage power supply (a.k.a., "voltage source.") Batteries are a rough approximation of a constant voltage source. An electrical generator that is driven at a constant speed is a good approximation of a constant voltage source, and the electric power supplied to most homes and businesses is an excellent approximation of a constant voltage source.

When the voltage is constant, then it's the load (i.e., the device/appliance) that determines the current and the power. (Power equals current times voltage.)

I'm not going to go into detail about how a device can be designed to take the right amount of current/power from a voltage source, but the reason why they are designed that way is because ever since the dawn of electric power, things have mostly been powered by batteries or constant-speed electrical generators.

Just envision electricity as water flowing, say in rivers across the landscape. Current means how many liters per minute of water are flowing past a point. Voltage is gravitational potential, or literally height. So picture a high voltage as a very steep hill the water flows down. You can have a trickle of water flowing down a very steep hill: high voltage, low current. This is a what a spark between your finger and the doorknob is. Or you can have an entire river flowing down a shallow slope: lower Voltage, higher current. Though the slope is low, that amount of water can knock stuff over or push a heavy boat a long way, whereas the steep trickle cannot. But remember, Voltage is simply the height at any point, not slope between two points. Electricity flows from high voltage to lower voltage just as water flows from high elevation to low elevation. It is that simple. Going back to the original topic, a transformer is basically as if a river were powering a paddlewheel mill, which was connected with a shaft to another paddlewheel that is pushing another parallel river that picks up where the first one left off. The rivers are not connected, but the energy flow is continuous. You can vary the gear ratio between the paddlewheels to change their relative speeds, and hence the flow speed between the streams. I think this will be helpful to you, and it is not just an imaginative gimmick – the math really is identical (up to a point) between water flow and electric circuits.

• Thank you, there are several confusing points actually still. In your example of a trickle of water following down a steep river, the speed of water increases thus increasing its energy. But I don't think voltage changes the speed of electricity, so what exactly causes it to have more energy? If it's the number of electrons, then that's represented by current already (making P=V*I pointless). But if it isn't the number of electrons (and their speed), then why does voltage affect the number of electrons (current), and what else could it be? Jan 27, 2022 at 3:41
• Actually electrons do move faster at higher voltage. And more electrons move at higher current. But don't focus on that. Although electrons are necessary to understanding atomic interactions, for simple circuits the 19th Century model of a electric "fluid" running thru the wires like water thru a pipe will serve just fine, and probably give you better intuition. Higher voltage means more energy because voltage is the energy. It's a measure of the energy of a charge in that location Jan 27, 2022 at 5:30

The Volt unit is equivalent to Joules/Coulomb. The Joule is a unit of energy, and the Coulomb is a unit of charge equal to the charge held by approximately $$6.24 \times 10^{18}$$ electrons. So what we really mean when we talk about voltage is the amount of potential energy stored by each electron. Potential energy is just one way to store energy, kinetic is another. When there is a potential difference across a load like a resistor, what that really means is that the load is absorbing some of the energy of the electrons passing through it. In the case of a resistor, this generally becomes heat, but in other loads, it could become mechanical energy or other things.

The Ampere unit is equal to Coulombs/second. The unit of charge, the Coulomb, in this case, physically represents a total number of electrons. So current measured in amps is really just a measure of how many electrons are passing a given point per unit time. Multiplying the electron flow rate, measured in Coulombs/second, by the energy carried by each electron, measured in Joules/Coulomb, gives the total energy passing through per unit time, or power, measured in Joules/second or Watts.

You are correct that you could reduce the voltage and current passing through a circuit by adding resistance, and you are also correct that doing so would waste a lot of energy in the form of heat. This is not what transformers do. A basic transformer has two coils of wire, and what it essentially does is transfer the energy from the electrons in one coil (the primary) to the electrons in the other coil (the secondary). Depending on the construction of the transformer, this energy may be divided over more or fewer electrons in the secondary than it was in the primary. For instance, if a transformer is stepping down the voltage on the output from what it was in the input, it will also result in increased current on the output compared to the input. What this basically means is that the total amount of energy being carried remains the same, but instead of being carried by a few electrons each with a large amount of energy, it is now being carried by a larger number of electrons with a smaller amount of energy.

With regard to measuring voltage, you are correct that it is technically impossible to measure the exact voltage of a source without knowing the impedance of it. Most voltmeters actually work by measuring the current through an internal resistor. However this internal resistor has a very large resistance (typically $$10 \text{ M} \Omega$$ for a standard digital multimeter) compared to the resistance of most power sources, which will usually be much less than $$1 \Omega$$. The resistance of the power source you are measuring can become significant if it is close to that of your voltmeter though. You can test this by putting a $$10 \text{ M} \Omega$$ resistor in series with your voltmeter. If your meter has $$10 \text{ M} \Omega$$ of internal resistance, you should see it measure half the voltage it would have without the resistor.

• There is only one question left. What changes physically in an electron with more energy that causes it to have more energy? Generally with potential energy, it means that said object would have greater speed if released (which really is because of how the setup is, like an object higher up would have more time before it hits the ground and thus can accelerate more, an spring would try to retain its state, so of course pulling it would cause it to move back, and I guess an atomic level explanation for why it tries to retain its shape is about how atom components attract or repel each other). Jan 27, 2022 at 4:18
• And I couldn't thank you due to comment space constraints, so I am making another comment. Thank you for the explanation. Jan 27, 2022 at 4:18
• Oh and one more, If voltage doesn't have to do with the number of electrons and is just the energy they have, then why does it directly affect the number of them (I=V*R)? I could understand if it somewhat was related, but this is a pretty direct relation. Jan 27, 2022 at 4:25
• You're welcome :). To answer your first question, when we talk about the potential energy of an electron, we are usually referring to its potential within an electric field, as the electric force is the primary force that acts on an electron within an electrical circuit. In the presence of an electric field, the electron will accelerate up the field (since it has negative charge), trading its potential energy for kinetic energy. Potential energy is not an absolute measure, but only a difference relative to some reference, which is why it only makes sense to talk about differences of potential. Jan 27, 2022 at 7:22
• As to your second question, the total number of electrons in a circuit does not change—that is a function of the materials making up the circuit—what changes is the rate at which they flow. Voltage in a circuit is also referred to as electromotive force or EMF, and it is basically a measure of how hard the electrons are being pushed through the circuit. It can be useful to envision this as flowing water in a pipe. The pipes are full of water regardless of whether it is actually flowing or not. If there is an obstruction in the pipe, it will slow down the water flowing through it. (1/2) Jan 27, 2022 at 7:29