"Circuits" - A convenient teaching tool?

Question

At school, we are taught that electricity requires a complete circuit for current to flow.

We are later taught that current flows from areas of high voltage to lower voltage.

So, current can always flow from a high-voltage line into ground since the earth is so large it essentially has an infinite amount of electrons.

Is the cumulative effect of this that circuits don't actually really exist in the sense of being the deciding feature in whether or not electricity can flow, and instead, all that is required is a medium through which charge can flow from high to low potential?

If that's not the case, and circuits are actually required, then how can the earth seamingly sink infinite current?

Answer 1

Current can flow from a high voltage power line through the Earth because the power plant is connected to the Earth. The Earth acts as a wire.

It is like things work in your car.

A light bulb must be connected to the car battery to work. You connect one terminal of the light bulb to the positive terminal of the battery, then you connect the other terminal of the light bulb to the vehicle chassis - the bulb lights up.

Where's the circuit? You've connected the bulb to the battery at one end, while the other end of the bulb is connected to the chassis. The vehicle chassis is connected to the negative terminal of the battery. That completes the circuit.

It works much like that with AC power lines.

If you touch a high voltage power line, you will connect yourself to one terminal of the generator through the power line. You will also be connected to the other terminal of the generator through the Earth. The Earth plays the part of a wire going back to the generator.

The Earth doesn't "sink" the current. It completes the circuit back to the source.

Answer 2

So, current can \$\color{red}{always}\$ flow from a high-voltage line into the ground since the earth is so large it essentially has an infinite amount of electrons.

With emphasis on the word \$\color{red}{always}\$, the above statement is false. There needs to be a closed circuit path for current to \$\color{red}{always}\$ flow.

all that is required is a medium through which charge can flow from high to low potential?

False or untrue in the context of \$\color{red}{always}\$.

If that's not the case, and circuits are actually required, then how can the earth seamingly sink infinite current?

It can't sink infinite current.

Answer 3

Yes it is true, that current can always flow from high to low voltage.

Because (drumroll) in order to define this high voltage you must already have a closed circuit.

Consider a 1.5 V battery on a conducting sheet and, next to it, a 9 V battery on the same sheet. Both batteries have the negative side connected to the sheet.

If you measure between the terminals with a voltmeter, you get 7.5 V because the voltmeter completes the loop with its input current.

If you take the sheet away and repeat the measurement, you now get 0 V because there is no loop that can sustain the necessary voltmeter current.

Answer 4

Is the cumulative effect of this that circuits don't actually really exist in the sense of being the deciding feature in whether or not electricity can flow, and instead, all that is required is a medium through which charge can flow from high to low potential?

If you were to take a space shuttle to Mars, stand on it's surface, and look at the Earth through binoculars, you'd have no evidence that any electrical charge was flowing at all.

i.e., all electrical current flow is a localized phenomena. Even in the case of a lightning strike (which is considerably bigger than a say, a hearing aid), all of the charge imbalance is still localized.

If that's not the case, and circuits are actually required, then how can the earth seemingly sink infinite current?

It can't, and doesn't. Even in the case of the largest thunderstorms ever recorded, they first didn't exist, then did, then dissipated. The net charge difference before and after is essentially zero, despite a lot of charge exchange in the process. The lightning bolt may be ten million degrees and carry thousands of Amperes for some fraction of a second - but the net effect is null.

A single lightning strike doesn't make the whole planet slightly more or less charged - the differential in the cloud versus the ground at the strike location simply equalized the charge at that location. The ground isn't a good conductor overall, so there is considerable conductive resistance from one city to another, let alone from one continent to another. If this weren't the case, then a lightning strike in one location would statistically increase the chances of another strike in distant locations, and that just isn't observed. What is observed, is the strikes only happening in the area of the storm cloud, since that is where the localized charge differential exists.

Answer 5

Electrical circuits (and associated laws) are a simplification of electromagnetism which fundamentally described by the Maxwell's equations. These equations don't require a circuit to be there, or even define what a circuit is. So yes, you can have a current without a circuit.

Circuits are very useful because they abstract away the physical dimensions of electrical components. Using Ohm's law you can calculate the current without knowing how long are the wires, what is their thickness, and whether the resistor is flat or round.

Quite obviously, there are cases where size starts to matter, as it is with very large objects such as Earth.

Answer 6

The term "ground" is just a name of a voltage potential, chosen by the engineer. This is the voltage potential that is used as a reference - when the engineer says "5 volts" it is shorthand for saying that it is a voltage 5 volts higher than the potential of the point he has chosen as "ground." Any current flow into ground must also flow back out, to the source of the current (power supply, battery, etc.) The ground does not "sink" the current.

The term "earth ground" or just "earth" often confuses people because they associate it with the earth itself, and in many cases, the "earth ground" is attached to a ground rod or similar. But it is just another named circuit ground. It would be a mistake to think that the entire earth is at one potential. In general, tying a circuit ground to a ground rod is done for safety reasons - to keep a circuit's potential from floating to a dangerous level above an object that you might be standing on or touching. But this type of "ground rod" earth ground never means for these objects to be a conductor of your circuit. When current goes through the actual earth instead of a wire, this is a "ground fault." In fact, ground fault interrupters measure the current in both directions, and switch the circuit off if they are not in balance, which would be an indicator that some current is flowing back to the source through another path than the circuit's wires.

Answer 7

The most commonly encountered electronic flow without apparent circuits is electrostatic discharge. That can range from a tiny spark when you touch your car to a lightning. In all cases there is an excess of electrons on one side, and the charge balances with the other side of the spark.

However, we already have a circuit element that models charge: a capacitor. So a lightning can be modeled as a capacitor formed by clouds and ground:

^{simulate this circuit – Schematic created using CircuitLab}

Like the other answers have explained, your example of high voltage line shorting to ground also has a full circuit if modeled as such:

^{simulate this circuit}

But there is some truth in calling it a teaching tool: Circuits are just a convenient way to model the real world. None of the electrons know or care whether there is a closed circuit or not, they are affected only by the local electric and magnetic fields. The circuit model works well in most practical cases, which is why it finds wide use both in teaching and in engineering.

And because of its convenience, many effects get represented as equivalent circuits. For example semiconductors cannot be fully analyzed using the circuit model (we need to get to the single electron level), but we can construct equivalent circuits that match the actual behavior close enough to be useful.

Answer 8

At school, we are taught that electricity requires a complete circuit for current to flow.

This is approximately true. Stated another way, current pretty much only flows in loops. The amount of current entering any region of space is almost exactly equal to the amount of current leaving that region of space.

If I do something like pick up a AA battery and then touch just one of its terminals, then some current will flow, but the amount of current is so very, very tiny that we can usually assume that it's 0.

We are later taught that current flows from areas of high voltage to lower voltage.

This is completely wrong. Current flows in loops, so if you have some current flowing from an area of high potential to an area of low potential, then you must also have an equal amount of current flowing the opposite way, from low potential to high potential.

What is true is that current "wants to" flow from an area of high potential to an area of low potential. To be precise, if some device has current flowing through it from high potential to low potential, then that device is receiving energy and can use that energy to do something useful (like emit light).

If I'm an electrical device and I want current to flow through me from low potential to high potential, then I need to expend energy in order to make it do that. This is what batteries do: they use the energy stored in chemicals in order to cause a current to flow through themselves, into the low (negative) end and out from the high (positive) end.

So, current can always flow from a high-voltage line into ground since the earth is so large it essentially has an infinite amount of electrons.

That's not quite true.

Suppose that I have an enormous 12,000 V battery. If I connect one of its terminals to the ground, then pretty much nothing will happen. Since the battery doesn't have an essentially infinite amount of charge in it, only a tiny amount of current will flow very briefly. If I connect the positive terminal to the ground, then the positive terminal will have a potential equal to ground potential, and the negative terminal will have a potential equal to ground minus 12,000 V. If, instead of doing that, I connect the negative terminal to the ground, then the negative terminal will have a potential equal to ground potential, and the positive terminal will have a potential equal to ground plus 12,000 V.

Now, if I connect both of the terminals to the ground, then a large amount of current will flow. But that's not because the earth is so large that it essentially has an infinite amount of electrons; it's because I made a complete circuit.

Is the cumulative effect of this that circuits don't actually really exist in the sense of being the deciding feature in whether or not electricity can flow, and instead, all that is required is a medium through which charge can flow from high to low potential?

The thing is that in order to maintain a voltage (that is, a high potential in one place and a low potential in another place), you need to have a closed loop—a circuit. If there's a voltage between two objects, and you connect those two objects together, but there's no loop, then the voltage between those two objects will almost instantly become 0, and no more current will flow.

If that's not the case, and circuits are actually required, then how can the earth seamingly sink infinite current?

It can't. We humans don't have any way to put a large amount of current into the earth without simultaneously taking the same amount out of the earth somewhere else.

Nature is able to make large amounts of current flow in one direction only—that's called lightning! But even lightning is ultimately a closed cycle. All the electrons that a lightning strike puts into (or takes out of) the ground are electrons that came out of (or went into) the ground at some earlier time.

Answer 9

The fundamental flaw in your reasoning is this:

So, current can always flow from a high-voltage line into ground since the earth is so large it essentially has an infinite amount of electrons.

It's amazing how wrong this is. The ability of the earth to absorb or release net charge is its "self-capacitance", and the self-capacitance of the earth is about 710uF. That means you raise the potential of the earth by 1 volt if you sink just 1A of current into it for only 0.7ms.

So, if you had an infinite sink of electrons at 12V above the earth, then you could draw an amp for 0.00852 seconds. But of course you do not have an infinite sink of electrons. On one side you have the whole earth. On the other side you are likely to have something much smaller, so that 12V will vanish much more quickly.

Even with the whole earth on one side and the sky on the other, and many kilovolts of potential difference, a lightning bolt erases that difference in a flash.