# How does the current/source know that the circuit has been closed?

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Recently when I was procrastinating, I have learned that an electric energy is not propagated via wires but through fields. Once a circuit is closed, a field is created and it carries the energy from a source to eg a light bulb. It proposed a question to me.

The question is, how does the current/source know that the circuit has been closed?

Let’s ask two similar questions, both assume ideal conditions.

1. we have a source and a switch on Earth and a light bulb on Mars. We close the circuit using the switch and the energy starts to be emitted from the source in an almost instant. After some time, once the field reaches the bulb, it starts to emit light.
2. we have the same situation, but the switch is moved to Mars. Will it take the same time for the bulb to emit light? How does the source know that the circuit has been closed and it can start to emit and electric field?

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It is not a matter of the source “knowing” it is just a matter of the source doing what is either natural or it is designed to do.

If you are not interested in electricity then you can think of any source with potential energy as analogous to an electric source. If you put a hole in a balloon, you know air will come out. If you drop a rock, you know it will fall. If you close a switch, you have created a path for electricity to flow.

Electricity flows at the speed of light, so in your earth-mars question, there will be a measurable delay based on the distance of each example

Electricity can be as intuitive to understand as a rock fall because of gravity if you understand the basics principles of electrons

The field is not always there, it required electrons to move.

The source doesn’t know a circuit is closed any more than a ball knows a hand is holding it up. The force to pull it down is always there until you let go.

Moving the switch changes nothing, as the electrons don’t physically need to move the full length. It bfhaves like marbles in a pipe filled to both ends. Push one in and the last one pops out. The marble didn’t move all the way but just displaced the one beside it.

Current flows from high potential to low potential. It’s sort of like how if you hold a ball over your head, and let go, it will fall to the ground. How does the ball know that it’s higher than the ground? It doesn’t… it’s just constantly attracted to the ground, and once you remove the thing that’s keeping it from going to the ground, it’ll fall.

In the case of a circuit, when you flick the switch, you connect an area of high potential (the power supply) to an area of low potential. Again, the high potential current always wants to flow, and when you give it a way to flow, it’ll flow.

I assume it doesn’t. Energy typically travels from a higher potential to lower potential, and the source is enginerd to be at a high potential and the light bulb has lower potential. The fucking cool as nuts part is that as energy passes through the lightbulb, some of it is converted from a field of energy to a source of heat and light at the cost of some of that fields’ strength, current and whatnot. You could get rid of the lightbulb entirely and current will still flow when the two potentials are communicating, but what communicates the difference is probably gonna get a lil warmer, and that’s why wires need to be thick. But I’m not sure I am allowed to guess the answer to your post

There’s no “knowing”. That’s what things that think do. You’re talking about physical phenomenon.

It is not much different than from when you have pressurized water in a hose but have closed off the end, then open up a nozzle. Water doesn’t “know” you opened the nozzle. Water was uniformly applying pressure against every surface, including the valve keeping the nozzle shut. That valve was pushing back against the pressure with equal and opposite force, so water could not get through. Then, the valve opened. Now the water pushes against nothing, encounters no resistance, and water is flowing through the hole, propelled by the force of all of the pressure behind it.