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


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?

In: 0

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.

So to answer your questions:

1. This isn’t a question. You stated something, but didn’t ask anything about it.
2. The time period is the same.

My Physics III professor explained it this way. We did some math and calculated the speed of electrons through a wire, and it’s actually really slow. The reason opening/closing a circuit is instantaneous despite that is because the field acts on the electrons that are already there.

Imagine a hose like above, but now I have filled it with marbles that are as big around as the hose. If I push a marble at one end, the marble at the other end moves instantaneously because force moves that way. This is how the wire exists: the marbles are like electrons in the wire. They’re already there whether the circuit is charged or not.

When one end has a source of electricity, it’s sort of like i pushed on a marble, but the open switch is like the valve I used to restrict water in the last example. Except now it’s a marble-restricting valve. No matter how hard I push the marble on one end, the force gets to the valve and can’t act on the marbles on the other side. So one end of my wire is “pressurized” with electrical charge and the other isn’t.

*As soon as I open the switch*, it’s the same thing as opening the water valve above. I’m already pushing on the marbles at one end. They immediately push on the marbles that were behind the switch. That immediately causes all the marbles in the system to move, even if the individual marbles aren’t moving quickly.

How an electrical field acts on electrons is *basically* the same.