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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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.
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