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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Electric charge flows (current) because there is a difference in voltage between parts of the circuit.
The current is maintained by charge constantly being pushed from areas of high voltage to low voltage, even within the same section of conducter between the elements of the circuit where the local voltage difference is tiny between one millimeter and the next, but is still present.
When you open the circuit, the charge doesn’t have an easy path to get from low voltage to high voltage, and since the charge at the point of the opened circuit, the charge further being that location cannot move into it’s new location on the conducter. This repeats all the way back to the terminal of the power source, as the charge the power source is pushing out has no where to go.
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