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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You can think of the circuit to be always closed if you consider air to be a part of the circuit. Now, the current cannot flow through this circuit because the resistance offered by air is too high to overcome for the voltage applied even though the voltage always ‘tries’ to overcome it. The moment you ‘close’ the circuit, you are providing a path of far less resistance which can be overcome by the applied voltage to start the movement of electrons or ‘current’.
Lightning strike is another ‘open’ circuit between clouds and earth but voltage is so high that it can take the current through the air to earth.
So the circuit is ‘open’ means it has too much of resistance to the current flow and nothing else.
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