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Which electron moves first if you connected a wire to both ends of a battery at the exact same time?

Physics Asked by ProGrammar on August 10, 2020

Question 1:

Which electron in the circuit moves first if you were to connect a wire to both ends of a battery simultaneously?

Is it:

  • The last electron in the wire, which is pulled into the cathode of the battery? And the rest of the electrons are "pulled"?
  • The first electron coming out of the electrolyte, and the rest of the electrons are "pushed"?
  • They all start moving at the same time? (…ok, but…)

Question 2:

If I put a resistor in the circuit in the middle of the wire, I get that the current drops over the entire circuit. But how does that "information" about the lower current get to the source of the push/pull? Is there an infinitesimally-quick initial pileup or void of electrons at the resistor that propagates back before it balances at the proper current?

Or does the first electron to move just "magically" know the proper current to push/pull the INSTANT the circuit with the resistor is closed? What if we had a circuit with a resister in the middle of a theoretical wire that had perfect conductance and was a light-year long. Would the information of the resistor be available to the first electron to move before light/information could have reached that first electron?

One Answer

A battery provides an electric potential across a wire. However, its electric field can only propagate at the speed of light. Typically, because the time it takes light to cross a circuit is so small compared to the other timescales of the system, it is safe to neglect the speed of light when evaluating circuit diagrams, and simply take the speed of light to infinity.

In reality, as the electric field propagates throughout the wire it will cause the electrons closest to the negative terminal of the battery to start moving first. As the field propagates throughout the wire, transient currents will oscillate and rapidly decay towards the equilibrium state that we normally think of in circuits.

This addresses your second question about the resistor. The resistor does not instantly transmit information throughout the circuit, but rather sets up electric fields which transmit that information at the speed of light.

Answered by David on August 10, 2020

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