# How does a circuit “know” the resistance? [duplicate]

This is obviously a very basic question, I'm pretty new to concepts of circuitry and so on. So, lets say you have a circuit with simply a battery, a resistor, and a multimeter measuring the current.

From my understanding, the circuit almost, well, knows(?) the resistance before any current travels. Obviously that's not really possible. But could someone shed some light on how this works? Thanks a ton.

• What do you mean? I mean, a circuit is a bunch of things stuck together, they do not know anything... – Vladimir Cravero Mar 22 '17 at 15:36
• What are the evidences of your assumtion? – Curd Mar 22 '17 at 15:36
• I think the best thing for you to do is google Ohm's Laws and understand that. I'm pretty sure that should answer your question – MCG Mar 22 '17 at 15:38
• Actually the question is good and not trivial to answer, see duplicate. – Rev1.0 Mar 22 '17 at 15:42
• Ohm's law doesn't cover the "how", it's empirically derived. As rev says the duplicate covers this in detail. – pjc50 Mar 22 '17 at 15:43

It does NOT know in advance.

When you connect a resistor to a battery, current starts to flow from zero and quickly ramps up until the voltage across the load (the resistor) matches the voltage of the battery.

Voltage Across Resistor = Current x Resistance Value


When that happens the current can no longer grow. It balances out.

NOTHING ever happens instantaneously. But it happens so fast in human terms, it appears that way.

Actually the math is a bit more complicated than that.. but it boils down to the same thing.

• Really? A down vote with no explanation? Sigh... – Trevor_G Mar 22 '17 at 15:52
• Funny that a cable with almost zero resistance, if long enough, may behave as 50R or something. Those waves in the cable are even more funny – Gregory Kornblum Mar 22 '17 at 19:49
• IMO your answer is not really addressing the actual question, similar to this one. But yes, explaining a down vote is good etiquette. – Rev1.0 Mar 23 '17 at 7:51
• @Rev1.0, I agree with you, the answer is not comprehensive, however, I have found over the years that you have to tailor answers to questions. If a child asks, "why is the sky blue" they will stop listening as soon as you mention something like wavelength or atoms. Better to round it up into a correct, if simplified, answer first, and delve deeper if they ask deeper questions. There is always a real danger of swamping and discouraging the OP with overly complex answers to, what appears to most of us, trivial and naive questions. We all started there after all. I see that a lot on here.. – Trevor_G Mar 23 '17 at 14:17