# Question about voltage source and current

How can one be sure that the current remains the same, i.e. 2 A, after passing through a voltage source and does not increase or become less?

I have the same question regarding the 3 A; does it mean that it was 3 A before passing through V1?

You have the impression that the current entering a two-terminal thing can be different from the current leaving the other end?

If so, that's your problem. We refer to KCL, Kirchhoff's current law, which (when applied to a two-terminal device) says that current entering at one terminal must be the same as the current emerging at the other.

It's as simple as plumbing. The rate (in litres per second) at which water enters a pipe at one end must be the same as the rate at which it leaves the other. Otherwise you have a leak somewhere, or a pipe that's ballooning.

If that's raised a whole bunch of other questions, and causing you to rethink your understanding of current, then good! There seems to be a common misconception that a resistor, for example, slows charge flow, so that charges enter fast, and leave slow, but that's so wrong.

If water is flowing around a loop of pipe, and then you created a restriction at some point in the loop, you slow the flow everywhere in the loop, not just immediately following the restriction.

If road traffic encounters a restriction, all traffic along the road is affected, on both sides of the restriction.

That's why we know that if 3 coulombs of charge passes every second (that's what 3A means) at some point prior to entering the two-terminal device, then 3C per second must be passing some point after that device, too.

• Thank you for explaining this - it helped me understand the concept much better. Mar 28 at 7:51

That is axiomatic - it's just conservation of charge dictating that you can't have anything you can put an enclosing surface around into which more current flows than it leaves, or vice versa.

If that wasn't the case, Kirchhoff's circuital current law couldn't exist and we could do nothing about networks like this.

Voltage and current sources are theoretical things, not real components. By definition the current entering and leaving these devices is the same, since there’s nowhere else for the (theoretical) electrons to go.