>>14417820Because even though you keep saying the word "flow", your intuition about current intensity nevertheless makes you think of it as the "speed" at which charge is going, which is throwing you off. You're thinking of pic related (i.e. resistance = constriction on the flow) and you're intuitively applying Bernoulli's law (even if you don't know what it is, you're applying intuitively it) which tells you
>ok, I can see the water is flowing at different speeds inside different constrictions. how can the current possibly be the same in all three restrictions?The part that you're forgetting about the definition of "flow" is the SURFACE. Yes, the current intensity depends on SPEED, but it also depends on SURFACE. No, it is not as intuitive as "large current = fast flow" unfortunately. Two restrictions can have water flow at different speeds and yet have the same water flow. And two resistors can have different resistance and yet same current intensity.
You need to differentiate between current density and current intensity.
Current density is defined as charge desnity moving at speed v and only depends on v.
Current intensity is defined as current density through a surface and depends on both v and S
In perpendicular case like pic related this reduces to
In the 2nd restriction, although the flow is faster the surface through which it flows is smaller.
In the 3rd restriction, although the flow is slower the surface through which it flows is larger.
According to continuity equation which also known as Kirchhoff's 1st law these balance out precisely in a way to keep the flow equal in all restrictions.
