ВЭФ 12 (Astrad VEF 12 Project) - including a transistor tutorial

[ppppenguin]

Mark has mentioned Kirchhoff's current law. It's one of those amazingly simple things that'a also very profound. At a commonsense level it just says that all the current flowing into a point on a circuit must be equal to the current flowing out. You can't lose any. It also is one of the foundations of analysing and understanding an electrical circuit. Works fine for water pipes too

It's usually expressed as the sum of currents flowing into a point is zero. Why is that the same as saying current going in is the same as current going out? Because by convention we say current flowing into a point is positive and current flowing out is negative. It wouldn't matter if the convention was the other way round.

In fact the whole business of conventional current flow (from positive to negative) causes a lot of confusion. The labelling of positive and negative was an arbitrary decision on which everyone happened to agree. And the agreement was before we knew about electrons carrying current. They happen to move in the oposite direction to conventional current so in a valve we have what seems like a very odd situation. Conventional current flows from anode to cathode but the electrons go the other way. It's just one of those things that we have to get used to. The answers would be just the same if the convention for positive and negative was the other way round but we're not going to change now.