# Current in series

1. Oct 20, 2011

### amaresh92

whats the reason behind that the current sources doesnt add in series?.

2. Oct 20, 2011

### phinds

Why would you think that they SHOULD add in series?

As a rough analogy, think of pulling on one end of a strong wire with a force F. The other end of the wire is attached to something that then feels the force F. Now make the wire twice as long by adding a wire in series with it and pull on one end with force F. What does the other end feel?

3. Oct 20, 2011

### waht

KCL at the node of two current source in series breaks down. Current into the node does not equal current out of the node. But if the current sources are in parallel, you can use KCL to add up their currents.

4. Oct 20, 2011

### sophiecentaur

Really ideal current sources, connected in series, constitute an impossible situation - like ideal voltage sources connected in parallel. The two will just fight each other and produce a lot of smoke and molten metal. You can't insist on 1A flowing into one end of a wire and 2A flowing out at the other end. Where would all the charge come from to make up the difference?

5. Oct 20, 2011

### phinds

Excellent point.

amaresh92, do you understand this?

6. Oct 20, 2011

### Jiggy-Ninja

Picking nits, but "ideal" sources will never cause smoke and molten metal, since they are purely theoretical.

As was said though, one of the most basic properties of series circuits, as codified in Kirchoff's Current Law, is that the current must be exactly the same through all of the components. If you think it through, it wouldn't make sense for it to be any other way, since the electrons going into and out of the line would not add up, and the cosmic account will get so pissed that you made him work through the weekend to rebalance the books he's liable to short-change you on your "metabolism" budget.

7. Oct 20, 2011

### waht

KVL also breaks for current sources in series. A current source can be viewed as having infinite source resistance. That's why when you turn off a current source, you are left with an open.

When two current sources are in series, one current source sees the other current source as a series resistor having an infinite resistance, and vice versa. As a result, a term in the KVL equation for an infinite resistance (due to one current source) will give you an infinity, since V = IR. So this is just undefined.

In other words, the first current source is forcing a constant current into an open circuit of the second current source. And the second current source is forcing a constant current into an open circuit of the first current source.

8. Oct 21, 2011

### amaresh92

ya got it

9. Oct 21, 2011

### sophiecentaur

Yes, of course but a bit of graphics always helps in pointing out absurdities.

10. Oct 21, 2011

### jim hardy

"" Yes, of course but a bit of graphics always helps in pointing out absurdities. ""

AMEN !

often it's a useful tool of logic to test some idea by extrapolation to its extreme.

ideal current sources in series would be an irrestible force meeting an immovable object,,
... and something's gotta give
but i'm showing my age....

11. Oct 21, 2011

### sandy.bridge

In terms of energy, however, heed the law of conservation of energy.

12. Oct 22, 2011

### sophiecentaur

"ideal current sources in series would be an irrestible force meeting an immovable object,,
... and something's gotta give
but i'm showing my age..."

That was a favourite of Wilfred Pickles on Workers' Playtime. How's your memory?

13. Oct 22, 2011

### jim hardy

14. Oct 22, 2011

### sophiecentaur

Wilfred Pickles used to have a popular daytime radio show (1950s) in which he used to interview 'ordinary people' and would ask them daft questions. One of which was "what happens when an unstoppable force meets an immoveable object?" AS none of them was academic, they couldn't give him a proper answer.

15. Oct 22, 2011

### Jiggy-Ninja

The real answer, if we use Newton's law and consider the immovable object to be of infinite mass and the unstoppable force to have infinite Newtons, is that the acceleration of the object can be any finite number.

Or something like that. It's been a few years since I had calculus class.