# I am confused. I need an explanation current flow in this circuit please!

#### subatomic particle

Joined May 8, 2018
36
Hello guys,
so i was studying for exams, and i came across a confusing excersice for me as a beginner.
so i have this circuit, s1 is open as you can see in the picture, and i need to find the current I in the circuit

I have the solution, but i dont understand it. Hier is the solution:

my question is: Why do we just take the current of the current source? is the voltage source not generating current as well??
My soulution would be to calculate the current of the voltage source and then add it to the courrent of the current source.
Can you explain to me please, why the sum of current is equal to I0? Its like we are ignoring the effect of U0 in the circuit. Is it because U0 is an ideal voltage source? or is it because of the 0 potential (i mean the ground)?

Last edited:

#### MrChips

Joined Oct 2, 2009
23,577
The current loop equation is correct.

There is only one current loop and hence the current must be the same around the loop.
Yes, U0 is an ideal voltage source. It has no control on the loop current which is totally determined by I0.

#### subatomic particle

Joined May 8, 2018
36
The current loop equation is correct.

There is only one current loop and hence the current must be the same around the loop.
Yes, U0 is an ideal voltage source. It has no control on the loop current which is totally determined by I0.

Why is I0 the only current in the circuit? if we disconnect the current source we will still have a current that is generated by the voltage source right?

#### subatomic particle

Joined May 8, 2018
36

#### MrChips

Joined Oct 2, 2009
23,577
1) If you disconnect the current source you have a totally different problem.

2) The current source is an ideal current source. It overrules everything else in that single current loop.
Insert any number of voltage sources in the loop and the current source wins every time.

#### subatomic particle

Joined May 8, 2018
36
1) If you disconnect the current source you have a totally different problem.

2) The current source is an ideal current source. It overrules everything else in that single current loop.
Insert any number of voltage sources in the loop and the current source wins every time.

#### RBR1317

Joined Nov 13, 2010
643
my question is: Why do we just take the current of the current source? is the voltage source not generating current as well??
In most cases the voltage source will be generating current; but it is also possible that a voltage source will absorb current if forced to do so. (In the real world it usually won't do this for very long.)

You might also ask, Is the current source not generating voltage as well?

Think of a current source as being just a voltage source with a special property; it adjusts its terminal voltage to make the specified current flow in the circuit.

#### crutschow

Joined Mar 14, 2008
27,243
Why do we just take the current of the current source? is the voltage source not generating current as well??
Another way to look at it is to note that all the component currents are the same in a series circuit.

Now, by definition, an ideal current source will generate its specified current, regardless of whatever other components are connected in series, (resistors, voltage sources, etc) so the series current has to be what the current source is generating, and whatever the voltage source is doing, has no effect on that current.

#### subatomic particle

Joined May 8, 2018
36
Thanks so much guys. That helped me a lot!

#### MrAl

Joined Jun 17, 2014
8,266
Hi,

Based on your original response, there is no such rule that "currents in series add". The only rules like that are:

Although the statements from the past are a little more concise than that:
1. The total sum of voltage drops in a series circuit equals zero.
2. The total sum of currents into or out of a node is zero.

There is one teaser that has to do with ideal current sources though, and that is what is the total current when we have two different ideal current sources in series. It becomes a matter of practicality then. You may not want to get into this right now though.

#### subatomic particle

Joined May 8, 2018
36
In most cases the voltage source will be generating current; but it is also possible that a voltage source will absorb current if forced to do so. (In the real world it usually won't do this for very long.)

You might also ask, Is the current source not generating voltage as well?

Think of a current source as being just a voltage source with a special property; it adjusts its terminal voltage to make the specified current flow in the circuit.
you are talking about ideal sources right?

#### MrChips

Joined Oct 2, 2009
23,577
you are talking about ideal sources right?
It is only ideal in the sense that an ideal current source has infinite impedance and can supply infinite voltage.
In contrast, an ideal voltage source has zero impedance and can supply infinite current.

#### RBR1317

Joined Nov 13, 2010
643
you are talking about ideal sources right?
Ideal sources are a mathematical abstraction. I was talking about a conceptual description that should make it easier to understand the abstraction.