# [Circuits] Calculating voltage and current in a circuit

1. Jan 20, 2014

### ainster31

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

How does the author do the below?

$$v_{ i }=2li=42V$$

What equation is he/she using?

2. Jan 20, 2014

### Staff: Mentor

That should be 21i. 21 Ω is the parallel combination of the 70 Ω and 30 Ω resistors. The author is calculating the voltage drop across that pair.

3. Feb 10, 2014

### ainster31

They seemed to have applied KCL at the middle node: $$i_1=i_2+I_0$$

But what about the other side of the same node? It looks like they only looked at the left-side of that node. Is that acceptable?

4. Feb 10, 2014

### Staff: Mentor

The current Io takes care of whatever is happening on the other side; the node was partitioned in order to "get at" the current Io . If you want, think of the horizontal wire as a zero ohm resistor separating two nodes