Homework Help: KCL to find voltage

1. Dec 21, 2015

Gbox

1. The problem statement, all variables and given/known data
Find the voltage between a and b

2. Relevant equations

KVL, KCL
3. The attempt at a solution

I did not understand how they have reach the result we just two equations, and how did they use KCL to get those equations

2. Dec 21, 2015

Staff: Mentor

There are only two essential nodes in the circuit, and they are labeled v and voc. The node at the top of the 20 V supply is not an essential node since its potential with respect to the reference node is fixed by that voltage source. Effectively that node's potential is already "solved".

Since there are only two essential nodes, only two node equations are required in order to completely analyze the circuit.

The KCL equations that were written are node equations. They sum the currents at a given node. Look at them term by term and you should recognize the currents for each branch leading into a given node.

3. Dec 22, 2015

Gbox

Why is the node on top on the 20V is "solved"? There are different currents coming out of it?

4. Dec 22, 2015

Staff: Mentor

Nodal analysis determines the potentials at the nodes with respect to the reference node. The potential at that node is already known: 20 V.

Given the node potentials the potential difference across every branch in the circuit can be determined, hence the currents can be determined, too, by Ohm/s law.