# Electrical networks(linear algebra)

1. Jan 12, 2010

### EV33

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

Determine the currents in the various branches.

Then there is a picture of the network which has batteries, resistors, and nodes.

Most of the problems have the currents labeled.

My question is when the currents are not labled how do you determine where to put them and what direction to give them?

2. Relevant equations
No equations.

3. The attempt at a solution

By looking at the the various electrical networks in the book it looks like you can put the currents anywhere and in any diretion.

2. Jan 12, 2010

### pgardn

Have you learned what role voltage plays in "producing" current? Are there any numbers indicating voltage or potential difference? And by networks, are you referring to circuits?

3. Jan 12, 2010

### EV33

To solve these problems he has us using Kirchhoff's first and second law.

We get the amount of ohms at each resistor, the amount of volts at each battery and like I said typically it will show us the currents direction.

I haven't truly been taught what voltage does in producing current but those are the two laws the book gives.

We didn't get much of an explanation of these problems in general but the explanation we got very much so lacked the concepts involved because it is for my linear algebra class rather than for a physics class.

4. Jan 12, 2010

### EV33

The book calls them networks but yes they are cicuits.

5. Jan 12, 2010

### pgardn

Well basically without seeing a diagram the idea is that current cannot be created or destroyed, so when a wire is split, the amount of current in each of the splits must be equal to the amount of current that came into the split... like a river forking.

When two or more wires join, its like rivers merging, the amount of current entering from each tributary must equal the total current in the large part of the river.

Voltage differences drive current like gravity drives the flow of water in a river.

If one then looks at the circuit as a whole and picks a particular spot in that circuit and then moves around the circuit all the way back to the spot, the voltage (or energy per coloumb of charge must be the same. So on your trip around the circuti you might "gain" or "lose" energy along the way, but by the time you have completed your route you must have the same energy per coloumb of charge. One usually "gains" electrical potential energy at batteries and "loses" electrical potential energy at resistors...

This is the simplified basics behind Kirchoffs stuff. Sorry for the delay in response, I went and got a hamburger.

6. Jan 12, 2010

### vela

Staff Emeritus
You give a current a distinct label if it's different from the other currents in the circuit, and you can choose whatever direction you want. When you work out the problem, if you guessed the wrong direction, the current comes out negative, but the magnitude is correct.