How to tell which way current flows in mesh analysis

[gtreg]

The problem can be seen @ http://gregsomers.com/elg.jpg


Depending on what way is chosen for the current going through the 50ohm resistor, there are completely different answers... how do i know which direction to choose?

I.E. 50(i2-i3) for current flowing to the right OR 50(i3-i2) for current flowing to the left

 

[The Electrician]

You need to show some more details of your attempted solution before your question can be answered.

 

[willem2]

You need to show some more details of your attempted solution before your question can be answered.

I think this is rather harsh, because if you didn't spot the 3 curved arrows labeled i1, i2 and i3, there's no way to do any work on the problem.

 

[mugaliens]

I think this is rather harsh...

I agree, for two reasons:

1. A cursory look at the problem indicates the HW question involves the values of R1 and R3, not "how do i know which direction to choose?" which is what the OP asked.

2. While we're not here to hand out final answers, there's nothing wrong with guiding a student towards the answer, or explaining how something works.

3. There is something very wrong when a forum member talls a newbie to "go fish" without taking any time to help the newbie with the question he asked.

So, on with the help!

The anwer to your question is easier than find than you might think, as all the Ix are given. You know Is=I2-I1+I3-I50, where I50 is the rightward current across the 50 Ohm resistor. As you use mesh analysis to determine R1 and R3, you can then use the currents across them to determine the voltages across them. Armed with the voltages across them, you can determine the voltages at all points, including right and left of the 50 Ohm resistor. Armed with the voltage across the 50 Ohm resistor, you can determine the both the current as well as its direction.

It may that your text explains mech current analysis poorly. I believe http://www.allaboutcircuits.com/vol_1/chpt_10/3.html" explains it quite well.

Good luck!

 

[The Electrician]

I agree, for two reasons:

1. A cursory look at the problem indicates the HW question involves the values of R1 and R3, not "how do i know which direction to choose?" which is what the OP asked.

Yes, the question does involve finding the values of R1 and R2. To do this requires solving the network, and choosing the proper directions for currents is vital to a correct solution. I found the OP's question ambiguous, so I asked him to provide more details about what he had done.

2. While we're not here to hand out final answers, there's nothing wrong with guiding a student towards the answer, or explaining how something works.

3. There is something very wrong when a forum member talls a newbie to "go fish" without taking any time to help the newbie with the question he asked.

I didn't tell the OP to "go fish"; I asked him to help me help him, but he seems not to be listening any more.

So, on with the help!

The anwer to your question is easier than find than you might think, as all the Ix are given. You know Is=I2-I1+I3-I50, where I50 is the rightward current across the 50 Ohm resistor.

Is = I2-I1 (which is 3 amps); it would only be true that Is=I2-I1+I3-I50 if (I3-I50) = 0. The rightward current through the 50 ohm resistor is I50 = I3-I2, so I3-I50 = I3 - (I3-I2) = I2 which is not zero.

You have shown just why it is so important to be careful about the choice of currents and their directions.

Your intentions may be good, but it is no help to the OP to give him wrong hints.