# When using the Mesh Current Method?

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When using the Mesh Current Method in circuit analysis and you encounter a loop with a current source that is going the opposite direction of the mesh current do you assign a negative number to that current source?

For example in this circuit there is a 7.5 amp current, I was taught to go clockwise with the current, it really doesn't matter which way you go as long as you get the sign conventions right and put negative and positive where they need to be but in this case I am dealing with whether it is negative or not since I am going clock wise. Do I treat it as negative number, -7.5.

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When using the mesh current method you should ahve been taught to replace the current source with an open circuit, since it has infinite impedance.

Then you work on the revised circuit, maintaining the original mesh currents.

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I am actually learning about Thevenin equivalent circuits and transforming circuits into a Thevenin equivalent circuit. That is what we do with Thevenin Theorem not the Mesh Current Method, to find Equivalent Thevenin Resistance.

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7 hours ago, eggman2 said:

I am actually learning about Thevenin equivalent circuits and transforming circuits into a Thevenin equivalent circuit. That is what we do with Thevenin Theorem not the Mesh Current Method, to find Equivalent Thevenin Resistance.

Yes there are many basic methods of circuit analysis and they have point of similarity as well as differences.
This includes node analysis, branch analysis, mesh analysis, Thevenin, Norton and superposition.
After all they must come up with the same answers.

It is common to take one basic simple circuit, using only simple componenets such as the one you posted, and compare the analyses by the different methods as you encounter them.

There is also theory as to which method is the most efficient for a particular purpose, but that is not the reason for understanding them all.

The point is that lecturers are also looking forward to the introduction of more complicated components and also the fact that most of our circuits ar AC not DC.

The constant current generator is very important as it forms the basis of the model of the transistor that is used to include transistors into circuit.

These methods you are learning have one common feature.

They are based on the physical fact that if you take two points in a circuit, say A and B where you could separate the circuit into two halves by cutting through A and B, you can

'lump' the entire characteristics in one half into an equivalent cicuit.

An equivelnt circuit is one which will behave the same way - there are always many, some more complicated, some simpler.

Lump means collect all the components into one impedance (and/or source) that will behave the same way (equivalently).

This is an incredibly powerful concept which has parallels in other areas of physics, eg mechanics but is much weaker there.

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Ok thank you. But I found that you treat as negative number for what I was asking.

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30 minutes ago, eggman2 said:

Ok thank you. But I found that you treat as negative number for what I was asking.

Not quite.

The constant current is only 'negative' because it is going against the chosen clockwise direction of mesh currents.

If it was in the same direction as any of the circulating mesh curent it would be positive.

Do you think the 5A and 7.5A sources oppose or add?

What would happen if you reversed one of them?

Edited by studiot

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That was weird I couldn't reply for a while...

anyway....

I don't know what you mean by oppose or add but 5 amps is positive and 7.5 amps is negative if your meshes are going clockwise. And if you reversed them and your meshes are going clock wise still then it would be negative 5 amps and positive 7.5 amps.

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