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Sometimes circuit analysis can get confusing as to which way currents go in and out of nodes , especially when there are many current / voltage sources. Is there a trick to see which way i should reference everything? I always see reference with respect to ground but I'm not too sure what that means. Here is an example from my professor, although im not too sure if he made a mistake on i2

enter image description here

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  • \$\begingroup\$ My guess is that i2 isn't an error, it was done on purpose to illustrate the point that choosing the wrong direction will only give you a negative result. It'll all work out in the end. \$\endgroup\$
    – s3c
    Commented May 16, 2014 at 14:50

2 Answers 2

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the trick is:

don't waste too much time trying to guess currents directions. if you set the wrong direction, then the current value will be negative. you can see currents as vectors. referencing to ground is not mandatory.

remember: wrong direction -> negative value -> no errors

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  • \$\begingroup\$ Yes, but how would I be able to check if I set it in the right direction or not ? I understand that referencing ground is not necessary, but im not tooo entirelysure because if the answer would be positive then on a test i would have to write it positive... I guess in a sense i'm kind of confused as when im setting the right direction or wrong direction. \$\endgroup\$
    – user42926
    Commented May 17, 2014 at 10:47
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is there a trick to see which way i should reference everything? I always see reference with respect to ground but I'm not too sure what that means.

Remember: A voltage is a potential difference between two nodes. A voltage by itself is not meaningful and needs to be referenced somewhere. Conveniently, we choose a location (and call this reference 'ground') that is common for the whole circuit. Current will flow from a high potential to the lower potential, so no matter what the reference that we use, we will see the current flow as either positive (in the direction we named the current) or negative (opposing the direction we named the current).

For circuit analysis, it does not matter what node we call our ground reference. Typically, it is convenient to have as many sources as possible touching this node, but it truly does not matter. V2 or V3 could just as easily been chosen as our ground reference.

Do not get hung up by the name 'ground' as when dealing with antennas or wireless devices, there is no concept of a ground connection as the device may literally be floating in air. In this case, the circuit or signal ground is only a reference for the circuit. By referencing everything to a common ground, different circuit components are able to interact with each other.

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  • \$\begingroup\$ So for example in the image above, 10A is considered "positive" ? \$\endgroup\$
    – user42926
    Commented May 17, 2014 at 10:48
  • \$\begingroup\$ Sorry for the questions I have basic understanding of circuit theory but its those confusing circuits that i get once in a while that confuse me greatly \$\endgroup\$
    – user42926
    Commented May 17, 2014 at 10:49

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