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schematic

simulate this circuit – Schematic created using CircuitLab

By calculating the equivalent Norton circuit between a and b, I first calculated \$R_{eq}\$ and after \$i_{eq}\$.

When I calculated \$i_{eq}\$, I short-circuited the terminals a and b, and so the circuit becomes:

schematic

simulate this circuit

So \$i_{eq}=-\frac{E}{R_1}\$, but the solution gives \$\frac{E}{R_1}\$. Is it a typo?

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  • \$\begingroup\$ Is the arrow representing I on the original problem? If you should consider the current between a and b its value is E/R1. If you should consider the current at the direction of arrow, then it's -E/R1. To avoid misunderstood, represent the direction of current by an arrow. \$\endgroup\$ – Pedro Quadros Jun 9 '15 at 18:57
  • \$\begingroup\$ This is a common thing that people get hung up on in early courses. If you say that current going into something is -X amps and the answer says current coming out of something is X amps, they're the same thing. PS - Look at the current source. It shows current flowing up and then down from a to b. It would make sense then that current is positive from a to b and negative from b to a (hence why you got negative answer). \$\endgroup\$ – I. Wolfe Jun 9 '15 at 21:00
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The difference in the signs of the solutions lies in the directions chosen for the current. So, the answers are equivalent, the current is just flowing in the opposite direction to which you have selected.

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  • \$\begingroup\$ Yes, but who decides what's the right direction? I reviewed the theory part and I think my result (with minus sign) is the right one. \$\endgroup\$ – sl34x Jun 9 '15 at 18:53
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    \$\begingroup\$ @sleax The direction chosen is arbitrary and doesn't matter. Think of what the two answers means: one says the current flows up with negative current, the other says it flows down with positive current. Two ways of saying the same thing. \$\endgroup\$ – helloworld922 Jun 9 '15 at 20:20
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    \$\begingroup\$ @sleax - books and profs will almost always choose the direction that gives a positive current. Just think about it logically. Why would you say "There are negative 2 bucks going into my piggy bank" when you can say "I took 2 bucks out of my piggy bank". It also just makes more sense to avoid negative signs when you can. I can't count the number of times a negative sign was the difference between me getting a problem right vs wrong. Not a huge issue with simple problems like this one, but on complex problems negative signs get tedious to carry around. \$\endgroup\$ – I. Wolfe Jun 9 '15 at 21:05

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