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Taken from here, 3rd problem.

As you can see, no matter from which point current goes, from a to b or from b to a, in either cases, magnetic field would be down, right?

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2 Answers 2


You can get the direction of the field without actually drawing it. The magnetic field of the current through the resistor is not just up or down. The field lines go in a circle around the resistor. You can use the right-hand rule to visualize which the way the lines go around, either clockwise or counterclockwise. If the current flows from $b$ to $a$, the lines will go around the resistor clockwise looking from $b$ toward $a$.

You can also get the direction of the magnetic field from the Biot-Savart law, which also requires application of the right-hand rule.


Use the right hand rule. Point your thumb in the direction of the current and your fingers will curl around the wire in the direction of the magnetic field.

For current flowing from a to b: Your thumb points down and to the right. Your fingers will be to the left of the loop pointing downward. If you curl them around you will see that the magnetic field points UP inside the loop and DOWN outside of the loop.

For current flowing from b to a: Your thumb points up and to the left. Your fingers will be inside the loop pointing downward. The magnetic field points DOWN inside the loop and UP outside of the loop.

You can do this at any point in the loop, not just the resistor and you will get the same result.


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