# Direction of Magnetic field caused by straight current-carrying conductor

The question: Two crisscrossing insulated wires contain currents as displayed. Find the total mag field at points P and Q, assuming the horiz current moves (a) right and (b) left. QUESTION is for part a.

I assumed going out of the page = + and going into the page = - (since it doesn't matter). The answer says that at point P, the field caused by the 12A current wire is into the page, while at point Q, the field is going out of the page.

I am having trouble understanding this. I know the RH rule for figuring out the direction of the field (thumb in direction of I, fingers curl in direction of B), but I have trouble applying it to points. I don't understand what the difference is at point Q and point P for the directions. It would be great if someone can clearly clarify for me, preferably using the RH rule!

• Commented May 21, 2020 at 14:04
• I’m voting to close this question because (beyond of being a homework-like question) there is no interest from any user and especially by the OP, a comet that appeared for a while and disappeared from the PSE planet. Commented Feb 5, 2022 at 8:56

First, recall that the field goes like $1/d$ with $d$ the distance from the wire. Using the RHR, the field for the vertical wire will be into the page at P (your finger curl into the page at that point) and out of the page at Q (your finger curl out the page a that point).