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I do understand that, but it's been put in a context where I can't find φ. An infinite wire carrying current I in the positive y direction generates a field at P, which is a distance a along the x axis. I want to cross y, which is infinite, with x, which is finite but changing. x is the position vector of point P, relative to the infinite wire. I don't know what to use as the angle between them. Is φ=90°? Is sinφ x/(x

^{2}+y

^{2})

^{1/2}, by Pythagoras theorem and the geometry of the situation, or should that be a y on the numerator?

My textbook explains this poorly, and I think it's still more maths than physics, but I'm sorry if I posted in the wrong place.