OpenStax College Physics for AP® Courses, Chapter 22, Problem 32 (Problems & Exercises)

This is College Physics Answers with Shaun Dychko. So we imagine in parts (a), (b) and (c) of this question that there is a current carrying wire in the presence of a magnetic field, which is indicated by dots coming out of the page in part (a), and that wire experiences a force upwards so what direction must the current be in? Now we take our right hand and point all the many fingers in the direction of these many magnetic field lines— there are many of each and that's how you remember that many fingers point in the direction of the many magnetic field lines. See fingers are pointing out of the page, your palm is what you use to push things and so it is the force, the palm is pointing in the direction of force, which is... it's upwards and so what direction is the thumb pointing that's gonna be the answer to our question because the thumb will point in the direction of current in this current carrying wire; the thumb will be pointing to the left and so the current is to the left. And then in part (b), we have all the fingers pointing to the right; our palm is still pointing upwards and our thumb is now pointing out of the page so there must be a wire coming out of the page so we draw a dot which indicates—like an arrowhead pointing towards us— this is the direction of current in the wire. In part (c), we have our many fingers pointing into the page, our palm is pushing to the left and our thumb is giving us the answer that the direction of the current is upwards so the wire must be going this way. So we can say left is our answer for part (a), out of the page is the answer for part (b) and upwards is the answer for part (c).