For years I’ve struggled with the best way to teach force on a moving particle in a field. Tom Balanowski first introduced me to the “slap” method and this year with the help of the 2016-2017 Higher Physics Group we have got it sorted!
Use the Left-hand Slap Rule where the four fingers of the left point in the direction of the magnetic field B and the thumb points in the direction of the moving charge or current, the direction of slapping would be the direction of force F on the conductor.
So how to deal with positive and negative charges. We decided that this method can be used for both. If the charge is negative the slap is the way the palm points. If the charges are positive then use the back of the hand, which is far more Painful
Pain = Positive
Thanks to Amy for these! Note the arrow for the force, should really be coming out of the hand or the way the slap goes; Amy drew this on her hand before I had told her about the × and • -kinaesthetic learner!