It is given in my book. hat F=B I l sin theta where theta is the angle between B and the current I... I guess this is no right.. I mean, how would the current make an angle with B, it should b the length of wire itself... Secondly F=Bqv is the reason behind a charged particle which enter perpendicularly to a magnetic field moves in a circle.. My doubt is that at point, this point charge will be inline with the field line.. From where will it get force at that point.. Talking about the quarter circle... I need more clarification and it would be more than amazing if there are some illustrative diagrams!