Physics · Motion In A Plane · NEET
The full magnitude is F = qvB sin theta, where theta is the angle between v and B. You only get the short form F = qvB when v and B are perpendicular (theta = 90 degrees, sin 90 = 1). NEET options often hide the sin theta, so always write it and then plug in the angle.
Because F = q(v x B) is a cross product, and a cross product is by definition perpendicular to both vectors that make it. So F is perpendicular to v (and also to B). Since force is always at 90 degrees to motion, the charge turns but its speed does not change.
No. Work = F . displacement, and the force is perpendicular to the velocity, so the dot product with the path is zero. The magnetic force changes only the direction of v, not its magnitude. This is why a charge in a uniform field moves in a circle at constant speed.
Use the right-hand rule for v x B: point your fingers along v, curl them toward B, and your thumb points along v x B. Multiply by the sign of q. For a positive charge F is along v x B; for a negative charge F is opposite to v x B.
Then theta = 0 or 180 degrees, sin theta = 0, so F = 0. A charge moving straight along the field lines feels no magnetic force and goes in a straight line. Maximum force happens when v is perpendicular to B.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the total force on a charge from electric and magnetic fields: F = qE + q(v x B). In this concept we focus on the magnetic part, F = q(v x B).
From F = qvB, B = F / (qv), which gives the tesla (T). One tesla = one newton per (coulomb x metre/second) = 1 N/(A.m).
It is a vector equation. Both v and B are vectors, their cross product v x B is a vector, and multiplying by charge q keeps it a vector. The force has both magnitude qvB sin theta and a definite direction.
Because the cross product is taught here as a vector operation, and F = qv x B is the standard NEET example of it. Learning it now makes Moving Charges and Magnetism in Class 12 much easier.
