Physics · Moving Charges And Magnetism · NEET
Force is F = qvB sin θ. It is ZERO when θ = 0° or 180° (charge moving parallel or anti-parallel to B, sin θ = 0). It is MAXIMUM (F = qvB) when θ = 90° (charge moving perpendicular to B, sin 90° = 1). A stationary charge (v = 0) also feels no magnetic force. This single rule answers most NEET one-liners.
θ is the angle between the velocity vector v and the magnetic field vector B, measured tail-to-tail. It is NOT the angle with any axis or with the force. Always slide the two vectors so their tails meet, then read the angle between them. If v and B are given in i, j, k form, use the cross product instead of guessing the angle.
Because F = qv x B is a cross product, and a cross product is always perpendicular to both vectors that make it. So F ⟂ v and F ⟂ B at every instant. Since F is perpendicular to v, it can only turn the charge, not speed it up or slow it down. That is why magnetic force does no work and speed stays constant.
First find the direction of v x B using the right-hand rule (fingers point along v, curl toward B, thumb gives v x B). For a positive charge, F points along v x B. For a NEGATIVE charge (like an electron), F points OPPOSITE to v x B. Missing the sign flip for electrons is the most common NEET mistake.
No. Magnetic force needs motion: F = qvB sin θ has v in it, so if v = 0 the force is 0. A charge must be moving (and not exactly along B) to feel a magnetic force. This is the key difference from the electric force qE, which acts even on a charge at rest.
A uniform electric field and a uniform magnetic field are acting along the same direction in a certain region. If an electron is projected in the region such that its velocity is pointed along the direction of fields, then the electron
Try the real previous-year questions from this chapter — each with the answer and a full solution.
F = qv x B (vector form), with magnitude F = qvB sin θ, where q is the charge, v the speed, B the field and θ the angle between v and B.
From F = qvB sin θ, B = F/(qv sin θ). Its SI unit is the tesla (T), where 1 T = 1 N per (ampere·metre) = 1 kg per (C·s).
No. Since F is always perpendicular to v, the force does no work, so kinetic energy and speed stay constant. It only changes the direction of motion.
The right-hand rule for v x B: point fingers along v, curl toward B, thumb gives v x B. For a positive charge F is along v x B; for a negative charge it is opposite.
It is the magnetic part. The full Lorentz force is F = qE + qv x B, which adds the electric force qE. When E = 0, only the qv x B magnetic part remains.
