Physics · Moving Charges And Magnetism · NEET
Doing work and applying a force are different things. The magnetic force is a real force, so it can change the direction of velocity, which is what circular motion needs. Work is only done when a force has a component along the direction of motion. Since F = qv x B stays perpendicular to v, it bends the path without ever adding or removing kinetic energy. So it curves you, but your speed stays the same.
It changes velocity but not speed. Velocity is a vector (size + direction). The magnetic force changes only the direction of v, never its magnitude. So |v| (speed) is constant, but the direction keeps turning. That is why the charge moves in a circle or helix at constant speed.
By the rule of the cross product, v x B is always perpendicular to both v and B. So the magnetic force F = qv x B is perpendicular to v no matter what angle v makes with B. Because the force is always at 90 degrees to the motion, the power delivered P = F.v = 0 at every instant.
No. Work-energy theorem says change in kinetic energy = work done. The magnetic force does zero work, so kinetic energy is unchanged, which means speed is unchanged. If you ever see a problem where a charge speeds up or slows down, an electric field (or some other force) must be doing that work, not the magnetic field.
Then the angle between v and B is 0 degrees, so v x B = 0 and the magnetic force is zero. The charge moves in a straight line with no deflection. This case appears often in NEET: a charge fired along B feels no magnetic force at all.
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 the fields, then the electron
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Because F = qv x B is always perpendicular to the velocity v. Power delivered is P = F.v = 0 at every instant, so no work is done and kinetic energy stays constant.
No. It only changes the direction of velocity. Speed and kinetic energy remain constant because the force is always at 90 degrees to the motion.
The perpendicular magnetic force acts as the centripetal force. It continuously turns the velocity without changing its magnitude, giving circular motion at constant speed.
Yes, an electric field can, because the electric force qE can have a component along the velocity. The magnetic force alone never does.
The angle is 0 degrees, so v x B = 0 and the magnetic force is zero. The charge moves straight with no deflection.
