Force on Charged Particle in Magnetic Field – Complete Notes, Revision, Important Questions & Downloads
Force on Charged Particle in Magnetic Field starts with Lorentz Force and Trajectory, where the magnetic part of the force fixes whether motion is straight, circular, or helical, and then moves to Velocity Selector and Cyclotron, where the same force law becomes a device-level application. NEET uses this topic for direct formula questions on F = qvB sin theta, for geometry-based trajectory questions using r = mv/qB, and for selector or cyclotron conditions such as v = E/B and nu = qB/2pi m. A standard example is a proton entering a uniform magnetic field perpendicularly: the force changes only the direction of motion, so the path becomes circular while kinetic energy stays unchanged. Hall effect is usually tested as a linked application of transverse force on charge carriers rather than as an isolated definition.
NEET Weightage & Exam Pattern
Magnetic Effect of Current| NEET Year | Questions from this Topic | Bar | Marks |
|---|---|---|---|
| 2024 | 1 | 4 | |
| 2023 | 2 | 8 | |
| 2022 | 1 | 4 | |
| 2021 | 2 | 8 | |
| 2020 | 1 | 4 | |
| Topic Weightage | 7 | 28 |
Cyclotron frequency and velocity-selector speed are standard one-step formulas, but the trap is deciding which force balance is active in the given setup.
Hall effect questions are usually application-based, asking what transverse emf reveals about charge carriers rather than asking for a memorised definition alone.
Preparation Strategy
Split the Motion by Angle First Before writing any formula, decide whether theta is 0 degree, 90 degree, or an intermediate value. That single geometry check tells you whether the path is straight, circular, or helical and prevents mixing r = mv/qB into a case where no magnetic deflection exists.
Use Perpendicular Force to Protect Energy Logic Memorise that magnetic force is always perpendicular to velocity, so in a pure magnetic field the speed, momentum magnitude, and kinetic energy stay unchanged. The trap is to think the particle speeds up because it is experiencing a force.
Keep Selector and Cyclotron Conditions Separate In a velocity selector, electric and magnetic forces cancel so qE = qvB and only one speed passes undeviated. In a cyclotron, the magnetic field bends the orbit while the alternating electric field accelerates the particle across the gap; do not merge these two devices into one formula set.
Assign Direction After Fixing the Charge Sign Find the direction for a positive charge first using the vector product or Fleming's left-hand convention, then reverse it for a negative charge. Many NEET errors come from using the correct rule with the wrong sign convention.
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Quick Revision
Concept → Trap → Example1) Lorentz Force and Trajectory
Motion GeometryMagnetic force on a moving charge is F = q(v x B), with magnitude qvB sin theta. In a pure magnetic field this force stays perpendicular to velocity, so it changes direction of motion but does not change speed or kinetic energy.
- If velocity is parallel or antiparallel to the field, magnetic force is zero and the particle keeps a straight-line path.
- If velocity is perpendicular to the field, magnetic force acts as centripetal force and gives r = mv/qB and T = 2pi m/qB.
- Trap: for an oblique entry, do not use the full speed in the radius formula; only the component perpendicular to B sets the circular part, while the parallel component builds the pitch of the helix.
2) Velocity Selector and Cyclotron
Device ApplicationIn crossed electric and magnetic fields, a charged particle passes undeflected only when qE = qvB, so the selected speed is v = E/B. In a cyclotron, the magnetic field bends the orbit while the alternating electric field accelerates the positive ion, giving nu = qB/2pi m and maximum kinetic energy q squared B squared r0 squared divided by 2m.
- Cyclotron frequency depends on charge, magnetic field, and mass, but not on the speed or radius of the orbit in the non-relativistic treatment used for NEET.
- Hall effect is the transverse emf produced when moving charge carriers are deflected sideways by a magnetic field perpendicular to the current.
- Trap: cyclotron does not accelerate neutrons because q = 0, and it is not used for electrons in this school-level treatment because the simple constant-frequency condition breaks down at high speed.
US Curriculum Gaps
Note for NRI/OCI students studying abroad.Cross Product Direction Under Speed Pressure
Many school courses discuss magnetic force conceptually, but NEET expects very fast sign-sensitive direction work for positive versus negative charges and for circular versus helical motion.
- reversing direction for electron motion
- splitting velocity into parallel and perpendicular components
Device-Level Applications from One Force Law
Velocity selector, cyclotron, and Hall effect are often taught as separate devices, while NEET expects them to be solved as direct consequences of the same Lorentz-force framework.
- force balance qE = qvB in crossed fields
- cyclotron frequency independent of orbital speed
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Physics Revision Checklist
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Frequently Asked Questions
Notes · Downloads · Revision · Important QuestionsWhy does a magnetic field not change the speed of a charged particle?
When does the path become a straight line in a magnetic field?
Why is the time period of circular motion independent of speed?
How do I decide the force direction for a negative charge?
What is the physical idea behind a velocity selector?
Why can a cyclotron accelerate only charged particles?
What does Hall effect tell us in exam problems?
What is the most common trap in helical-motion questions?
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