Magnetic Force Does No Work
Because q(v cross B) is perpendicular to v, the instantaneous magnetic power F dot v is zero. In a purely magnetic field, kinetic energy and speed are constant although momentum direction may change.
Why this shows up in the exam
Checking energy conservation in magnetic deflection · Separating speed change from direction change · Rejecting impossible magnetic acceleration claims
Learn the idea
A magnetic force can bend a path but cannot change a particle's speed or kinetic energy. The magnetic push is always sideways to the instantaneous motion, like steering without pressing an accelerator. Direction can change continuously even while speed and kinetic energy remain fixed.
🧠 Memory hook: Magnetism steers; it does not speed up.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- P_B = F_B dot v = 0 — magnetic power is zero because force is perpendicular to velocity
- Delta K = 0 — kinetic energy is unchanged when no electric or other work-producing force acts
How to approach it
- 1List all fields and forces
- 2If only magnetic force acts, keep speed fixed
- 3Use geometry or dynamics only for the changing direction
Common slip-ups that cost marks
- •Concluding that zero work means zero force
- •Holding momentum vector constant instead of only its magnitude
- •Using this result when an electric field is also doing work
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A charge of 2 microC moves perpendicular to a 3 T magnetic field at 4 x 10^5 m/s. Find the magnetic force.
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