Charged-Particle Motion in a Uniform Field
With constant E and no other force, a = qE/m. Apply constant-acceleration kinematics component-wise, reversing acceleration for negative q.
Why this shows up in the exam
Electron deflection between plates · Trajectory through a finite field region · Time and stopping-distance calculations
Learn the idea
A uniform electric field gives a charged particle constant acceleration qE/m. The motion is the electric analogue of projectile motion: one component can remain uniform while the field changes the parallel velocity linearly.
🧠 Memory hook: Use projectile equations with g replaced by qE/m.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- a = qE/m — constant acceleration in a uniform field
- v = u + at — velocity component along constant acceleration
- r = r₀ + u t + (1/2) a t² — trajectory components
How to approach it
- 1Set axes parallel and perpendicular to E
- 2Find time from the unaffected motion when possible
- 3Update velocity and position only in the field region
Common slip-ups that cost marks
- •Using |q| and losing the acceleration direction
- •Applying field acceleration after the particle exits
- •Mixing horizontal transit time with total speed
🌟 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.
Two point charges 1 microC and 2 microC are 1 m apart in vacuum. Take k = 9 x 10^9 SI. Find the force magnitude.
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