Mechanical Equilibrium of Charged Bodies
Static equilibrium requires vector sum F = 0. Resolve tension along convenient axes and calculate electric force from the actual field or other charges.
Why this shows up in the exam
Charged pendulums · Charges on inclined planes · Weightless charged balls
Learn the idea
At rest, electric force must balance weight, tension, normal force, and any other real forces. A suspended charged ball is just a force-balance problem with an electric force added. Geometry of the string converts force ratios into angle relations.
🧠 Memory hook: Draw the free-body diagram before writing Coulomb's law.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- sum F_x = 0 — horizontal equilibrium
- sum F_y = 0 — vertical equilibrium
- tan theta = F_e/(effective weight) — common suspended-body relation when axes fit
How to approach it
- 1Isolate one body
- 2Draw and resolve every force
- 3Use geometry only after equilibrium equations
Common slip-ups that cost marks
- •Balancing magnitudes that are not collinear
- •Using the full string angle instead of half-angle
- •Omitting buoyancy or a normal force
🌟 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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