Coulomb Force Between Point Charges
For stationary point charges in a homogeneous medium, the force magnitude is k|q_1q_2|/r^2 with k = 1/(4 pi epsilon); its direction lies on the separation vector.
Why this shows up in the exam
Two-ion force · Force changes with distance · Force in a dielectric medium
Learn the idea
Two point charges exert equal and opposite inverse-square forces along their joining line. Doubling separation makes the force one fourth, while changing either charge scales the force directly. The sign decides attraction or repulsion.
🧠 Memory hook: Multiply charges, divide by distance squared, then set the direction.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- F = k |q₁ q₂|/r² — magnitude for point charges
- k = 1/(4 pi epsilon) — medium-dependent Coulomb constant
How to approach it
- 1Convert all data to SI
- 2Compute the magnitude
- 3State attraction or repulsion separately
Common slip-ups that cost marks
- •Using r instead of r squared
- •Putting charge signs into a magnitude
- •Using epsilon₀ inside a dielectric without adjustment
🌟 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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