Charge Quantization, Conservation, and Mass
For ordinary charging, q = ne with integer n. Total charge of an isolated system is constant, while a body's mass changes by the transferred electron mass when that effect is asked.
Why this shows up in the exam
Counting transferred electrons · Checking possible charge values · Comparing masses after charging
Learn the idea
Net charge changes in integral multiples of e and is conserved in an isolated system. Charging moves electrons rather than creating charge. Adding electrons makes a body more negative and very slightly heavier; removing them does the reverse.
🧠 Memory hook: Charge comes in e-sized steps; electrons carry both charge and mass.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- q = n e — n is an integer and e is the elementary-charge magnitude
- Delta m = N m_e — mass change from N transferred electrons, with sign handled physically
How to approach it
- 1Identify the carrier transfer
- 2Use N = |q|/e
- 3Apply conservation and the correct mass-change direction
Common slip-ups that cost marks
- •Using a non-integer electron count
- •Treating proton transfer as ordinary metallic charging
- •Forgetting whether electrons were added or removed
🌟 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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