Conserve charge during isolated electron transfer
Keep the combined charge constant while calculating each object's change from a stated electron transfer.
Section P4.1 · Electrical quantities
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Track negative electron units.
- Add signed initial charges.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
A solid loses one electron. What change occurs in its net charge?
What you will learn
- Account for equal and opposite charge changes in an isolated pair.
- Distinguish total charge conservation from each object's individual charge.
Choose the system boundary
For an isolated pair, every electron leaving A arrives at B. A's net charge increases by one elementary unit per transferred electron and B's decreases by the same amount. The individual charges change, but their sum stays constant. Rubbing can separate charge; it does not create a net charge for the isolated pair.
If both objects begin neutral, equal and opposite final charges follow. If they begin charged, equal and opposite changes still occur but the final charges need not be equal and opposite. For A initially +2 units and B initially −1, a transfer of three electrons A to B leaves +5 and −4, whose sum is still +1.
Check isolation before applying the pair result
A connection to a person, conducting support or Earth can let electrons cross the chosen two-object boundary. The pair alone may then change total charge. Charge conservation is retained when the larger system and all transfers are accounted for; the error is omitting a connected reservoir from the account.
Use the sign of the transferred charge consistently. Five electrons moving A to B cause a +5 change for A and a −5 change for B. Do not add a negative change to both objects or substitute a mechanical-energy calculation. The elementary-unit count is supplied here; no current rate or electrical potential is inferred.
Sources for this lesson
Pause and explain
In an isolated A–B pair, four electrons move A to B. What are the charge changes?
Worked example
A starts at +2 units and B at −1 unit. Three electrons move A to B in an isolated pair. Find both final charges and the total.
Show the worked solution
- A loses three negative units: +2 + 3 = +5.
- B gains three negative units: −1 − 3 = −4.
- The total is +5 − 4 = +1, matching +2 − 1.
Answer A +5 units, B −4 units; total +1 elementary charge unit.
Try a topic test
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Initially neutral A and B exchange five electrons from A to B. Find final charges and total.
Give me a hint
Apply opposite changes to the initially zero individual values.
Compare my reasoning
- A loses electrons and changes to +5 units.
- B gains electrons and changes to −5 units.
- The total remains +5 − 5 = 0.
A +5; B −5; total 0 elementary charge units.
Look for these in your work
- Use opposite signed changes.
- Check the final sum against the initial sum.
A begins at −4 units and B at +1. Two electrons move B to A. Find final charges and total.
Give me a hint
Reverse the donor and recipient relative to the earlier example.
Compare my reasoning
- A gains two electrons: −4 − 2 = −6.
- B loses two electrons: +1 + 2 = +3.
- The total remains −6 + 3 = −3 units.
A −6; B +3; total −3 elementary charge units.
Look for these in your work
- Identify B as the donor.
- Preserve the non-zero initial total.
A rod and cloth are rubbed while a conducting connection lets electrons flow to Earth. Can the pair-only total be assumed constant?
Give me a hint
Check what can cross the chosen rod-and-cloth boundary.
Compare my reasoning
- The Earth connection is an external transfer path.
- The pair is not isolated.
- Include the transferred charge and Earth in a complete account.
No; account for the external Earth transfer or use a larger system boundary.
Look for these in your work
- Name the missing reservoir.
- Keep conservation rather than claiming it fails.
Common mistakes
- Conserve the combined charge, not each individual charge.
- Only neutral initial pairs imply equal opposite final values.
- Include any external connection in the boundary.
What can you explain now?
Must a rubbed isolated pair have total charge zero?
Compare with the explanation
Only if its initial total was zero.
Conservation preserves the initial signed total, which may be non-zero.
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Make it stick
Balance equal and opposite electron-transfer changes and apply conservation to the stated system.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.