Detect electrostatic charge with an electroscope
Explain leaf divergence, distinguish non-contact redistribution from contact transfer, and use a known reference when identifying sign.
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall like-charge repulsion.
- Distinguish electron movement inside an object from transfer across its boundary.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Why can two like-charged leaves separate?
What you will learn
- Describe simple electroscope charge detection.
- Explain why initial state and contact determine the interpretation.
Approach without touching
A leaf electroscope connects a conducting cap and stem to light conducting leaves, held on an insulating support. Begin with a neutral detector and closed leaves. Bring a charged rod near the cap without touching it. The detector's electrons redistribute; the leaves acquire like charge and repel, so they diverge.
For a positive nearby rod, electrons move toward the cap, leaving the leaves relatively positive. For a negative rod, electrons are pushed away from the cap toward the leaves, which become negative. In either ideal isolated non-contact case, the detector's total charge remains zero while its regions have different balances.
Control the initial condition
Remove the rod: an initially neutral isolated detector can return to its original distribution and the leaves close. Touching the cap with a charged object is a different procedure that can transfer charge and leave persistent divergence after removal. Do not treat every observation of divergence as evidence of net charge transferred to the detector.
Divergence of initially neutral leaves detects a response but does not determine an unknown sign. A known precharged electroscope can help compare a weak approaching charge under controlled conditions, but large approach can redistribute enough charge to make a naive leaf rule ambiguous. A clear repulsion test against a known charge is a simpler sign reference. Humidity, leakage and detector sensitivity remain practical limitations.
Sources for this lesson
Pause and explain
A rod approaches a neutral isolated cap without contact. Must the detector gain net charge?
Worked example
A positive rod approaches an initially neutral isolated electroscope without contact. Explain the cap and leaf responses.
Show the worked solution
- Electrons are attracted toward the cap.
- The leaves are left with an electron deficit and like positive charge.
- They repel while the whole detector remains net neutral.
Answer Cap becomes relatively negative; both leaves relatively positive and divergent; total detector charge stays zero.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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.
Describe how a neutral electroscope can detect a nearby charged rod.
Give me a hint
State the initial leaves, approach method and observed response.
Compare my reasoning
- Begin with closed leaves on a neutral isolated detector.
- Approach the cap with the rod without touching it.
- A charge response can redistribute electrons and cause leaf divergence.
Non-contact approach produces like-charge leaves and divergence in a responding detector.
Look for these in your work
- Describe the observation.
- Keep contact separate.
For separate positive-rod and negative-rod approaches, give the leaf signs in the ideal neutral detector.
Give me a hint
Track whether electrons move toward or away from the cap.
Compare my reasoning
- A positive rod draws electrons toward the cap, leaving positive leaves.
- A negative rod pushes electrons toward the leaves, making them negative.
- Both like-sign leaf pairs repel.
Positive rod: positive leaves; negative rod: negative leaves; both can diverge.
Look for these in your work
- State both cases.
- Explain the electron redistribution.
Leaves remain divergent after a rod touches the cap and is removed. Is this the same evidence as a non-contact approach?
Give me a hint
Check whether charge could cross the detector boundary.
Compare my reasoning
- Contact permits charge transfer.
- Retained net charge can sustain leaf divergence after removal.
- Non-contact divergence alone need not change the total charge.
No; contact can leave transferred net charge, whereas non-contact redistribution alone need not.
Look for these in your work
- Name the changed boundary condition.
- Avoid identifying the unknown sign from persistence alone.
Common mistakes
- Initial neutrality is a stated condition.
- Non-contact redistribution is not net transfer.
- Leaf divergence alone does not reveal an unknown sign.
What can you explain now?
Can both positive and negative rods make initially neutral leaves diverge?
Compare with the explanation
Yes.
Each can produce like charges on the two leaves, with opposite signs in the two trials.
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Make it stick
Explain detection using electron redistribution and separate charge sign, net transfer and leaf divergence.
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.