Separate charge first; disconnect ground at the right time
You will be able to: Explain polarization and the sequence of grounded induction.
How can a conductor be charged without touching the nearby charged rod?
Bring a negative rod near a neutral metal sphere. Electrons in the sphere move away from the rod. The sphere is still neutral until a conducting path lets some electrons leave.
A useful starting point: Track the electrons across the system boundary →
Words and symbols before equations
- Induction
- Redistributing charge in response to a nearby charged object.
- Ground
- A connection to a much larger charge reservoir such as Earth.
- Grounded induction
- Charging by allowing charge exchange with ground while an inducing object is nearby.
- Electrostatic equilibrium
- A state with no continuing bulk motion of charge under a static field.
What this picture assumes
Qualitative grounded-induction sequence with a fixed illustrative 2 nC transfer. The magnitude is not calculated from rod geometry. Stage order disconnects ground before removing the rod. Earth is part of the charge-conservation ledger.
Read the picture in three steps
- Locate the labeled sources, system boundary or graph axes. Read the units before comparing values.
- Rod approaches: polarization only. Sphere 0 nC; Earth change 0 nC. Charge is separated while the rod is nearby.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the physics
Step 1: approach with a negative rod without contact. The sphere’s electrons redistribute; the near side is relatively positive and the far side relatively negative. This polarization alone leaves the total unchanged.
Step 2: connect ground while holding the rod nearby. Electrons can leave the sphere. Step 3: disconnect the ground while the rod remains. The remaining positive net charge is now trapped on the isolated sphere.
Step 4: remove the rod. The sphere’s positive excess redistributes. If the rod is removed before the ground connection, a route remains for neutralization. For a positive rod all charge-transfer signs reverse. The sphere-plus-Earth system conserves charge throughout.
A worked example, step by step
A negative rod induces a grounded sphere to lose 2 nC of electron charge. Ground is disconnected first and then the rod removed. Describe the sphere and Earth.
- The initial approach polarizes the sphere but does not change its net charge.
- During grounding the sphere loses negative charge of magnitude 2 nC.
- After disconnecting ground, the sphere has +2 nC; removing the rod does not provide a path for charge transfer.
- Earth gained −2 nC. Sphere plus Earth still has the same total charge as before.
Induced separation is not the same as net charging. A charge-transfer path and the removal order matter.
Does a nearby negative rod give a neutral isolated sphere a positive net charge?
Compare with an explanation
Not by approach alone. It polarizes the sphere; net charge requires transfer.
Predict. Change one thing. Explain.
Advance the stage control through approach, grounding, ground removal and rod removal. Reverse the rod sign and explain the reversed electron flow. Ledger values are illustrative, not a solved geometry.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Rod approaches: polarization only. Sphere 0 nC; Earth change 0 nC. Charge is separated while the rod is nearby.
Qualitative grounded-induction sequence with a fixed illustrative 2 nC transfer. The magnitude is not calculated from rod geometry. Stage order disconnects ground before removing the rod. Earth is part of the charge-conservation ledger.
Explain what you noticed: Which quantity changed? Which stayed fixed? Use the relevant charge, vector superposition, electric field, flux or symmetry relationship to justify your prediction.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionExplain how a positive rod can leave an initially neutral metal sphere negatively charged without touching it. Account for charge in Earth.
This response is not submitted or saved. Copy it before leaving.
Compare with the answer and four-point rubric
- 1 point: Approach with the positive rod to polarize the neutral sphere.
- 1 point: Ground the sphere; electrons flow from Earth into it.
- 1 point: Disconnect ground while keeping the rod in place, then remove the rod.
- 1 point: The sphere retains negative charge; Earth has the corresponding positive change, conserving total charge.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1Does polarization require grounding?
No. Neutral isolated objects can polarize.
RECALL 2Why disconnect ground first?
To prevent further charge exchange when the rod is removed.
RECALL 3Is the model’s amount of transferred charge universal?
No. It depends on geometry and environment; the ledger uses illustrative values.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Separate charge first; disconnect ground at the right time
- Isolated polarization: ΔQ_net = 0.
- Grounding allows charge to cross the sphere’s boundary.
- Disconnect ground before removing the inducing rod to retain charge.
Remember: Induced separation is not the same as net charging. A charge-transfer path and the removal order matter.
Conditions: Qualitative grounded-induction sequence with a fixed illustrative 2 nC transfer. The magnitude is not calculated from rod geometry. Stage order disconnects ground before removing the rod. Earth is part of the charge-conservation ledger.
Refresh Kid · AP Physics C: Electricity and Magnetism Unit 1 (official Unit 8) · Objectives 8.2.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 8.2, objectives 8.2.A. CED effective Fall 2024, current PDF ©2026; checked September 16, 2026. This is E&M Unit 1: Electric Charges, Fields, and Gauss’s Law, numbered Unit 8 in the official combined Physics C sequence. Topics 8.1–8.6 retain their official identifiers. Quantitative force examples use at most four point charges. Field integrals use the specified rods, ring, arc and infinite wire; Gauss-law field calculations use spherical, cylindrical or planar symmetry. Optional projected 3D views clarify area normals and geometry; camera rotation never changes the physics. Checked with the Fall 2026 clarifications. The lesson breakdown and questions are original Refresh Kid work, not official topic subdivisions.
Implementation and automated checks are separate from independent teacher review and observation of students. Both human review stages remain pending. This is a review edition, not a certified or validated assessment.
Optional further resource: College Board’s released questions and scoring guides. Papers can combine units; this link is an archive, not an assignment of every question to this lesson.
Our learn, explore, practice and recall sequence is informed by the IES learning guide. The exact Refresh Kid implementation has not been evaluated for learning effectiveness.
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