Charge by induction: the order matters
You will be able to: Explain induction with grounding and track charge across the system boundary.
Can a conductor gain charge without touching the charged rod?
Bring a negative rod close to a neutral conductor without touching it. Electrons in the conductor move away from the rod. The conductor is polarized but still neutral—until a path to ground permits charge to leave.
A useful starting point: Track charge through contact →
Words and symbols before equations
- Induction
- Redistribution caused by a nearby charged object; net charging requires a transfer route.
- Ground
- A much larger approximately neutral conductor that can supply or receive charge.
- Ground connection
- Conducting path permitting charge exchange with Earth.
- Sequence
- Keep the rod nearby while disconnecting ground, then remove the rod.
What this picture assumes
Negative rod induces a neutral conducting sphere. Symbols show qualitative charge separation or excess positive charge, not a quantitative surface density. Ground can exchange electrons only in step 2; step 3 retains the rod.
Read the picture in three steps
- Locate the labeled sources, system boundary or graph axes. Read the units before comparing values.
- Step 1: Rod nearby; conductor neutral. Charge separation alone does not change net charge.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the physics
With a negative rod nearby, mobile electrons shift to the farther side. Adding a ground connection can let some electrons leave the conductor. The conductor then has positive net charge because electrons crossed its boundary.
Disconnect ground before removing the rod to trap the positive net charge. After the rod is removed, the remaining charge redistributes over the conductor’s surface. If the rod is removed while ground remains connected, electrons can return and the intended net charge may disappear.
A positive rod reverses the electron-transfer direction. Grounding does not always mean removing electrons, nor does an object need to touch the rod. In all cases include Earth if you want the larger isolated system’s charge account.
A worked example, step by step
A conductor near a negative rod loses 2.0×10¹⁰ electrons through ground. Ground is disconnected first. Find its net charge and describe removal of the rod.
- Each lost electron removes charge −1.60×10⁻¹⁹ C.
- The conductor’s charge becomes +(2.0×10¹⁰)(1.60×10⁻¹⁹)=+3.2 nC.
- Disconnecting ground prevents further charge exchange.
- Removing the rod lets charge spread, but net +3.2 nC remains.
Polarization alone leaves a neutral object neutral. Disconnecting the ground at the wrong time changes the outcome.
What if the rod never touches and there is no ground path?
Compare with an explanation
A neutral isolated conductor polarizes but keeps zero net charge.
Predict. Change one thing. Explain.
Step through rod approach, grounding, ground removal, and rod removal. At each step identify whether electrons can cross the conductor’s boundary. The charge pattern is schematic; its density is not quantitative.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Step 1: Rod nearby; conductor neutral. Charge separation alone does not change net charge.
Negative rod induces a neutral conducting sphere. Symbols show qualitative charge separation or excess positive charge, not a quantitative surface density. Ground can exchange electrons only in step 2; step 3 retains the rod.
Explain what you noticed: Which quantity changed? Which stayed fixed? Use the relevant charge, field or energy 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 questionA positive rod is brought near a grounded conductor. (a) Predict electron flow. (b) State the conductor’s resulting net sign. (c) Give the removal order. (d) Explain how total charge is conserved.
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Compare with the answer and four-point rubric
- 1 point: Electrons can enter from Earth.
- 1 point: The conductor becomes negative.
- 1 point: Disconnect ground, then remove the rod.
- 1 point: Earth loses the negative charge gained by the conductor; the larger system’s charge is conserved.
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 induction require rod contact?
No.
RECALL 2Does grounding always remove electrons?
No. Direction depends on the surrounding charge configuration.
RECALL 3Why disconnect ground first?
To prevent further exchange when the inducing rod is removed.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Charge by induction: the order matters
- Electron transfer across a boundary changes net charge.
- Negative inducing rod: electrons can leave through ground.
- Disconnect ground before removing rod to retain induced charge.
Remember: Polarization alone leaves a neutral object neutral. Disconnecting the ground at the wrong time changes the outcome.
Conditions: Negative rod induces a neutral conducting sphere. Symbols show qualitative charge separation or excess positive charge, not a quantitative surface density. Ground can exchange electrons only in step 2; step 3 retains the rod.
Refresh Kid · AP Physics 2 Unit 2 (official Unit 10) · Objectives 10.2.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 10.2, objectives 10.2.A. CED effective Fall 2024, current PDF ©2026; checked September 16, 2026. Refresh Kid calls this the second AP Physics 2 unit; College Board numbers it Unit 10; the first unit in this course is official Unit 9. 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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