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CED objectives

Conservation of Electric Charge and the Process of Charging

▶︎  Watch it animatedinteractive step-through · ~3 min · optional ⚙︎  Open the appletCharging Bench · give the wool and the earth a row of their own and watch the total refuse to move, then pull in a scrap that has no net charge at all

Charging processes move electrons between systems and never create or destroy charge, so every transfer has two sides that sum to what they started at. Friction moves electrons between two materials, contact spreads excess charge over the objects in contact and splits it evenly between identical conductors, and grounding connects an object to a large neutral reservoir so electrons can flow either way through the wire. Induction charges an object without touching it, by polarizing it with a nearby charge, grounding it while that charge is still in place, then removing the ground before the charge, which leaves the object with the opposite sign.

Two errors dominate. Describing a charging process as making or destroying charge, so the totals before and after do not match and a grounded object is said to have had its charge annihilated. And ruling out any force between a charged object and a neutral one, when polarization brings the opposite sign closer and the like sign farther away, so attraction wins on the $1/r^2$ difference. The same gap makes students report that induced separation changed the object's net charge, when only the arrangement moved.

induction: the order of the last two steps is the whole trick + − + 1. polarized net charge 0 + − − 2. grounded: electrons flow UP into the sphere + − − 3. ground removed FIRST, rod still there − − − 4. rod removed: sphere is permanently negative remove the rod before the ground wire and the electrons drain straight back out the rod ends with exactly the charge it started with: it was never touched result carries the sign OPPOSITE the charging rod
Step 2 is what changes the sphere's total. Skip it and the sequence is polarization, which undoes itself the moment the rod leaves.
net charge zero, net force not zero + − − + + near side far side attraction: closer, so stronger repulsion: farther, so weaker 1/r² decides the contest, and the near side always wins it, so the pull is always toward the rod
Both sides of the neutral object respond. The near one is closer, and inverse square makes that the whole story.

The work

3 ways in · any order
Lesson
Conservation of Electric Charge and the Process of Charging ›

Balances the charge books across friction, contact, grounding and induction, and shows why polarization pulls a neutral object toward a charged one without changing its net charge.

Skill check · 10 scenarios
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Diagnostic
10-item topic check ›

Ten items spanning the failure modes of this topic: describing a charging process as creating or destroying charge, losing track of the other side of a transfer, and ruling out a force on a neutral object. Take it cold to find which one is yours, or after the lesson to confirm it is not.

Not started · 10 items · ~15 min
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Targeted Practice
Drill a single misconception ›

Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears it for now and moves you to the next. Two in a row is a checkpoint, not proof: if the error resurfaces later, the misconception comes back.

Take the diagnostic to identify your misconceptions
Diagnostic first