Mistake Master
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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 Lab · build a charging procedure and watch the ledger refuse to invent charge

Charging is transfer, not creation. Friction moves electrons between two neutral objects and leaves them oppositely charged; contact shares charge between conductors and leaves them with the same sign; induction never touches the charged object at all and, with a grounding step, leaves the conductor with the sign opposite to it. In every case the total charge of the isolated system before equals the total after. Whether the charge can move once delivered is a property of the material: a conductor redistributes it over the outer surface, an insulator holds it in the patch where it landed.

Two errors run through the whole topic. The first is describing a process as making charge: both objects coming out negative from a neutral start, or a grounded conductor having its charge destroyed rather than conducted away. Total the charge before and after and the story either balances or it is wrong. The second is letting charge flow across an insulator, which turns a rubbed patch on a plastic rod into a uniform coating, or lets a single grounded touch neutralize the whole rod. Naming the material first settles both.

contact between IDENTICAL conductors: the total is split, not changed +8 nC −2 nC before: total = +6 nC touch, then separate +3 nC +3 nC after: total = +6 nC (+8) + (−2) = +6, and +6 split two ways is +3 each NOT 0 and 0 (that destroys +6 nC) and NOT +6 and +6 (that mints another +6)
The only arithmetic contact charging ever needs: add the charges, then divide by two if the conductors are identical. The total on the last line has to match the total on the first.
induction: nothing ever touches the rod, and the ground comes off FIRST −−− ++ −− 1. near: polarized net q = 0 −−− ++ e− leave 2. ground it −−− ++ 3. ground OFF first + + + + + 4. rod away net q > 0 the sphere ends OPPOSITE in sign to the rod, and the rod never lost any charge reverse steps 3 and 4 and the electrons flow straight back: the sphere ends neutral the ground is part of the system: the charge that left is on the earth, not gone
Induction is the one process whose outcome depends on the ORDER of two steps. Removing the rod before the ground undoes everything the ground accomplished.

The work

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

Balances the charge books for friction, contact and induction, fixes the four-step induction sequence including which connection comes off first, and separates what a conductor does from what an insulator cannot.

Skill check · 10 scenarios
Diagnostic
10-item topic check

Ten items spanning the failure modes of this topic: describing a charging process as creating charge, giving two rubbed objects the same sign, letting charge migrate across an insulator, and grounding an insulator as though it were metal. Take it cold to find which one is yours, or after the lesson to confirm it is not.

Not started · 10 items · ~15 min
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