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CED objectives

Electric Current

▶︎  Watch it animatedinteractive step-through · ~3 min · optional ⚙︎  Open the appletCurrent Lab · an ammeter in every gap of one series loop, then a race between the signal and one drifting electron

Current is the rate at which charge passes a cross-section, $I = \Delta q/\Delta t$, and along an unbroken series path that rate is identical everywhere because charge cannot leave the wire. What an element takes from each passing charge is energy, which appears as a potential difference across it. The carriers are already spread through the circuit before the switch closes, so the battery supplies energy per unit charge rather than a stock of charges, and the response of the circuit is nearly immediate even though the drift is slow. An ideal battery fixes the terminal potential difference, and the network then sets the current through $I = \Delta V/R_{\text{eq}}$.

Three errors dominate. Treating current as fuel that each element burns, so an ammeter after a bulb should read less and the second bulb in a series loop should glow dimmer. Picturing the battery as a tank of charges that have to travel out to the bulb, which predicts a delay before a distant bulb lights and calls a dead battery one that has run out of electrons. And treating the battery as a source of fixed current, so a change to the circuit only redistributes a supply that was never fixed.

two ledgers around one loop 12 V bulb 1 bulb 2 A 0.5 A A 0.5 A same rate everywhere position V drop drop energy per charge falls in steps; charge per second does not fall at all
The two readings are equal and the two steps are not. Whatever the bulbs took, it came off the lower graph, not the upper measurement.
the carriers were already all the way round push here, the whole loop moves drift is slow; the response is immediate a dead battery has run out of chemistry, not of electrons NOT this: a tank emptying toward the bulb it predicts a delay, a hot near wire, and a bulb waiting for delivery
Every prediction on the right is testable, and each one fails. The loop on the left is already full before anything is switched on.

The work

3 ways in · any order
Lesson
Electric Current ›

Keeps charge per second constant around a series loop while energy per charge falls in steps, replaces the battery-as-tank picture, and makes the current a consequence of the network rather than of the source.

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

Ten items spanning the failure modes of this topic: treating current as fuel that elements consume, storing the carriers inside the battery, and holding the current fixed when the circuit changes. 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