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CED objectives

Simple Circuits

▶︎  Watch it animatedinteractive step-through · ~3 min · optional ⚙︎  Open the appletCircuit Builder · series or parallel from the nodes, a loose terminal, an open switch and a plain wire laid across one bulb

Current flows only around a closed conducting loop, so every element needs two connections that lead somewhere and a path traced from one terminal has to return to the other. A short circuit is a zero-resistance path across an element: it puts both of that element's ends at the same potential, so the current through it is zero and the current the battery delivers rises. A gap stops the current in the loops that contain it and leaves any still-closed loop carrying current, which is why a burned-out bulb in a parallel branch leaves its partner lit while one in a series loop does not.

Three errors dominate. Expecting a bulb to light with only one terminal connected, which treats charge as something delivered to the element rather than passed through it. Leaving a bulb dimly lit after a plain wire is connected across it, on the grounds that current still arrives at the junction, and missing that the same change raises the battery current sharply. And declaring the whole circuit dead when a single branch opens, which is the one-chain picture applied to a circuit that offers more than one path.

compare the potentials at the element's two ends lit one closed loop, one current the bulb's two ends sit at different potentials plain wire: 0 Ω DARK both ends are now ONE node at one potential ΔV = 0 across the bulb, so its current is zero and R(eq) fell, so the BATTERY current rose sharply
Current still arrives at the left junction on the right-hand circuit. All of it turns down the resistanceless route, which is why the bulb goes out rather than dimming.
which loops did the gap belong to? still lit burned out parallel: the other loop is still closed R(eq) rose, so the battery current fell dark too burned out series: the only loop is broken both go dark, and the battery current is zero
The same failure, two topologies, opposite outcomes. Erasing the broken branch and looking for what still closes is the whole method.

The work

3 ways in · any order
Lesson
Simple Circuits ›

Requires a traced closed path before any current is assigned, works out what a short circuit does to the element it bridges and to the battery, and sorts out which branches a single gap stops.

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

Ten items spanning the failure modes of this topic: lighting a bulb with one wire, leaving a shorted bulb dimly lit, and killing a whole circuit with one broken branch. Take it cold to find which one is yours, or after the lesson to confirm it is not.

Not started · 10 items · ~15 min
Start diagnostic →
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