Follow connections before following the drawing
You will be able to: Recognize nodes, closed loops, open switches and resistor bypasses.
How do open paths and bypass wires change a circuit?
A battery and lamp need a complete path for sustained current. Bending a connecting wire does not change that path, but opening a switch or adding a bypass can.
A useful starting point: Add current through concentric area strips →
Words and symbols before equations
- Node
- Points joined by ideal wire with no intervening element; they share a potential.
- Open switch
- A break that prevents conduction through that branch.
- Short or bypass
- An ideal wire joining two points, forcing their voltage difference to zero.
What this picture assumes
A 2 Ω series resistor always remains in the conducting route. The bypass shorts only the load, not the source. Ideal wires; steady DC states. A directly shorted ideal nonzero-voltage source is excluded.
Read the picture in three steps
- Locate the labeled sources, system boundary or graph axes. Read the units before comparing values.
- Source current = 1 A; load current = 1 A; load voltage = 4 V. Trace the conducting path shown.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the physics
Read circuit topology: which elements connect to which nodes. A filled junction dot marks a connection; a crossing without a junction is not automatically a connection. Symbol position alone does not determine series or parallel.
An open branch carries no current, but it can support a voltage difference. Closing the loop allows current set by the source and resistances; charge is not consumed by a resistor.
In the investigation a battery always has a 2 Ω series resistor. The switch can open the loop, include a load resistor, or bypass only the load. Bypassing the load makes its voltage and current zero while source current increases through the remaining resistance. A nonzero ideal voltage source directly shorted by an ideal wire has no finite current solution.
A worked example, step by step
A 6 V battery feeds 2 Ω in series with a 4 Ω load. Compare normal, open and load-bypassed states.
- Normal: total R = 6 Ω, so I = 1 A.
- The load voltage is I(4 Ω) = 4 V.
- Open: I = 0; the open gap can support the source voltage.
- Load bypassed: I_source = 6/2 = 3 A, while load voltage and load current are zero.
A shorted load does not mean zero current everywhere. An open path does not mean zero voltage everywhere.
Is the load consuming charge in the normal state?
Compare with an explanation
No. It transfers electrical energy; the steady current is the same entering and leaving it.
Predict. Change one thing. Explain.
Compare the three switch states. Trace the conducting route and identify the resistance that still limits current in the bypass state.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Source current = 1 A; load current = 1 A; load voltage = 4 V. Trace the conducting path shown.
A 2 Ω series resistor always remains in the conducting route. The bypass shorts only the load, not the source. Ideal wires; steady DC states. A directly shorted ideal nonzero-voltage source is excluded.
Explain what you noticed: Which quantity changed? Which stayed fixed? Use the relevant current, circuit topology, charge or energy conservation, or RC 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 questionExplain all three states of the 6 V, 2 Ω plus 4 Ω circuit, including where current flows and why the bypass case remains finite.
This response is not submitted or saved. Copy it before leaving.
Compare with the answer and four-point rubric
- 1 point: Normal current passes through both resistors: 1 A.
- 1 point: Open state breaks the route, giving zero steady current.
- 1 point: Bypass state routes 3 A through the 2 Ω resistor and wire; the load has zero voltage.
- 1 point: The unbypassed 2 Ω resistance prevents an ideal-source short.
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 1What defines a node?
Connectivity by ideal wire.
RECALL 2Does a resistor use up current?
No; it dissipates energy while conserving charge.
RECALL 3When is a source short singular in the ideal model?
When a nonzero ideal voltage is imposed across a zero-resistance loop.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Follow connections before following the drawing
- Ideal-wire-connected points form one node.
- Open branch: I = 0.
- Ideal bypass: ΔV = 0 across the bypassed element.
Remember: A shorted load does not mean zero current everywhere. An open path does not mean zero voltage everywhere.
Conditions: A 2 Ω series resistor always remains in the conducting route. The bypass shorts only the load, not the source. Ideal wires; steady DC states. A directly shorted ideal nonzero-voltage source is excluded.
Refresh Kid · AP Physics C: Electricity and Magnetism Unit 4 (official Unit 11) · Objectives 11.2.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 11.2, objectives 11.2.A. CED effective Fall 2024, current PDF ©2026; checked September 16, 2026. This is E&M Unit 4: Electric Circuits, numbered Unit 11 in the official combined Physics C sequence. Topics 11.1–11.8 retain their official identifiers. Models include signed charge flow, prescribed current-density and resistivity integrals, DC resistor networks, nonideal batteries and meters, capacitor combinations and finite-resistance RC transients. Circuit schematics use conventional-current references and explicit node connectivity. Initial capacitor voltage and positive time constants are stated. Unequal ideal sources are never directly wired in parallel. The optional spatial wire view supplements a complete 2D current-density explanation. Checked with the Fall 2026 clarifications. 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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