Read a complete circuit path
You will be able to: Identify closed, open and shorted paths from circuit connections.
Why does a gap stop a simple circuit?
A flashlight needs both battery terminals connected through its lamp. One loose contact breaks that route. Follow the actual connected path before trying any equation.
A useful starting point: Current counts charge per second →
Words and symbols before equations
- Node
- Points joined by ideal wire share the same electric potential.
- Closed path
- An uninterrupted conducting route around the circuit.
- Open switch
- A gap that prevents steady current in that branch.
- Short circuit
- A very low-resistance connection that bypasses a component or connects supply terminals.
What this picture assumes
Ideal wires and voltage source; positive, fixed resistances. Circuit geometry is schematic, not a physical length or speed scale. Open-switch values are steady-state values. A shorted ideal source is deliberately excluded.
Read the picture in three steps
- Locate the labeled sources, system boundary or graph axes. Read the units before comparing values.
- Closed: I=2 A; resistor voltage=6 V; gap voltage=0 V.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the physics
A schematic describes connectivity, not the physical shape or length of the wires. Battery symbols use a long line for the positive terminal and a short line for the negative terminal; zigzags or rectangles represent resistors. A dot marks a joined wire intersection.
With one ideal battery and one resistor, closing the switch gives I=V/R. Opening it gives zero steady current. The battery still has a potential difference; the open switch can support that difference while no current crosses its gap.
A wire placed across a resistor connects its two ends to the same node, so that resistor has nearly zero voltage and is bypassed. An ideal zero-resistance path directly across an ideal voltage source has no finite-current solution. Real wire and source resistance limit the resulting large current. The Explore model includes only open and properly closed paths.
A worked example, step by step
An ideal 6 V battery, 3 Ω resistor and switch form one loop. Compare closed and open states.
- Closed: trace battery → switch → resistor → battery.
- I=6 V / 3 Ω=2 A; the resistor has a 6 V drop.
- Open: no complete path, so steady I=0 and IR=0 across the resistor.
- The 6 V is across the open gap, rather than disappearing from the circuit.
Zero current does not imply zero voltage everywhere: an open gap can have a potential difference.
Can an ideal voltmeter read a voltage across an open switch?
Compare with an explanation
Yes. It can measure the potential difference without completing a conducting path.
Predict. Change one thing. Explain.
Toggle the switch. Compare current, resistor voltage and gap voltage. Trace the connected wire rather than judging where symbols sit on the page.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Closed: I=2 A; resistor voltage=6 V; gap voltage=0 V.
Ideal wires and voltage source; positive, fixed resistances. Circuit geometry is schematic, not a physical length or speed scale. Open-switch values are steady-state values. A shorted ideal source is deliberately excluded.
Explain what you noticed: Which quantity changed? Which stayed fixed? Use the relevant current, voltage or energy 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 questionA 12 V ideal battery connects to a 6 Ω resistor through one switch. (a) Find closed current. (b) Find open current. (c) Find open-state resistor voltage. (d) Find open-switch voltage magnitude.
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Compare with the answer and four-point rubric
- 1 point: Closed I=2 A.
- 1 point: Open I=0.
- 1 point: Resistor drop IR=0 V.
- 1 point: The switch gap supports 12 V.
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 does a schematic preserve?
Connections between elements, rather than physical distances.
RECALL 2Which battery-symbol line is positive?
The longer line.
RECALL 3Why is an ideal source short not assigned a finite current?
The model would require nonzero voltage across zero resistance; real resistance must be included.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Read a complete circuit path
- One closed resistive loop: I=V/R.
- Ideal wire connects equal-potential points.
- Open branch: no steady conduction current across its gap.
Remember: Zero current does not imply zero voltage everywhere: an open gap can have a potential difference.
Conditions: Ideal wires and voltage source; positive, fixed resistances. Circuit geometry is schematic, not a physical length or speed scale. Open-switch values are steady-state values. A shorted ideal source is deliberately excluded.
Refresh Kid · AP Physics 2 Unit 3 (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. Refresh Kid calls this the third AP Physics 2 unit; College Board numbers it Unit 11; the first unit in this course is official Unit 9. 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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