Measure current through; voltage across
You will be able to: Place ideal meters correctly and predict qualitative loading effects of nonideal meters.
Where should a meter connect?
To count cars on one road, a counter must lie on that road. To compare the heights of two places, you need both endpoints. Similarly, an ammeter belongs in the current path; a voltmeter compares two nodes.
A useful starting point: A real battery loses some voltage internally →
Words and symbols before equations
- Ammeter
- Measures current through its terminals, in A; connected in series.
- Voltmeter
- Measures voltage between its terminals, in V; connected across the element.
- Ideal meter
- An ammeter has zero resistance; a voltmeter has infinite resistance.
- Loading
- A measuring instrument changes the circuit it is trying to measure.
What this picture assumes
Ideal wires and voltage source; positive, fixed resistances. Circuit geometry is schematic, not a physical length or speed scale. Ideal ammeter has zero resistance; ideal voltmeter has infinite resistance and carries no current. Nonideal loading is discussed qualitatively in Learn.
Read the picture in three steps
- Locate the labeled sources, system boundary or graph axes. Read the units before comparing values.
- Ammeter in the resistor path: 2 A. Voltmeter across the resistor: 6 V. Its ideal current is zero.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the physics
Insert an ideal ammeter into the chosen branch so all that branch’s current passes through it. Connect an ideal voltmeter across the two ends of the chosen element. Label which branch and which voltage you are measuring.
A real ammeter adds some series resistance and therefore tends to lower current in a simple voltage-driven resistive loop. A real voltmeter draws some current because its resistance is finite; it changes the equivalent resistance across the measured element. Its effect on a measured voltage depends on the rest of the circuit.
Across one resistor driven directly by an ideal voltage source, a finite-resistance voltmeter leaves that resistor’s voltage fixed but increases total source current. With source or upstream resistance, loading can also reduce the measured voltage. AP-level nonideal-meter reasoning here is qualitative.
| Property | Ammeter | Voltmeter |
|---|---|---|
| Measured quantity | Current through a branch | Voltage across two points |
| Connection | Series | Parallel |
| Ideal resistance | Zero | Infinite |
A worked example, step by step
An ideal 6 V source drives 3 Ω. Place ideal meters to measure resistor current and voltage.
- Put the ammeter in series with the resistor.
- Put the voltmeter across the resistor’s two ends.
- Ammeter reads I=6/3=2 A.
- Voltmeter reads 6 V; it draws zero current in the ideal model.
Connecting a very low-resistance ammeter across a supply creates a short path instead of measuring the intended branch current.
Must a finite-resistance voltmeter always lower the voltage it measures?
Compare with an explanation
No. An ideal voltage source directly across those same nodes maintains voltage; source current changes instead.
Predict. Change one thing. Explain.
Vary supply voltage and resistor value. Trace the route through A and the two endpoints connected to V. Predict both readings before changing a control.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Ammeter in the resistor path: 2 A. Voltmeter across the resistor: 6 V. Its ideal current is zero.
Ideal wires and voltage source; positive, fixed resistances. Circuit geometry is schematic, not a physical length or speed scale. Ideal ammeter has zero resistance; ideal voltmeter has infinite resistance and carries no current. Nonideal loading is discussed qualitatively in Learn.
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 source drives 4 Ω. (a) Place an ideal ammeter. (b) Place an ideal voltmeter. (c) Give both readings. (d) Predict the direction of change if the ammeter has appreciable resistance.
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Compare with the answer and four-point rubric
- 1 point: Ammeter in series with the 4 Ω resistor.
- 1 point: Voltmeter across the resistor.
- 1 point: A reads 3 A and V reads 12 V.
- 1 point: Added series resistance lowers current and the load voltage.
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 1Current through or voltage through?
Current through an element; voltage across two points.
RECALL 2Why does an ideal voltmeter not divert current?
Its resistance is infinite.
RECALL 3Does a measurement leave every real circuit unchanged?
No. Finite meter resistances cause loading.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Measure current through; voltage across
- Ammeter: series; ideal R_A=0.
- Voltmeter: parallel; ideal R_V→∞.
- Real instruments can alter the circuit; identify the source model.
Remember: Connecting a very low-resistance ammeter across a supply creates a short path instead of measuring the intended branch current.
Conditions: Ideal wires and voltage source; positive, fixed resistances. Circuit geometry is schematic, not a physical length or speed scale. Ideal ammeter has zero resistance; ideal voltmeter has infinite resistance and carries no current. Nonideal loading is discussed qualitatively in Learn.
Refresh Kid · AP Physics 2 Unit 3 (official Unit 11) · Objectives 11.5.C · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 11.5, objectives 11.5.C. 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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