Current counts charge per second
You will be able to: Relate current to transferred charge and distinguish conventional current from electron motion.
What actually flows through a wire?
Imagine counting people passing through one doorway. Current uses a similar counting idea, but counts electric charge crossing a wire’s cross section. If 6 coulombs pass in 3 seconds, the average current is 2 amperes.
A useful starting point: Electric potential and energy per charge →
Words and symbols before equations
- Current I
- Rate of charge transfer; 1 ampere (A) = 1 coulomb per second (C/s).
- ΔQ and Δt
- Charge passing a chosen cross section and the elapsed time. Delta means change or interval.
- Conventional current
- Direction positive charge would move. In a metal, electron drift is in the opposite direction.
- Drift
- Small average motion superimposed on the much faster random motion of charge carriers.
What this picture assumes
Constant conventional current in the reference direction. Transferred charge is counted at one cross section, not accumulated in the entire wire.
Read the picture in three steps
- Locate the labeled sources, system boundary or graph axes. Read the units before comparing values.
- ΔQ=IΔt=1×6=6 C. Q–t slope is 1 A.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the physics
For a steady current, ΔQ=IΔt. More generally, ΔQ/Δt gives average current over the interval. A graph of transferred charge against time has slope equal to current; a current–time graph has signed area equal to transferred charge.
A battery establishes a potential difference that drives charge motion around a conducting path. Charge carriers are already distributed throughout the metal: a lamp does not wait for one electron to travel from the battery.
Current has an assigned direction around a circuit, but is not a spatial vector added with ordinary x- and y-components. Zero current means no net drift across the section, not that every electron has stopped. A resistor transfers energy while steady charge flow continues.
A worked example, step by step
A steady current of 0.50 A runs for 8.0 s. Find the charge passing a cross section.
- Choose one cross section and the 8.0 s interval.
- Use ΔQ=IΔt=(0.50 C/s)(8.0 s).
- ΔQ=4.0 C. The units of seconds cancel.
- This is charge transferred through the section, not charge consumed by the lamp.
A resistor does not use up current; it transfers electrical energy to other forms.
At the same current, does twice the time mean twice the charge?
Compare with an explanation
Yes: for steady I, transferred charge is proportional to elapsed time.
Predict. Change one thing. Explain.
Change current and elapsed time separately. Predict how the slope and endpoint of the charge–time graph change.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
ΔQ=IΔt=1×6=6 C. Q–t slope is 1 A.
Constant conventional current in the reference direction. Transferred charge is counted at one cross section, not accumulated in the entire wire.
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 steady 0.20 A current runs for 15 s. (a) Find transferred charge. (b) Sketch transferred Q versus t. (c) State the graph slope with units. (d) Relate electron drift to conventional current.
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Compare with the answer and four-point rubric
- 1 point: ΔQ=3.0 C.
- 1 point: Straight line through (0 s,0 C) and (15 s,3 C).
- 1 point: Slope 0.20 C/s = 0.20 A.
- 1 point: Electron drift is opposite conventional current in a metal.
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 is an ampere?
One coulomb per second.
RECALL 2What does the slope of Q versus t represent?
Current; a changing slope means changing current.
RECALL 3Is charge consumed by a resistor?
No. Energy is transferred, while charge is conserved.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Current counts charge per second
- I_avg=ΔQ/Δt.
- For constant I, ΔQ=IΔt.
- Metal electrons drift opposite conventional current.
Remember: A resistor does not use up current; it transfers electrical energy to other forms.
Conditions: Constant conventional current in the reference direction. Transferred charge is counted at one cross section, not accumulated in the entire wire.
Refresh Kid · AP Physics 2 Unit 3 (official Unit 11) · Objectives 11.1.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 11.1, objectives 11.1.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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