Current transfers energy without being used up
You will be able to: Calculate electrical power and compare fixed-current with fixed-voltage changes.
How do voltage and current determine power?
A 6 V supply moving 2 coulombs each second transfers 12 joules each second. A resistor turns that electrical energy into thermal energy while charge keeps circulating.
A useful starting point: Read the axes before calling a slope resistance →
Words and symbols before equations
- Power P
- Energy transfer per time, in watts: 1 W = 1 J/s.
- Voltage drop V
- Energy transferred per coulomb across the consuming element.
- Passive convention
- Current entering the higher-potential terminal gives positive absorbed power.
What this picture assumes
Ideal resistor; only the selected supply setting applies. The other setting is inactive. Positive absorbed power; fixed conditions, no heating feedback.
Read the picture in three steps
- Locate the labeled sources, system boundary or graph axes. Read the units before comparing values.
- Selected supply: fixed voltage. V = 6 V; I = 0.5 A; absorbed P = 3 W. The other supply setting is inactive.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the physics
Multiplying energy per charge by charge per time gives P = IV. For a resistor, V = IR gives P = I²R = V²/R.
At fixed current, larger R increases resistor power. At fixed voltage, larger R decreases it. There is no contradiction: the constraints differ.
Over a constant-power interval, energy is Pt. Power can become heat, light or mechanical output. Bulb brightness comparisons use dissipated power qualitatively; real bulbs need not have constant resistance.
A worked example, step by step
A 12 Ω resistor is connected to 6 V for 10 s. Find current, power and energy.
- I = 6/12 = 0.5 A.
- P = VI = 6(0.5) = 3 W.
- Energy = Pt = 3(10) = 30 J.
- Charge is conserved; the resistor dissipates energy, not current.
State whether voltage or current is fixed before comparing brightness or power.
Can all electrical power become useful mechanical power in a real motor?
Compare with an explanation
Usually not; losses mean useful output is less than electrical input.
Predict. Change one thing. Explain.
Switch between a fixed-voltage supply and a fixed-current supply. Increase R in each case and explain the opposite power trends.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Selected supply: fixed voltage. V = 6 V; I = 0.5 A; absorbed P = 3 W. The other supply setting is inactive.
Ideal resistor; only the selected supply setting applies. The other setting is inactive. Positive absorbed power; fixed conditions, no heating feedback.
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 questionA motor receives 12 V at 2 A and delivers 18 W of mechanical power. Find electrical input, loss power and energy lost in 5 s.
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Compare with the answer and four-point rubric
- 1 point: Input power is 12(2) = 24 W.
- 1 point: Loss power is 24−18 = 6 W.
- 1 point: Over 5 s, energy lost is 30 J.
- 1 point: This is energy conversion; incoming and outgoing steady charge flow still balance.
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 a watt?
One joule per second.
RECALL 2Why can larger R mean less P?
If voltage stays fixed, current falls and P = V²/R.
RECALL 3Is brightness always proportional to current alone?
No. Electrical power depends on both current and voltage.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Current transfers energy without being used up
- P_absorbed = IV with the passive sign convention.
- Resistor: P = I²R = V²/R.
- Constant power: energy = Pt.
Remember: State whether voltage or current is fixed before comparing brightness or power.
Conditions: Ideal resistor; only the selected supply setting applies. The other setting is inactive. Positive absorbed power; fixed conditions, no heating feedback.
Refresh Kid · AP Physics C: Electricity and Magnetism Unit 4 (official Unit 11) · Objectives 11.4.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 11.4, objectives 11.4.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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