Power is an energy-transfer rate
You will be able to: Calculate power and energy while keeping watts distinct from joules.
How much energy does a circuit transfer?
A 6 V device carrying 0.50 A transfers 3 joules each second. Its power is 3 watts. Running it longer transfers more energy even if its power stays constant.
A useful starting point: Read an I–V graph before calculating →
Words and symbols before equations
- Power P
- Energy transferred per unit time; 1 watt (W)=1 J/s.
- Potential difference V
- Energy transferred per unit charge; 1 V=1 J/C.
- Energy E
- Total transfer over an interval, measured in J.
- Dissipation
- Conversion of electrical energy into thermal energy, not destruction of energy.
What this picture assumes
Ideal wires and voltage source; positive, fixed resistances. Circuit geometry is schematic, not a physical length or speed scale. Constant power over the displayed interval; all load power becomes thermal energy.
Read the picture in three steps
- Locate the labeled sources, system boundary or graph axes. Read the units before comparing values.
- I=2 A; P=12 W; energy in 10 s=120 J. Source voltage 6 V, load 3 Ω.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the physics
Combine joules per coulomb with coulombs per second: P=VI. A passive resistor’s absorbed power is positive when V is its voltage drop in the current direction. A battery supplies that energy from a chemical store.
Using V=IR for a resistor gives P=I²R and P=V²/R. These are equivalent when the same operating values are used. To predict changes, first establish whether the supply fixes voltage or the circuit fixes current.
For constant power, E=PΔt. If power changes, total energy is the area under a power–time graph. Charge remains conserved while electrical energy is transferred to heating, light or mechanical motion.
A worked example, step by step
A 12 V ideal supply drives a 6 Ω resistor for 10 s. Find I, P and transferred energy.
- I=V/R=12/6=2 A.
- P=VI=(12)(2)=24 W.
- E=PΔt=(24)(10)=240 J.
- Check P=V²/R=144/6=24 W; the same current returns to the source.
Watts describe a rate. Joules describe an amount; a longer operating time does not by itself raise power.
At fixed V, double R: what happens to power?
Compare with an explanation
It halves because P=V²/R; current also halves.
Predict. Change one thing. Explain.
Change R while holding voltage fixed, then vary the run time. Explain why changing time affects energy but not the instantaneous power.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
I=2 A; P=12 W; energy in 10 s=120 J. Source voltage 6 V, load 3 Ω.
Ideal wires and voltage source; positive, fixed resistances. Circuit geometry is schematic, not a physical length or speed scale. Constant power over the displayed interval; all load power becomes thermal energy.
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 9 V source drives a 3 Ω resistor for 20 s. (a) Find I. (b) Find power. (c) Find energy. (d) Predict power if resistance doubles with V unchanged.
This response is not submitted or saved. Copy it before leaving.
Compare with the answer and four-point rubric
- 1 point: I=3 A.
- 1 point: P=27 W.
- 1 point: E=540 J.
- 1 point: P halves to 13.5 W because voltage is fixed.
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 1Volt × ampere equals what?
Watt: (J/C)(C/s)=J/s.
RECALL 2Which formula is convenient at fixed voltage?
P=V²/R for a resistor.
RECALL 3Where does dissipated energy go?
Into thermal energy of the resistor and surroundings.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Power is an energy-transfer rate
- P=VI.
- For a resistor: P=I²R=V²/R.
- Constant power: E=PΔt.
Remember: Watts describe a rate. Joules describe an amount; a longer operating time does not by itself raise power.
Conditions: Ideal wires and voltage source; positive, fixed resistances. Circuit geometry is schematic, not a physical length or speed scale. Constant power over the displayed interval; all load power becomes thermal energy.
Refresh Kid · AP Physics 2 Unit 3 (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. 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.
Want to work through this with a tutor?
Bring your question about Power is an energy-transfer rate. Your explanation and answers remain free to access.