Compare bulbs using power
You will be able to: Compare the power of identical model bulbs in series and parallel with an ideal source.
Does adding a bulb always dim the first one?
Two identical flashlight bulbs can be connected one after the other or on separate branches. The battery label alone does not tell you each bulb’s voltage. Draw the connections, find the current or voltage, then compare power.
A useful starting point: Power is an energy-transfer rate →
Words and symbols before equations
- Series
- One route through both bulbs; their current is shared.
- Parallel
- Both bulbs connect across the same two nodes; their voltage is shared.
- Brightness model
- For otherwise identical bulbs, greater electrical power generally means greater brightness.
- Fixed-R bulb
- A simplified bulb represented by a constant resistance; a real filament’s resistance changes with temperature.
What this picture assumes
Ideal wires and voltage source; positive, fixed resistances. Circuit geometry is schematic, not a physical length or speed scale. Identical bulbs are fixed-R models. Power compares heating rates; the illustration does not quantify real light output.
Read the picture in three steps
- Locate the labeled sources, system boundary or graph axes. Read the units before comparing values.
- Series: each bulb 3 V, 0.5 A, 1.5 W. Source current=0.5 A; total power=3 W.
- 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 two equal resistances R in series across V, total current is V/(2R). Each bulb’s power is V²/(4R). Compared with one bulb alone on the same ideal source, each series bulb has one quarter the power.
For two equal resistances in parallel across an ideal V source, each has full V across it and power V²/R. Adding that second branch raises the source’s total current while leaving the first branch’s voltage unchanged.
These statements assume the same ideal source voltage and identical constant-resistance bulbs. A real battery’s internal resistance can lower terminal voltage when extra current is drawn; real filament brightness is not an exact linear watt-to-light conversion.
A worked example, step by step
Two 6 Ω model bulbs connect to a 6 V ideal source. Compare power per bulb in series and parallel.
- Series equivalent resistance is 12 Ω, so source I=0.50 A.
- Each series bulb has P=I²R=0.25×6=1.5 W.
- Parallel: each has 6 V, so P=36/6=6 W.
- Each parallel bulb has four times the model power; total powers are 3 W and 12 W.
P=I²R and P=V²/R do not conflict: series shares current, whereas parallel shares voltage.
Remove one parallel bulb while keeping the ideal supply. What happens to the other?
Compare with an explanation
Its voltage and model power remain unchanged; source current falls.
Predict. Change one thing. Explain.
Switch the identical bulbs between series and parallel. Compare per-bulb power and total source power before discussing brightness.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Series: each bulb 3 V, 0.5 A, 1.5 W. Source current=0.5 A; total power=3 W.
Ideal wires and voltage source; positive, fixed resistances. Circuit geometry is schematic, not a physical length or speed scale. Identical bulbs are fixed-R models. Power compares heating rates; the illustration does not quantify real light output.
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 questionTwo 4 Ω model bulbs are connected to an 8 V ideal source. (a) Find series current. (b) Find series power per bulb. (c) Find parallel power per bulb. (d) Explain the comparison’s main limitation.
This response is not submitted or saved. Copy it before leaving.
Compare with the answer and four-point rubric
- 1 point: Series current 8/8=1 A.
- 1 point: Each dissipates 4 W.
- 1 point: Each dissipates 64/4=16 W.
- 1 point: Fixed resistance and ideal supply are approximations; real heating and internal resistance can change results.
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 1Why start with connectivity?
Connections determine which currents and voltages are shared.
RECALL 2Is brightness a direct measurement of charge consumption?
No. It reflects energy transfer.
RECALL 3What does an ideal battery hold constant?
Its terminal voltage, independent of current in this model.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Compare bulbs using power
- Identical series pair: P_each=V²/(4R).
- Identical parallel pair: P_each=V²/R.
- Brightness comparisons require an explicit bulb and source model.
Remember: P=I²R and P=V²/R do not conflict: series shares current, whereas parallel shares voltage.
Conditions: Ideal wires and voltage source; positive, fixed resistances. Circuit geometry is schematic, not a physical length or speed scale. Identical bulbs are fixed-R models. Power compares heating rates; the illustration does not quantify real light output.
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.
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