Equal Brightness in Series and the 'Current Is Used Up' Model
In a conceptual physics class, students build a circuit with a 6.0 V battery, a switch, and two identical flashlight bulbs rated 6.0 V, 0.30 A in series. Before closing the switch, most students predict the first bulb will be brighter because 'the current gets used up.' When the switch is closed, both bulbs glow equally but dimly. A student insists the battery must be weak because each bulb is dimmer than a single bulb was in an earlier activity. Another group connects the same two bulbs in parallel; both are bright, and the battery feels warm after a minute. The teacher has digital multimeters, a 6.0 V battery, spare bulbs, and a clamp-on ammeter. One student asks whether the bulbs 'share the voltage' or 'share the current,' and another asks why the parallel circuit drains the battery faster.
1. Describe a measurement sequence using the multimeters that would let students test the 'current is used up' idea and the 'share voltage/current' question. Include meter placement and expected readings.
Read the worked answer
Series: break the circuit at three points-before bulb 1, between the bulbs, and after bulb 2-and insert the ammeter in series at each point. Students should read approximately 0.15 A at all three points. Connect the voltmeter in parallel across each bulb (about 3.0 V each) and across the battery (about 6.0 V). If current were 'used up,' the reading after bulb 2 would be smaller; it is not.
Parallel: connect each bulb across the battery. Measure each branch current (about 0.30 A) and the total current from the battery (about 0.60 A). Measure the voltage across each branch (about 6.0 V). The branch currents add; the voltages do not.
Safety and technique: start on the highest DC current range, never connect an ammeter directly across the battery, and have students sketch meter placement before wiring. The measurements support conservation of charge (current is the same in a series loop and splits in parallel) and energy transfer (each coulomb loses potential energy in the bulb; energy, not current, is what the bulb converts to light and heat).
2. A student says the battery is 'weak.' Use the observations (dim in series, bright in parallel, warm battery) to explain what is actually happening, including internal resistance and power.
Read the worked answer
Use the numbers to replace the 'weak battery' story. Each bulb has R ≈ 6.0 V / 0.30 A = 20 Ω. In series, total resistance is 40 Ω, so I ≈ 6.0/40 = 0.15 A; each bulb dissipates P = I²R ≈ 0.45 W, one-quarter of the 1.8 W a single bulb dissipates at 6.0 V. In parallel, each bulb still sees about 6.0 V, draws 0.30 A, and dissipates 1.8 W, so both are bright. The battery supplies about 0.60 A total, four times the series current, so it drains faster and warms more.
The warmth is not evidence of a 'weak' battery; it is consistent with internal resistance: P_internal = I²r, and r produces a larger internal drop at higher current. If students measure terminal voltage under load, they may see it sag slightly in parallel. The explanatory chain is: bulb brightness → power → voltage across the bulb and current through it; battery heating → total current and internal resistance.
3. What instructional move would you avoid, and what would you do instead, to produce durable conceptual change rather than a memorized slogan?
Read the worked answer
Avoid the quick correction 'current is not used up,' which students can repeat without changing their model. Also avoid introducing the term 'current' as a substance that flows and gets consumed. Instead use a predict–observe–explain cycle: students commit to a prediction, measure the three series currents, and reconcile the prediction with the readings. Use a concrete loop analogy-students pass a ball around a closed loop; each student does work on the ball (energy transfer) but the ball is not consumed-and explicitly map the analogy to charge and energy.
Then have groups whiteboard a rule that fits both series and parallel data, and hold a consensus discussion. Revisit the idea in the power unit with P = VI. The goal is a model that explains equal brightness in series and undimmed brightness in parallel, not a memorized slogan.