Experiment 12: Series vs. Parallel Circuits — Bulb Brightness

Wire two bulbs one way and they share; wire them another way and they shine independently. Series and parallel are the two fundamental circuit architectures, and bulb brightness — power made visible — exposes the difference instantly. This experiment measures that difference, and the numbers are striking: each bulb in parallel gets four times the power of each bulb in series.

The physics behind the practical

In a series circuit there is one loop: the same current passes through every component, resistances add (R_total = R₁ + R₂), and the battery voltage is shared between the bulbs. Two identical bulbs each receive V/2. In a parallel circuit each bulb has its own branch across the battery: every bulb receives the full voltage, draws its own current, and the total current is the sum of the branches.

Power converts voltage and current into brightness: P = V × I, or for a bulb of resistance R, P = V²/R. Now the factor of four falls out: a series bulb gets V/2, so its power is (V/2)²/R = V²/4R — exactly one quarter of the V²/R it would get in parallel. That is why the sim's bar chart shows a clean 4:1 ratio. The trade-off is battery life: parallel draws twice the total current here (two branches at full power), so the battery's stored energy drains twice as fast. Homes are wired in parallel because every appliance needs the full mains voltage and must work — and fail — independently: in a series house, unscrewing one lamp would black out everything downstream of the break.

Variables and controls

In the virtual lab

Start in Series mode at Battery voltage 6.0 V: both bulbs glow dimly, and the readouts show the mode, each bulb's power and the Total current. Switch to Parallel: the same battery and bulbs blaze, each at full power. Read the numbers and confirm the ratio — parallel power per bulb is 4× series. Raise the battery to 12 V in both modes and watch the gap widen in watts (though brightness perception is roughly logarithmic, one reason by-eye scales saturate). Watch the electron dots: one shared stream in series, two branch streams in parallel. Log mode, voltage, total current and power per bulb for at least the two modes at two voltages.

Hands-on: three circuits

Build the single-bulb circuit and call its brightness "normal" (5 on your scale). Rewire both bulbs in series and record each bulb's brightness; then in parallel. Unscrew one bulb in each arrangement and note what happens to its partner. Repeat each arrangement once more to confirm your observations are stable — and disconnect the battery while rewiring. Low-voltage batteries only, never wall power.

A worked investigation

Take bulbs of resistance 20 Ω each on a 6.0 V battery (the sim's arithmetic):

Step 1 — Single bulb. I = V/R = 6.0 ÷ 20 = 0.30 A. P = V × I = 6.0 × 0.30 = 1.8 W. Brightness 5/5 — our reference.

Step 2 — Series. R_total = 20 + 20 = 40 Ω, so I = 6.0 ÷ 40 = 0.15 A through both. Each bulb gets V = IR = 0.15 × 20 = 3.0 V (the battery voltage splits equally) and P = 3.0 × 0.15 = 0.45 W. Brightness about 2/5. Unscrew one: the single loop is broken, the other dies too.

Step 3 — Parallel. Each bulb sits across the full 6.0 V: each draws 0.30 A and converts 1.8 W — full brightness, 5/5. Total current = 0.30 + 0.30 = 0.60 A, double the single-bulb draw: the battery drains twice as fast. Unscrew one: the other branch is untouched and stays lit.

Step 4 — The ratio. 1.8 ÷ 0.45 = 4 — parallel bulbs get exactly four times the power of series bulbs (same bulbs, same battery), because power goes as V² and the series bulb's voltage is halved.

Conclusion and evaluation

Brightness, power calculations and the sim agree: series shares voltage and quarters each bulb's power; parallel grants full voltage, full power and independence, at the cost of total current. The bench method's weakness is brightness-by-eye: human vision is logarithmic, so 0.45 W vs 1.8 W looks like "dim vs bright" (we scored it 2 vs 5) while the electrical ratio is a clean 4. Battery sag is the hidden trap — a tiring pack's voltage falls between arrangements, so test order can fake an effect; resting or swapping in a checked pack, and running arrangements in different orders across groups, defends against it. A light-meter app reading in lux turns the judgement into a number for the follow-up investigation.

Safety

Low-voltage battery packs only — never mains. Disconnect the battery while rewiring, and if a bulb or wire feels warm, disconnect and check for a short circuit.

Common mistakes

• Mismatched bulbs — different resistances split series voltage unequally and spoil the clean comparison.
• Judging brightness from different distances or angles — apparent brightness depends on where you stand; fix the viewing position.
• Leaving the circuit connected between arrangements — bulbs heat, resistance shifts and the battery drains, so later conditions are tested on a weaker pack.
• Calling parallel "brighter bulbs" without the cost — total current doubled; the energy bill shows up in battery life.
• Assuming series bulbs each get full voltage because each has "its own" wires — they share one loop and one voltage.

Exam Experience

Question 1 — calculation [4 marks]. Two identical bulbs of resistance 10 Ω are connected to a 6.0 V battery, first in series, then in parallel. Calculate the power of each bulb in each arrangement.

Scoring guide:

• Series: R_total = 20 Ω, I = 6.0 ÷ 20 = 0.30 A [1]
• Each series bulb: V = 3.0 V, P = V × I = 3.0 × 0.30 = 0.90 W [1]
• Parallel: each bulb gets 6.0 V, I = 0.60 A [1]
• Each parallel bulb: P = 6.0 × 0.60 = 3.6 W (four times the series value) [1]

Question 2 — evaluation and design [4 marks]. A student compares series and parallel brightness "by eye" and concludes parallel bulbs are "a bit brighter". Evaluate the method and design a more quantitative comparison.

Scoring guide:

• Evaluation: the eye judges brightness roughly logarithmically and from varying positions, so a 4× power ratio is compressed into "a bit" [1]
• Improvement: measure each bulb's voltage and the current through it, computing P = V × I [1]
• Or use a light-sensor/lux app at a fixed distance in a darkened room for a numerical brightness [1]
• Keep bulbs identical, battery checked/rest between arrangements, and repeat in swapped order to expose battery sag [1]

Multiple choice 1. In a series circuit of two bulbs, unscrewing one bulb turns the other off because:

• A. The battery runs out instantly
• B. Series is a single loop — breaking it anywhere stops the current everywhere
• C. The remaining bulb's resistance becomes infinite
• D. The voltage moves to the empty socket

Answer: B. One path means one point of failure kills the whole circuit; parallel branches are independent loops.

Multiple choice 2. A battery drains faster with two bulbs in parallel than with one bulb because:

• A. Parallel bulbs have more resistance
• B. Each branch draws its own full current, so the total current — and total power — doubles
• C. Electricity leaks out of the extra wires
• D. The battery voltage doubles

Answer: B. Two branches at full voltage draw twice the total current, converting the battery's stored energy twice as fast.

Going deeper: the battery's side of the bargain

Brightness is only half the story — the battery keeps the other half of the books. Total power drawn from the 6.0 V battery in this lesson's three arrangements: single bulb 1.8 W, series pair 0.15 A × 6.0 V = 0.9 W, parallel pair 0.60 A × 6.0 V = 3.6 W. Feed all three from the same battery pack holding, say, 10 watt-hours of energy, and the lifetimes divide accordingly: single ≈ 10 ÷ 1.8 = 5.6 hours, series ≈ 10 ÷ 0.9 = 11 hours, parallel ≈ 10 ÷ 3.6 = 2.8 hours. The series pair, dim as it looks, is the long-distance runner — half the total power buys double the endurance.

Torch designers live inside this triangle: brightness, runtime, battery size — pick two generously and pay for the third. It is also why "the bulbs are brighter in parallel" is an incomplete lab conclusion until the cost column is filled in: parallel spends the battery four times as fast per bulb-hour as series does. A results table with a cost column — energy, time, money, clips, charge — is the mark of an investigation that has grown up.

One more angle: why houses fuse the whole, not the parts

Household wiring extends this lesson's logic: every appliance is a parallel branch drawing its own current, and the fuse or breaker watches the sum. Three branches drawing 0.6 A each present 1.8 A to the supply; add a heater's 10 A and a kettle's 9 A and the total marches towards the breaker's limit — trip it, and every branch dies together, the one series-like behaviour in a parallel house. That is the design working as intended: the wiring is protected from the total current your independence creates. Parallel buys freedom per branch and bills it, with interest, at the meter.