Source: "Some Properties of Electric Circuits" lab (CCK simulation), Sections III–IV
Tags: series circuit, parallel circuit, light bulbs, voltage division, current division, Kirchhoff's laws, junction rule, loop rule, brightness, ammeter, PHY-222, classical physics II
Difficulty: Introductory to Intermediate | Prerequisites: Voltage fundamentals (batteries in series/parallel), basic idea of current and resistance.
Once you know that batteries supply voltage and bulbs consume it, the next question is: what happens when you wire several bulbs together? The answer depends entirely on whether they are in series (one path) or in parallel (multiple paths). This distinction runs through the entire circuits unit and shows up on virtually every exam. You need to be comfortable with how voltage, current, and brightness each behave in the two configurations. This lab section uses identical bulbs, so differences in behaviour come purely from the wiring topology.
In a series circuit, each additional bulb shares the battery voltage equally, so individual bulb voltage drops and all bulbs get dimmer, while current stays the same everywhere. In a parallel circuit, every bulb sees the full battery voltage and glows at full brightness, but the total current drawn from the battery increases with each added bulb.
Series circuit
A circuit in which components are connected along a single conducting path so the same current flows through every component. In simple terms, think of it as a single queue: everyone passes through every checkpoint.
Parallel circuit
A circuit in which components are connected across the same two nodes, giving current multiple paths. Each branch carries its own current, but they all share the same voltage. Think of it as multiple lanes on a motorway: traffic splits, but the speed limit (voltage) is the same in every lane.
Kirchhoff's current law (junction rule)
At any junction in a circuit, the total current flowing in equals the total current flowing out. This is conservation of charge.
Kirchhoff's voltage law (loop rule)
Around any closed loop, the algebraic sum of all voltage gains and drops is zero. This is conservation of energy per unit charge.
Ammeter
An instrument connected in series with a component to measure the current through it. It has very low internal resistance so it does not significantly alter the circuit.
Voltage drop (across a component)
The decrease in electric potential as current passes through a resistive element such as a bulb. In a series circuit the battery voltage is divided among the components.
With a 12 V battery (the lab used 9 V readings, but the setup specifies 12 V; the principle is the same), adding bulbs in series splits the voltage equally among identical bulbs.
1 bulb: full voltage across it (9 V in lab data), high brightness.
2 bulbs: half voltage each (4.5 V), noticeably dimmer.
3 bulbs: one-third voltage each (3 V), dimmer still.
Current through the circuit stays the same everywhere (0.9 A in the lab data) because there is only one path.
Each additional bulb adds resistance. By Ohm's law, if total resistance rises and the battery voltage is fixed, the voltage across each individual bulb must fall.
Brightness drops because the power dissipated by each bulb (P = V × I, or P = V²/R) decreases when the voltage across it decreases.
# of Bulbs | Voltage per Bulb (V) | Current (A) | Brightness |
|---|---|---|---|
1 | 9 | 0.9 | High |
2 | 4.5 | 0.9 | Dimmer |
3 | 3 | 0.9 | Even dimmer |
Every bulb in parallel sits directly across the battery terminals, so each one sees the full battery voltage (9 V in the lab data).
Brightness stays the same regardless of how many bulbs are added.
The total current drawn from the battery increases with each bulb added, but the current through each individual branch decreases proportionally to the number of branches (conservation of charge at the junction).
1 bulb: 0.9 A total.
2 bulbs: 0.45 A per branch, 0.9 A total. (The lab recorded 0.45 A from the battery, which represents the current per branch rather than total; the key point is each branch carries less.)
3 bulbs: 0.33 A per branch.
Removing a wire to one bulb in a parallel circuit does not affect the other bulbs. They still have a complete path through the battery and continue to operate.
# of Bulbs | Battery Voltage (V) | Battery Current (A) | Brightness |
|---|---|---|---|
1 | 9 | 0.9 | Bright |
2 | 9 | 0.45 | Just as bright |
3 | 9 | 0.33 | Just as bright |
Series circuit: removing any wire breaks the single path. Current drops to zero everywhere and all bulbs go dark. The battery still holds its voltage across its terminals, but no current flows.
Parallel circuit: removing a wire to one bulb only kills that branch. The remaining bulbs keep working because they still have a closed loop through the battery.
A voltmeter reads the same value whether placed on the left side or right side of the battery, because both leads still span the same two nodes.
Swapping the voltmeter leads flips the sign of the reading (positive becomes negative) but the magnitude stays the same. This reflects the direction convention for potential difference.
In a parallel circuit, a voltmeter reads the same voltage across any bulb, confirming that parallel components share voltage.
An ammeter, by contrast, gives different readings in different branches of a parallel circuit, because current divides among paths.
Series total resistance
R_total = R₁ + R₂ + R₃ + ...
Parallel total resistance
1/R_total = 1/R₁ + 1/R₂ + 1/R₃ + ...
Voltage division (series, identical components)
V_each = V_battery / n
where n is the number of identical components.
Power dissipated by a component
P = IV = V²/R = I²R
Christmas tree lights wired in series all go dark when one bulb burns out, because the single path is broken. Modern LED strings use parallel wiring so one dead bulb leaves the rest lit. Household mains wiring is parallel: every outlet sees the same voltage, and unplugging one appliance does not affect the others.
Students often believe that adding bulbs in series "uses up" current so less reaches the later bulbs. Current is the same everywhere in a series loop; what gets divided is the voltage.
A common error is thinking that bulbs in parallel must be dimmer because the current splits. Each bulb still receives full battery voltage, so each dissipates the same power and glows just as brightly.
Some students assume that if the ammeter reading drops when bulbs are added in parallel, the total current from the battery drops. In fact the ammeter may be measuring a single branch. Total battery current increases as more parallel paths open up.
Students sometimes think swapping voltmeter leads gives a different magnitude. It does not; only the sign changes.
⚠️ Expect a question asking you to predict what happens to brightness, current, and voltage when a bulb is added in series versus in parallel. Know both directions.
⚠️ A classic exam question: "One bulb in a series string burns out. What happens to the rest?" (They all go out.) Same question for parallel. (The rest stay on.)
⚠️ Be prepared to explain why voltage divides in series but current divides in parallel, using Kirchhoff's laws.
⚠️ Know the difference between where you place a voltmeter (in parallel with the component) and where you place an ammeter (in series with the component).
True or false: In a series circuit with three identical bulbs, each bulb carries the same current.
True or false: In a parallel circuit with three identical bulbs, each bulb has the same voltage across it.
Fill in the blank: Adding a bulb in series ______ (increases/decreases) the total resistance of the circuit.
True or false: Removing one bulb from a parallel circuit turns off all the other bulbs.
Fill in the blank: Swapping the leads of a voltmeter changes the ______ of the reading but not the ______.
Answers: 1. True. 2. True. 3. Increases. 4. False (only that branch is affected). 5. Sign; magnitude.
Q: A 12 V battery powers three identical bulbs in series. What is the voltage across each bulb, and what happens to brightness compared to a single-bulb circuit?
A: Each bulb drops 12/3 = 4 V. Brightness is noticeably lower than in the single-bulb case because each bulb dissipates less power (P = V²/R, and V per bulb has fallen).
Q: You add a fourth identical bulb in parallel with three existing parallel bulbs across a 9 V battery. Does the brightness of the original three change? Explain.
A: No. Each bulb still sits across the full 9 V, so each dissipates the same power as before. The battery must supply more total current to feed the extra branch, but individual bulb conditions are unchanged.
Q: In a series circuit, the ammeter reads 0.9 A regardless of whether there is one bulb or three. Explain why.
A: There is only one path for current. Adding identical bulbs increases total resistance, but the lab data shows the battery voltage also adjusts (the measured voltage per bulb drops proportionally). The constant 0.9 A reading reflects the ratio V_battery / R_total staying the same when identical bulbs share the fixed supply, because each bulb's resistance is the same and the battery's total output voltage divides among them.
Q: Explain, using Kirchhoff's laws, why all bulbs in a parallel circuit have the same voltage.
A: By the loop rule, trace a loop from the battery through any one bulb and back. The voltage gained at the battery must equal the voltage dropped across that bulb. Since every bulb forms its own loop with the same battery, every bulb must drop the same voltage.
Series and parallel analysis is the foundation for mesh and nodal analysis methods used later in the course. It also connects to the study of resistors (covered in the next set of notes), where Ohm's law (V = IR) quantifies the relationships you observe qualitatively here with bulbs. Power dissipation (P = IV) ties these ideas to energy and thermal physics.
series circuit, parallel circuit, voltage division, current division, Kirchhoff's current law, KCL, junction rule, Kirchhoff's voltage law, KVL, loop rule, ammeter, voltmeter, bulb brightness, power dissipation, resistance in series, resistance in parallel, PHY-222, classical physics II, CCK simulation