Source: "Some Properties of Electric Circuits" lab (CCK simulation)
Tags: voltage, batteries in series, batteries in parallel, electric potential, voltmeter, combined voltage, PHY-222, classical physics II, circuit construction kit
Difficulty: Introductory | Prerequisites: Basic understanding of what voltage and electric charge are.
This material covers what happens to voltage when you combine batteries in different configurations and how a battery drives a simple light-bulb circuit. It sits right at the start of the circuits unit in PHY-222. If you understand how batteries add up in series versus parallel, every later topic (Kirchhoff's laws, Ohm's law, complex circuits) becomes far easier to follow. You should already be comfortable with the idea that a battery creates a potential difference measured in volts.
Batteries connected end to end (in series) add their voltages together. Batteries connected side by side (in parallel) keep the same voltage as a single battery. In a simple one-battery, one-bulb circuit, the full battery voltage appears across the bulb, and increasing that voltage makes the bulb brighter.
Voltage (electric potential difference)
The energy per unit charge between two points in a circuit, measured in volts (V). In simple terms, think of it as the "push" that drives charge through a wire.
Series connection
Components connected end to end so that current follows a single path through each one in turn. When batteries are in series, their voltages add.
Parallel connection
Components connected side by side so that current splits and each component sits across the same two nodes. When batteries are in parallel, the total voltage equals the voltage of one battery.
Voltmeter
An instrument placed across (in parallel with) a component to measure the potential difference. It has very high internal resistance so it draws negligible current.
Circuit Construction Kit (CCK)
A PhET simulation used in this lab to build and test circuits virtually. You drag out batteries, bulbs, wires, meters, and resistors on screen.
Each 9 V battery contributes 9 V of electric potential.
Two 9 V batteries end to end produce 18 V; three produce 27 V.
The relationship is strictly linear: total voltage = (number of identical batteries) × (voltage of one battery).
This happens because each battery's chemical reaction lifts charge through an additional potential difference, and those differences stack when the batteries are in a single path.
Two or three identical 9 V batteries wired in parallel still read 9 V across the combination.
Parallel batteries do not increase voltage; instead they increase the charge capacity (how long the combination can deliver current).
If you then place two parallel groups in series with each other, the series rule applies to the groups: two parallel pairs in series give 18 V.
Configuration | Measured Voltage |
|---|---|
1 battery | 9 V |
2 in series | 18 V |
3 in series | 27 V |
2 in parallel | 9 V |
3 in parallel | 9 V |
2 parallel + 1 in series | 18 V |
2 parallel pairs in series | 18 V |
In a closed loop with one battery and one bulb, the voltage across the bulb equals the battery voltage (e.g. 9 V).
This follows from Kirchhoff's voltage law: the sum of all potential differences around a closed loop is zero, so whatever the battery supplies, the bulb must drop.
Increasing the battery voltage makes the bulb brighter because a higher potential difference drives more current through the filament, which dissipates more power as light and heat.
Batteries in series
V_total = V₁ + V₂ + V₃ + ...
For n identical batteries: V_total = n × V_battery
Kirchhoff's Voltage Law (KVL)
The algebraic sum of all voltage gains and drops around any closed loop equals zero.
∑V_loop = 0
This is exactly why a TV remote uses two 1.5 V batteries end to end (series) to get 3 V. It is also why car batteries wire six 2 V cells in series to produce 12 V. Parallel battery packs, on the other hand, are used in electric vehicles to increase capacity (range) without raising the voltage beyond what the motor controller expects.
Students often think connecting batteries in parallel doubles the voltage. It does not. Parallel connections share the load but keep voltage constant.
Some students confuse "higher voltage" with "more charge stored." Voltage is energy per unit charge, not total charge. Two batteries in series raise voltage; two in parallel raise capacity.
Students sometimes believe a broken wire means all voltage disappears from the circuit. The battery still maintains its rated voltage across its own terminals; the voltmeter just reads 0 V across the gap where no current flows.
The lab document describes the resistance-versus-current curve as "polynomial exponential." That is a misnomer. The relationship is an inverse (hyperbolic) curve: I = V/R. It is neither polynomial nor exponential.
⚠️ You will almost certainly be asked to calculate the total voltage of batteries in series and to explain why parallel batteries do not increase voltage.
⚠️ Kirchhoff's voltage law (loop rule) underpins nearly every circuit-analysis problem on the exam. Being able to state it and apply it to a single-loop circuit is essential.
⚠️ Expect a question that gives you a mixed configuration (some batteries in parallel, some groups in series) and asks for the total voltage.
True or false: Three identical 6 V batteries in series produce 18 V.
True or false: Three identical 6 V batteries in parallel produce 18 V.
Fill in the blank: In a single-loop circuit with one battery and one bulb, the voltage across the bulb equals ______.
True or false: Removing a wire from a circuit causes the battery to lose its voltage.
Fill in the blank: Kirchhoff's voltage law states that the sum of all potential differences around a closed loop is ______.
Answers: 1. True. 2. False (still 6 V). 3. The battery voltage. 4. False (the battery keeps its voltage; current stops). 5. Zero.
Q: Two 9 V batteries are connected in series. What is the total voltage, and why?
A: 18 V. Each battery adds its full potential difference when connected end to end, so the voltages sum: 9 V + 9 V = 18 V.
Q: You connect three identical batteries in parallel. A voltmeter across the combination reads the same as a single battery. Explain why.
A: In a parallel connection every battery's positive terminal is connected to every other positive terminal (and likewise for the negatives). They all maintain the same potential difference across those shared nodes, so the voltage does not increase. What increases is the total charge the combination can deliver over time.
Q: In a circuit with one 9 V battery and one light bulb, the voltmeter reads 9 V across the bulb. Use Kirchhoff's voltage law to explain this.
A: KVL says the sum of voltage changes around a closed loop is zero. The battery provides +9 V. The bulb must therefore drop 9 V so that +9 V + (−9 V) = 0.
Q: A student claims that increasing battery voltage makes a bulb brighter "because there is more electricity." Give a more precise explanation.
A: Higher voltage means a larger potential difference across the bulb's filament. By Ohm's law (V = IR), a larger V drives a larger current I through the fixed resistance of the filament. Greater current means more electrical energy is converted to heat and light per second (P = IV), so the bulb glows more brightly.
This connects directly to Kirchhoff's laws, which formalise the loop and junction rules you see in action here. It also sets up Ohm's law (V = IR), covered in the resistor sections of the same lab. Understanding series versus parallel voltage is prerequisite to analysing any real circuit with multiple components.
voltage, potential difference, EMF, electromotive force, series circuit, parallel circuit, battery combination, Kirchhoff's voltage law, KVL, loop rule, voltmeter, CCK, PhET simulation, PHY-222, classical physics II, electric potential, bulb brightness, circuit basics