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D2.6 · Compare current, voltage, and resistance in series and parallel circuits

Learn to compare current, voltage, and resistance in series and parallel circuits through clear examples and targeted practice.

Ontario Grade 9 Science

Physics: Principles and Applications of Electricity

How current, voltage, and resistance behave in two common circuit paths

A string of older holiday lights may go dark when one bulb is removed. In a parallel arrangement, other bulbs can often stay lit when one branch stops working. These everyday observations raise a science question: how does the path electricity can follow affect current, voltage, and resistance? A circuit is a complete path through which electric charge can move. A source, such as a battery, provides voltage. Voltage is the electrical push that can move charge. Current is the flow of electric charge. Resistance describes how much a material or component opposes that flow. We will compare these three ideas in series and parallel circuits using simple models. The models explain patterns; they do not show every detail of a real circuit.

What you will learn

Describe the difference between a series circuit and a parallel circuit.
Compare current, voltage, and total resistance in series and parallel circuits.
Use circuit models and stated readings to explain what changes when the circuit arrangement changes.

Start with the paths

A series circuit has one path through its components. Imagine two lamps connected one after the other in a loop with a battery. Charge moving around the loop passes through both lamps. If that single path is broken, the circuit is open, which means there is a gap and current cannot continue around the loop.

A parallel circuit has more than one path. Each separate path is called a branch. Imagine two lamps on separate branches connected across the same battery. If one branch is open, the other branch can still provide a complete path.

The source’s voltage is shared among components in a series circuit. The voltage across each branch in a parallel circuit is the same as the source voltage. Current is the same at every point in a simple series loop. In parallel, the current from the source divides among the branches. The source current equals the combined branch currents.

Resistance also depends on the arrangement. Total resistance means the resistance of the entire circuit as seen by the source. Adding components in series increases total resistance. Adding another branch in parallel lowers total resistance compared with the same circuit having only one of those branches. These are comparisons of the whole circuit, not claims that a component’s own resistance changes.

Series: one path, same current through each component.
Parallel: separate branches, same voltage across each branch.
The source current in a parallel circuit is shared among the branches.
Series components increase total resistance; adding a parallel branch lowers total resistance.

Use a model to compare the patterns

A labelled circuit diagram is a model. It shows how components are connected, but it is not a photograph of charge moving. The circuit comparison figure uses two loads in each arrangement. A load is a component that uses electrical energy, such as a lamp or resistor. The source is shown as a low-voltage source. The drawing is simplified and is not to scale.

The rules can be summarized in a table. In the table, “same” means equal for the ideal circuit pattern being described. “Shared” means the source’s voltage or current is divided among parts of the circuit. The size of each share depends on the circuit and its components.

These rules help interpret evidence. For example, readings from a suitable classroom circuit model may be compared to see whether current is equal at different points in a series path. A stated set of readings can also be used for practice. Such readings are not evidence from an experiment unless they were actually collected and their source is identified.

Circuit models show connections and comparisons; they simplify real circuits.
A measurement or stated reading is evidence only when its source is clear.
Do not treat a diagram as a record of an experiment.

Read relationships, not just numbers

Before comparing circuits, check that the source and components are the same. Then ask which feature changed: the number of paths, the current, the voltage across a component, or total resistance. This prevents a common mix-up: a change in total circuit resistance is not automatically a change in the resistance of every individual component.

A useful symbolic summary uses

I

for current,

V

for voltage, and

R

for resistance. The symbols are labels for quantities, not new kinds of electricity. For series components, the current has one value throughout the path, and the source voltage is shared across the components. For parallel branches, each branch has the source voltage, while the total current is the sum of the branch currents.

This lesson compares the three quantities by arrangement. It does not assume that a particular lamp or resistor will have a specific numerical resistance. A number can be compared only when it is given or measured in a suitable investigation.

I_{\text{source}}=I_1+I_2

Use $I$ for current, $V$ for voltage, and $R$ for resistance.
In series, current stays the same along the path; voltage is shared.
In parallel, branch voltage is the same; current divides among branches.

Evidence and limits

In a science investigation, evidence could include carefully recorded current or voltage readings from a suitable low-voltage circuit setup. The comparison should state the circuit arrangement and where each reading was taken. This lesson does not provide experimental readings, and it does not ask learners to build or test a circuit at home.

The rules here describe simple circuit patterns. Real components can behave in ways that a basic model does not show. A model is useful because it highlights relationships, but it is not a complete account of every detail. A social or ethical opinion about where electricity should be used is a judgment, not a circuit measurement.

Identify the arrangement and source of any readings before using them as evidence.
A simplified model helps compare patterns but leaves out detail.
Separate measurements from opinions about how electricity should be used.

Circuit patterns

Quantity	Series circuit	Parallel circuit
Paths	One path	More than one branch
Current	Same through each component	Divides among branches; source current is the branch total
Voltage	Shared among components	Same across each branch as the source
Total resistance	Increases when components are added in series	Adding a branch lowers total resistance compared with one branch

Worked example

Two loads in series

A hypothetical classroom data set describes two loads connected in series to a 6 V source. It reports a current of 0.20 A through each load, with voltage readings of 2 V across the first load and 4 V across the second. Use the series rules to check whether the readings fit the model.

Two loads in series

Simplified circuit model with one low-voltage source and two loads in one path. Connections are shown; sizes and internal details are not to scale.

Identify the path
The two loads are in series, so there is one path. The model predicts the same current through both loads.
$I_1=I_2=0.20\,\mathrm{A}$
Check the voltage sharing
The source voltage is shared across the loads. The two stated voltage readings add to the source voltage, so they fit the simple series model.
$2\,\mathrm{V}+4\,\mathrm{V}=6\,\mathrm{V}$
State the comparison
The readings match the expected series pattern: equal current in one path and voltage shared across the components. This checks consistency with the model; it does not prove that an experiment produced these values.

Answer: The readings fit the series pattern: both loads have 0.20 A, and their voltage readings total 6 V.

Check: The values are explicitly hypothetical practice data, not invented experimental evidence.

Worked example

Current in two parallel branches

A hypothetical practice set gives a parallel circuit with a 6 V source. Each of two branches has a voltage reading of 6 V. The branch currents are 0.15 A and 0.25 A. Find the source current and explain the voltage readings.

Two loads in parallel

Simplified circuit model with two separate branches across one low-voltage source. Connections are shown; sizes and internal details are not to scale.

Use the branch pattern
In parallel, each branch is connected across the source, so each branch has the source voltage in the simple model.
$V_1=V_2=6\,\mathrm{V}$
Combine the branch currents
The source current is shared among the branches. Add the stated branch currents to find the total current supplied by the source.
$I_{\text{source}}=0.15\,\mathrm{A}+0.25\,\mathrm{A}=0.40\,\mathrm{A}$
Interpret the result
The source provides 0.40 A in total, while the two branches carry different currents. Equal branch voltage does not mean equal branch current.

Answer: The source current is 0.40 A. Both branches have 6 V, but their currents differ.

Check: The total current equals the sum of the branch currents.

Worked example

Compare the same loads in two arrangements

Two identical loads are connected first in series and then in parallel to the same source. Without calculating numerical values, compare the current pattern, voltage pattern, and total resistance.

Same loads, two arrangements

Simplified comparison of two loads in series and in parallel, each with one low-voltage source. The figure shows connection patterns only; sizes and internal details are not to scale.

Compare current
In series, one path carries the same current through both loads. In parallel, current divides between the two branches, and the source current is their sum.
Compare voltage
In series, the source voltage is shared across the two loads. In parallel, each branch has the source voltage.
Compare total resistance
Putting the loads in series increases total resistance. With the same loads, adding a second parallel path lowers total resistance compared with having only one branch.

Answer: Series has one current path and shared voltage, with greater total resistance. Parallel has the source voltage across each branch, divided current, and lower total resistance than a single branch with the same load.

Check: The comparison changes the arrangement while keeping the source and loads the same.

Common mistakes and how to avoid them

Thinking current is shared in series.

Correction: A series circuit has one path, so current is the same through each component. It is voltage that is shared.

Thinking parallel branches have different voltages just because their currents differ.

Correction: In a simple parallel circuit, each branch has the source voltage. Branch currents may still differ.

Assuming the source current in parallel equals the current in just one branch.

Correction: Add the branch currents to describe the total current from the source.

Saying that adding a parallel branch increases total resistance.

Correction: In the simple comparison taught here, adding another parallel path lowers total resistance.

Lesson summary

A series circuit has one path; a parallel circuit has separate branches.
Series: current is the same through components, voltage is shared, and added components increase total resistance.
Parallel: each branch has the source voltage, current divides among branches, and added branches lower total resistance.
Use circuit models to compare patterns, and identify whether numerical readings are measured evidence or hypothetical practice data.

Check your understanding

Question 1

Two components are in one series loop. What is the expected current pattern?

The current is the same through both components.
The current is always zero through the second component.
The current splits into separate branch currents.
The current is different at every point in the loop.

Show answer and explanation

The current is the same through both components.

A series circuit has one path, so the current is the same through each component.

Question 2

A parallel circuit has a 5 V source. In the simple model, what is the voltage across each branch?

The voltage is 0 V in every branch.
Each branch has 5 V.
The 5 V is always divided equally among branches.
The branch voltage must be larger than 5 V.

Show answer and explanation

Each branch has 5 V.

Each parallel branch is connected across the source, so each has the source voltage.

Question 3

Two parallel branches carry 0.10 A and 0.30 A. What is the source current?

0.20 A
0.30 A
0.40 A
0.03 A

Show answer and explanation

0.40 A

The source current is the sum of the branch currents: 0.10 A plus 0.30 A equals 0.40 A.

Question 4

What happens to total resistance when a second branch is added in parallel to the same simple circuit?

It increases compared with one branch.
It becomes zero.
It stays exactly the same in every case.
It decreases compared with one branch.

Show answer and explanation

It decreases compared with one branch.

An added parallel path lowers the total resistance compared with having only one branch.

Key terms

Circuit: A complete path through which electric charge can move.
Current: The flow of electric charge.
Voltage: The electrical push that can move charge through a circuit.
Resistance: How much a material or component opposes electric current.
Series circuit: A circuit arrangement with one path through its components.
Parallel circuit: A circuit arrangement with separate branches that provide more than one path.
Branch: One separate path in a parallel circuit.
Total resistance: The resistance of the whole circuit as seen by its source.

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About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 9 Science (SNC1W), expectation D2.6. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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