Series and parallel circuits Revision Notes for AQA GCSE Physics

Series and parallel circuits

What are series and parallel circuits?

Series circuits have only one path for current to flow. All components are connected in a single loop, one after another.

Parallel circuits have more than one path for current to flow. Components are connected in separate branches, and current splits at junctions then rejoins.

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Understanding the difference between these two circuit types is fundamental to electrical theory. The way current flows determines how all other electrical properties behave in the circuit.

Series circuits

How current behaves in series circuits

In a series circuit, current is the same at every point. This happens because there's only one path for the current to follow.

An ammeter measures current and has very low resistance
All ammeters in a series circuit will show the same reading
Current cannot build up anywhere in the circuit

chatImportant

Because there's only one path for current, if any component in a series circuit fails or is removed, the entire circuit stops working. This is why old Christmas lights would all go out if one bulb burned out!

How voltage behaves in series circuits

In series circuits, voltage is shared between components.

Each component gets part of the total voltage from the cell
The voltages across all components add up to equal the cell voltage
Formula: $V_{total} = V_1 + V_2$ (for two components)
Voltmeters have very high resistance so no current flows through them

lightbulbExample

Worked Example: Voltage in Series

In a series circuit with a 12V battery and two resistors:

If the first resistor has 7V across it
Then the second resistor must have: $V_2 = V_{total} - V_1 = 12V - 7V = 5V$

This demonstrates how voltages always add up to the supply voltage.

How resistance behaves in series circuits

When you add more resistors in series, total resistance increases.

More components means more opposition to current flow
Current decreases when resistance increases (using $V = IR$ )
Formula: $R_{total} = R_1 + R_2$ (for two resistors)

Parallel circuits

How current behaves in parallel circuits

In parallel circuits, current splits up at junctions.

Current divides between the different branches
Current rejoins after passing through the branches
Formula: $I_{total} = I_1 + I_2$ (current entering = sum of currents in branches)

infoNote

Think of current in parallel circuits like water flowing through multiple pipes. The total flow splits between the pipes but rejoins afterward, and the total amount is conserved.

How voltage behaves in parallel circuits

In parallel circuits, voltage is the same across each branch.

Each branch gets the full cell voltage
All voltmeters connected across different branches show the same reading
Each component receives maximum voltage from the cell

chatImportant

This is why household electrical outlets provide the same voltage to every appliance - they're all connected in parallel branches from the main supply.

How resistance behaves in parallel circuits

When you add more resistors in parallel, total resistance decreases.

More branches give current more paths to flow through
This makes it easier for current to flow overall
Total resistance is always less than the smallest individual resistor

lightbulbExample

Worked Example: Parallel Resistance

If you have two 10Ω resistors in parallel:

Each provides a path for current
Total resistance = $\frac{1}{\frac{1}{10} + \frac{1}{10}} = \frac{1}{\frac{2}{10}} = 5Ω$
The total resistance (5Ω) is less than either individual resistor (10Ω)

Key differences

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Comparison of Circuit Types

The table below summarises the fundamental differences between series and parallel circuits:

Series circuits	Parallel circuits
Current is the same everywhere	Current splits between branches
Voltage is shared between components	Voltage is the same across each branch
Adding components increases total resistance	Adding components decreases total resistance
If one component breaks, the whole circuit stops	If one component breaks, others keep working

bookmarkSummary

Key Points to Remember:

Series circuits: Same current everywhere, voltage shared, resistance adds up
Parallel circuits: Current splits, same voltage across branches, resistance decreases
Ammeters go in series with components to measure current
Voltmeters go in parallel with components to measure voltage
In series: $V_{total} = V_1 + V_2$ , $R_{total} = R_1 + R_2$
In parallel: $I_{total} = I_1 + I_2$ , voltage same across all branches

Series and parallel circuits (AQA GCSE Physics): Revision Notes