Series and parallel circuits Revision Notes for AQA GCSE Physics Combined Science

Series and parallel circuits

Understanding how electrical components can be connected is crucial for GCSE Physics. Components can be arranged in two main ways: series or parallel. Each type follows different rules for current, voltage, and resistance.

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Mastering series and parallel circuits is essential for understanding more complex electrical systems you'll encounter in your GCSE Physics exam. These concepts form the foundation for analysing household wiring, electronic devices, and electrical safety.

What are series circuits?

A series circuit has just one path for the electric current to flow around. Think of it like a single loop of string - there's only one way to go around it.

In a series circuit:

All components are connected one after the other
There are no branches or alternative paths
If one component breaks, the whole circuit stops working

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The single-loop analogy is helpful: imagine Christmas lights where if one bulb fails, the entire string goes dark. This is because they're wired in series, creating a single continuous path for electricity.

How current behaves in series circuits

Current is the same everywhere in a series circuit. This is because there's only one path for the electrons to flow through.

Ammeters are connected in series with components to measure current
All ammeters in a series circuit will show the same reading
Current cannot split up because there's nowhere else for it to go

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This is a fundamental rule: Current in series circuits is constant throughout the circuit. No matter where you measure it, you'll get the same value because electrons have no alternative path to take.

How voltage behaves in series circuits

The voltage from the power supply gets shared between all the components in a series circuit.

Each component gets a portion of the total voltage
Voltmeters are connected in parallel across components to measure their voltage
The voltages across all components add up to the total supply voltage
Formula: $V_1 + V_2 + V_3 = V_{total}$

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Worked Example: Voltage in Series

If you have a 9V battery connected to three resistors in series, and the first resistor has 3V across it and the second has 4V across it:

Step 1: Apply the voltage rule $V_1 + V_2 + V_3 = V_{total}$

Step 2: Substitute known values $3V + 4V + V_3 = 9V$

Step 3: Solve for the unknown $V_3 = 9V - 3V - 4V = 2V$

How resistance behaves in series circuits

Adding more resistors in series increases the total resistance of the circuit.

The total resistance equals the sum of all individual resistances
Formula: $R_{total} = R_1 + R_2 + R_3$
More resistance means less current flows through the circuit

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Think of this like obstacles in a pipe: each resistor adds another obstacle, making it harder for current to flow. The more obstacles you add in series, the more difficult the journey becomes for the electrons.

What are parallel circuits?

A parallel circuit has more than one path for current to flow. Think of it like a river that splits into several streams and then joins back together.

In parallel circuits:

Components are connected on separate branches
Current can take different paths
If one component breaks, the others can still work

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This is why household appliances continue to work even when one device is switched off or breaks down. Each appliance is connected in parallel, giving it an independent path to the mains supply.

How current behaves in parallel circuits

Current splits up at junctions in parallel circuits and then recombines.

The total current leaving the power supply equals the sum of currents in all branches
Formula: $I_{total} = I_1 + I_2 + I_3$
Each branch can have a different amount of current flowing through it

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Worked Example: Current in Parallel

A 12V battery is connected to three parallel branches with currents of 2A, 3A, and 1A respectively.

Step 1: Apply the current rule for parallel circuits $I_{total} = I_1 + I_2 + I_3$

Step 2: Substitute the values $I_{total} = 2A + 3A + 1A = 6A$

Therefore, the battery supplies a total current of 6A.

How voltage behaves in parallel circuits

The voltage is the same across each branch of a parallel circuit.

Each parallel branch receives the full supply voltage
All voltmeters connected across parallel branches show the same reading
This is why household appliances all work at mains voltage (230V in the UK)

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This is why your TV, computer, and lights all operate at the same mains voltage despite being different appliances. Each receives the full 230V because they're connected in parallel branches.

How resistance behaves in parallel circuits

Adding more resistors in parallel decreases the total resistance of the circuit.

The total resistance is always less than the smallest individual resistor
This is because you're providing more paths for current to flow
More paths = easier for current to flow = less total resistance

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This might seem counterintuitive, but think of it like traffic: adding more lanes to a highway makes it easier for cars to flow, even though you've added more road. Similarly, adding parallel branches gives electrons more routes, reducing the overall resistance to current flow.

Key differences between series and parallel

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Key Circuit Characteristics:

Series circuits:

One path only
Same current everywhere
Voltage splits between components
Resistances add up

Parallel circuits:

Multiple paths
Current splits at junctions
Same voltage across each branch
Total resistance decreases

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Essential Rules to Remember:

Series = Single path - current stays the same but voltage splits
Parallel = Multiple paths - voltage stays the same but current splits
Ammeters go in series with components to measure current
Voltmeters go in parallel across components to measure voltage
Series resistance adds up, parallel resistance gets smaller

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