Current in Series & Parallel (Edexcel IGCSE Physics (Modular))
Revision Note
Written by: Ashika
Reviewed by: Caroline Carroll
Current in series circuits
There are two ways of joining electrical components:
in series
in parallel
Current in series
A series circuit is a circuit that has only one loop, or one path that the electrons can take
In a series circuit, the current has the same value at any point
This is because the electrons have only one path they can take
Therefore, the number of electrons passing a fixed point per unit time is the same at all locations
This means that all components in a series circuit have the same current
The current is the same at each point in a series circuit
The amount of current flowing in a series circuit depends on:
the voltage of the power source
the number (and type) of components
Increasing the voltage of the power source drives more current around the circuit
So, decreasing the voltage of the power source reduces the current
Increasing the number of components in the circuit increases the total resistance
Hence less current flows through the circuit
Increasing the voltage and number of components in series
Current will increase if the voltage of the power supply increases and decreases if the number of components increases
Current in parallel circuits
A parallel circuit is a circuit that has two or more loops, or more than one path that electrons can take
Parallel circuits contain junctions and branches
Junctions are points where two or more wires meet to form a new branch
Branches are the sections of wire between junctions
Current in parallel
In a parallel circuit, the current has different values at different points in the circuit
This is because the current splits at a junction
Therefore, the electrons have different paths they can take
The sum of the current in the individual branches is equal to the total current before (and after) the branches
Current splits at a junction into individual branches
Why is current conserved at a junction in a circuit?
At a junction, the current is always conserved
This means the amount of current flowing into the junction is equal to the amount of current flowing out of it
This is because the charge is conserved
Current does not always split equally – often there will be more current in some branches than in others
The current in each branch will only be identical if the resistance of the components along each branch is identical
Current behaves in this way because it is the flow of electrons:
Electrons, or any charge, cannot be created or destroyed
This means the total number of electrons (and hence current) going around a circuit must remain the same
When the electrons reach a junction, however, some of them will go one way and the rest will go the other
Worked Example
In the circuit below, ammeter A0 shows a reading of 10 A, and ammeter A1 shows a reading of 6 A.
What is the reading on ammeter A2?
Answer:
Step 1: Recall what happens to the current at a junction
At a junction, the current splits, but is always conserved
This means that the total amount of current flowing into a junction is equal to the total amount flowing out
Step 2: Consider the first junction in the circuit where the current splits
The diagram below shows the first junction in the circuit
Step 3: Calculate the missing amount of current
Since 10 A flows into the junction (the total current from the battery), 10 A must flow out of the junction
The question says that 6 A flows through ammeter A1 so the remaining current flowing through ammeter A2 must be:
10 A − 6 A = 4 A
Therefore, 4 A flows through ammeter A2
Examiner Tips and Tricks
The direction of current flow is super important when considering junctions in a circuit.
You should remember that current flows from the positive terminal to the negative terminal of a cell / battery. This will help determine the direction current is flowing 'in' to a junction and which way the current then flows 'out'.
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