Series & Parallel Circuits (AQA A Level Physics)
Revision Note
Series & Parallel Circuits
Current
In a series circuit, the current is the same for all components
In a parallel circuit, the current is split across the different branches (or junction). The total current into a junction must equal the total current out of a junction
The amount of current in each branch depends on the total resistance of the components within that branch
Potential Difference
In a series circuit, the e.m.f of the power supply is shared amongst all the components in different amounts, depending on their resistance
In a parallel circuit, the voltage of all the components in each branch is equal to the e.m.f of the power supply
Cells can also be connected in series or parallel
The total voltage of the combined cells can be calculated in the same way as voltage
If the cells are connected in series, the total voltage between the ends of the chain of cells is the sum of the potential difference across each cell
If the cells are connected in parallel, the total voltage across the arrangement is the same as for one cell
A summary of the current, voltage and resistance within a series and parallel circuit are summarised below:
Table of Voltage, Current & Resistance in Series & Parallel Circuits
Examiner Tips and Tricks
The best way to practice these calculations is to understand why you have made a mistake. Common mistakes are:
Thinking the current is the same through every branch in a parallel circuit
Thinking the voltage is the same through all components in a series circuit
Not taking into account multiple resistors
Not calculating the total resistance using the appropriate parallel or series resistance equation
The sum of the voltages of all the components in a series circuit not adding up to the e.m.f of the supply
The current into a junction not being equal to the current out of a junction (Kirchoff’s First Law)
Don’t be afraid to annotate circuit diagrams to help with this. The more information you have about all the components, the easier it is to calculate values that are missing
Conservation of Charge & Energy
Conservation of Charge
Charge is never used up or lost in a circuit - this is known as conservation of charge
As a result of this, for current in a parallel circuit:
The sum of the currents entering a junction always equal the sum of the currents out of the junction
This is sometimes known as Kirchhoff's First Law
In a circuit:
A junction is a point where at least three circuit paths meet
A branch is a path connecting two junctions
If a circuit splits into two branches, then the current before the circuit splits should be equal to the current after it has split
In the circuit below, I = I1 + I2+ I3, where I represents the current in the circuit before it branches, and I1, I2 and I3 represent the current in the respective three branches:
The current I into the junction is equal to the sum of the currents out of the junction
The charge is conserved on both sides of the junction
In a series circuit, the current is the same through all components
The current is the same at each point in a series circuit
In a parallel circuit, the current divides at the junctions and each branch has a different value
Kirchhoff’s First Law applies at each junction
The current divides at each junction in a parallel circuit
Conservation of Energy
Energy is never used up or lost in a circuit - this is known as conservation of energy
As a result of this, for voltage in any circuit:
The total e.m.f. in a closed circuit equals the sum of the potential differences across each component
This is sometimes known as Kirchhoff's Second Law
Each closed circuit can be treated like a series circuit
Below is a circuit explaining Kirchhoff’s Second Law with the sum of the voltages in the closed series circuit equal to the total e.m.f:
The sum of the voltages is equal to the total e.m.f from the batteries
In a series circuit, the voltage is split across all components depending on their resistance
The sum of the voltages is equal to the total e.m.f of the power supply
In a parallel circuit, the voltage is the same across each closed loop
The sum of the voltages in each closed circuit loop is equal to the total e.m.f of the power supply:
The sum of the e.m.fs in each closed loop is equal to the total e.m.f of the power supply
A closed-circuit loop acts as its own independent series circuit and each one separates at a junction. A parallel circuit is made up of two or more of these loops
Each circuit loops acts as a separate, independent series circuit
This makes parallel circuits incredibly useful for home wiring systems
A single power source supplies all lights and appliances with the same voltage
If one light breaks, voltage and current can still flow through for the rest of the lights and appliances
Worked Example
For the circuit below, state the readings of ammeters A1, A2 and A3.
Answer:
Worked Example
For the circuit below, state the readings of the voltmeters V1, V2 and V3.All the lamps and resistors have the same resistance.
Answer:
Examiner Tips and Tricks
Junctions only appear in parallel circuits and as circuits become more complex, it can be confusing as to which currents are into the junction and which are out. Drawing arrows on the diagram for the current flow (making sure it’s from positive to negative) at each junction will help with this
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