There are only two ways to connect components into a circuit: one after another, or side by side. Old Christmas lights died completely when one bulb blew; the lights in your home keep working when one fails. The difference is entirely about whether they are wired in series or in parallel, and learning to read those two patterns in a circuit diagram unlocks every circuit you will ever analyse.
Two identical globes are connected to one battery. In one circuit they are joined one after the other; in the other they are joined side by side across the battery.
Before reading: in which arrangement does removing one globe stop the other from lighting? Write your prediction and a one-sentence reason.
Series means a single loop; parallel means separate branches
Series and parallel are simply the two ways charge can travel: through everything in turn along one road, or splitting between several roads that rejoin.
In a series circuit the components form a single unbroken loop, so the same current must pass through each one in turn. In a parallel circuit the components are connected across the same two points, giving the current a choice of branches; the current divides among them and recombines.
In a series circuit, components are in one loop and the same current flows through each. In a parallel circuit, components are on separate branches across the same two points, so each branch has the same voltage across it.
Pause, copy the highlighted statement into your book before moving on.
Did you get this? True or false: in a single series loop, the current is the same at every point.
Standard symbols and how to trace the connections
We just saw the difference between one path and many. That raises a question: how do we record and recognise these connections on paper? This card answers it → use standard circuit symbols and trace the wires from one terminal of the source to the other.
A circuit diagram strips away the messy real wiring and shows only what is connected to what, using a small set of agreed symbols.
Key symbols: a cell is a long thin line (+) beside a short thick line (−); a resistor is a rectangle; a globe is a circle with a cross; a switch is a break in the line with a hinged lever. To classify a circuit, trace from one battery terminal: if you pass through every component before returning, it is series; if the path splits at a junction and the parts rejoin, those components are in parallel.
A circuit has a cell connected to a junction. From the junction, one wire goes through resistor $R_1$ and another goes through resistor $R_2$; both wires rejoin before returning to the cell. Are $R_1$ and $R_2$ in series or parallel, and what do they share?
To analyse a circuit, trace the current path from the source. A single unbroken path = series (shared current). A path that splits into branches and rejoins = parallel (shared voltage). Draw circuits with the standard symbols for cells, resistors, globes and switches.
Add the highlighted method to your notes before the check below.
Quick check: Two resistors are connected so the current must pass through the first, then the second, with no branching. They are connected in:
Current is not used up round a loop, and adding a branch lowers the total resistance
Use the simulation. Set Series, two lamps, $12\ \text{V}$, with Lamp 1 at $2\ \Omega$ and Lamp 2 at $10\ \Omega$. The ammeter reads $1.000\ \text{A}$ wherever you put it. Which lamp is brighter, and why?
Use the simulation. Start with one $6\ \Omega$ lamp on $12\ \text{V}$, then add a second identical lamp in parallel. True or false: the current through the first lamp halves, just as it would if the second lamp were added in series.
Pattern B, Construct then compare
Using standard symbols, in your book draw (1) a cell with two globes in series, and (2) a cell with two globes in parallel. For each: label the current path(s), then write one sentence stating what happens to the second globe if the first globe is removed, and explain why using the idea of a single path versus branches.
Two truths, one lie: three of these statements COMPARING series and parallel connections are correct. Which one is the lie?
Dot point: Compare the connection of components in series circuits with that in parallel circuits
Current, voltage and what happens when one component fails
We just learned to recognise the two connections in a diagram. That raises a question: what are the practical consequences of choosing one over the other? This card answers it → they share current and voltage in opposite ways, and they fail differently.
The single-path versus many-paths difference decides everything else: which quantity is shared, and whether one broken component shuts down the rest.
In series: there is one current, so the same $I$ flows through every component, and the supply voltage is shared between them. If any component breaks, the single loop is broken and everything stops, this is why old fairy lights all went out together.
In parallel: each branch is connected straight across the supply, so every branch gets the full voltage, and the total current divides between the branches. If one branch breaks, the others still have a complete path, so they keep working, which is why household circuits are wired in parallel.
Series: one current shared path, voltage divides between components, one break stops all. Parallel: full supply voltage on every branch, current divides, one break leaves the others working. Homes use parallel so appliances run independently at the same voltage.
Pause, copy the highlighted comparison into your book before the check below.
Last check: Decide if the statement is true or false.
Household lighting is wired in parallel so that each light receives the full supply voltage and one light failing does not switch off the others.
Fill the blanks: Complete the rules for READING a circuit diagram.
On a circuit diagram, components drawn one after another on a single wire with no junction between them are in . Where the wire divides at a junction and the paths later rejoin, the components in those paths are in .
Dot point: construct and analyse diagrams of series circuits and parallel circuits
A fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
UnderstandBand 4(4 marks) 1. Compare a series circuit and a parallel circuit, referring to how the current and the voltage are shared, and to what happens to the rest of the circuit when one component is removed.
1 mark: series has one current path, same current through each component · 1 mark: parallel has branches, each with the full supply voltage and current divides · 1 mark: in series, supply voltage is shared between components · 1 mark: removing a component stops a series circuit but parallel branches keep working
AnalyseBand 5(3 marks) 2. A string of decorative lights is wired so that when one lamp fails, all the lamps go out. A second string is wired so that one failure leaves the rest lit. Identify the connection used in each string and explain the difference.
1 mark: the first string is in series, the second is in parallel · 1 mark: a series break interrupts the single path, so no current flows anywhere · 1 mark: in parallel each lamp has its own path, so the others still form complete circuits
Dot point: Compare the connection of components in series circuits with that in parallel circuits
ApplyBand 5(3 marks) 3. Describe how you would decide, by tracing a circuit diagram alone, whether two resistors are connected in series or in parallel. State the feature you look for in each case, and describe how you would then redraw a parallel pair as a single equivalent component.
1 mark: series if a single traced path passes through both with no junction between them · 1 mark: parallel if the path divides at a junction and rejoins, so both lie across the same two points · 1 mark: a parallel pair is redrawn as one component between those same two junctions, carrying the equivalent resistance
Dot point: construct and analyse diagrams of series circuits and parallel circuits
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Q1 (4 marks): In a series circuit the components are joined in a single loop, so there is only one current path and the same current flows through every component; the supply voltage is shared between the components. In a parallel circuit the components are on separate branches connected across the same two points, so each branch experiences the full supply voltage while the total current divides among the branches. The two arrangements also fail differently: removing or breaking any one component in a series circuit breaks the single loop and stops the current everywhere, so nothing works. In a parallel circuit, removing one branch still leaves complete paths through the other branches, so the remaining components keep operating, which is why household appliances are wired in parallel.
Q2 (3 marks): The first string is wired in series and the second in parallel. In a series string all the lamps lie on one single conducting path, so when one lamp fails its filament breaks and the path is interrupted. There is then no complete circuit anywhere, no current can flow, and every lamp goes out regardless of its own condition. In a parallel string each lamp is connected across the same two supply points and therefore has its own independent path. If one lamp fails, only that branch is broken; every other branch is still a complete circuit connected across the full supply voltage, so the remaining lamps continue to operate normally at their usual brightness. This is why household wiring and modern light strings use parallel connection.
Q3 (3 marks): Start at one supply terminal and trace the conducting path. If you can pass through the first resistor and then the second without ever reaching a junction, there is only one route and the two are in series. If instead the path divides at a junction, with one resistor in each of the two branches, and the branches meet again at a second junction, then both resistors are connected between the same pair of points and they are in parallel. The two junctions are the feature to look for: their presence identifies parallel, their absence identifies series. To simplify the diagram, a parallel pair can be redrawn as a single component placed between those same two junction points, labelled with the equivalent resistance of the pair. The rest of the diagram is unchanged, and the redrawn circuit behaves identically as far as the supply is concerned.
You can now tell series from parallel, draw both with standard symbols, and describe how each shares current and voltage. Next lesson shows where to connect ammeters and voltmeters to measure them.
↩ Back to focus-area hubAt the start you predicted which arrangement loses both globes when one is removed. The answer is the series arrangement: a single loop means breaking it anywhere stops all the current. In parallel each globe sits on its own branch, so the other keeps lighting. If you predicted parallel, revisit Card 3, it is the single shared path that makes series fragile.
A short spaced-retrieval set, mixing this lesson with earlier ones. It gets harder to guess and easier to remember.
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