This grade 7 science lesson on series and parallel circuits notes gives the key ideas first, then worked examples and diagrams. After that, practise with the worksheet on series and parallel circuits and check yourself with the quiz.
Quick answer: In a series circuit you join the components in one single loop, so there is only one path for the current and the bulbs share the supply. However, in a parallel circuit they sit on separate branches, so each bulb works on its own and gets the full voltage of the cells.
Level: Grade 7 science (Year 8 · Class 7) · Curriculum: MS-PS2-3
Series and parallel circuits notes: the key ideas
What is the difference between series and parallel circuits?
A circuit is a complete loop that lets charge flow. Also, when you have more than one component, such as two bulbs, there are two basic ways to join them.
A series circuit joins the components one after another in a single loop, so there is only one path for the current.
By contrast, a parallel circuit puts the components on separate branches, so there is more than one path for the current.
How do bulbs behave in series and in parallel?
The pattern is easy to see in a circuit with identical bulbs. In series, the bulbs depend on each other. In parallel, each bulb works on its own branch.
| Feature | Series circuit | Parallel circuit |
|---|---|---|
| Paths for the current | One | Two or more |
| One bulb blows or is removed | All bulbs go out | The other bulbs stay on |
| Adding more bulbs | Every bulb gets dimmer | Every bulb stays as bright |
| A switch | Anywhere in the loop turns everything off | In a branch, it turns off only that branch |
| Typical use | Old fairy lights, safety interlocks | House lights, sockets, car headlamps |
Emma in Leeds has a string of four old fairy lights wired in series. In addition, one bulb sits loose in its holder. Predict what the other three bulbs do and explain why.
The loose bulb breaks the loop, so the current has no path. All four bulbs go out.
Femi in Lagos has three ceiling lamps wired in parallel. Moreover, one lamp blows. Predict what the other two do and explain why.
Each lamp has its own branch connected straight to the supply. The other two stay on at the same brightness.
Many students think the first bulb “uses up” the current, so the later ones are dimmer. In reality, bulbs in series are dimmer because they share the energy from the supply, not because the first bulb takes the current.
How do cells join together?
You can join cells in series or in parallel. In series, with the + end of one touching the − end of the next, their voltages add. In parallel, the voltage stays the same as one cell, but the cells share the work, so the supply lasts longer.
V total = V₁ + V₂ + V₃ + …
V total is the supply voltage of cells in series (V). V₁, V₂, V₃ are the voltages of the individual cells (V).
Wanjiru in Nairobi has a torch that needs 6.0 V. Similarly, each of her cells gives 1.5 V. Calculate how many cells she must join in series.
Number of cells = 6.0 V ÷ 1.5 V = 4 cells.
What is a short circuit and why is it dangerous?
A short circuit is an easy path, often a wire or damaged insulation, that lets current bypass the components. As a result, a very large current can flow, and the wires and cells get hot.
- A fuse contains a thin wire that melts if the current gets too large. Also, it must be in series with the appliance.
- Also, a circuit breaker switches off automatically and you can reset it.
- Never join a wire straight across a battery or the mains, and never use cables with damaged insulation.
- Keep water away from electrical sockets and plugs.
To decide whether bulbs are in series or parallel, put your finger on the + end of the cell and follow the wire. In addition, if you reach a point where the path splits, the circuit has parallel branches.
- Series: one path; if one bulb fails, all go out.
- Parallel: separate branches; the other bulbs stay lit.
- Cells in series: voltages add (4 × 1.5 V = 6.0 V).
- Also, a short circuit is an easy path that bypasses the components.
- You place a fuse in series, and it melts if the current is too large.
Series and parallel circuit patterns: frequently asked questions
What is the main difference between series and parallel circuits?
In a series circuit there is one path for the current, so the components share the supply and depend on each other. In a parallel circuit there are separate branches, so each branch works independently.
Why are houses wired in parallel?
Parallel wiring gives each lamp or socket the full supply voltage, so you can switch each one on or off without affecting the others. Also, if one lamp fails, the rest keep working.
What happens if one bulb blows in a series circuit?
The loop breaks, so the current stops and all of the bulbs go out. However, in a parallel circuit the other bulbs stay lit.
What is a short circuit?
A short circuit is an easy path that bypasses the components, so a very large current flows. Moreover, it can overheat wires and cells and cause a fire, which is why we fit fuses and circuit breakers.
Do bulbs stay bright when added in parallel?
Yes, because each bulb has its own connection to the supply, so each gets the full voltage of the cells. The cells run down faster, though, since they supply more current.
