2.7-2.8 - Series and parallel circuits
Series and parallel circuits are two different ways to connect components to a supply. The choice is not just a drawing style: it changes whether components depend on each other and how the current in the circuit responds when the supply or components change. In this lesson you will learn how to choose the more appropriate circuit for an application, including domestic lighting, and how to reason qualitatively about current in a series circuit.
One path and branches
A circuit must make a complete conducting path from one terminal of the supply to the other. Series and parallel circuits differ in the number of paths available through the components.
Series circuit
A series circuit has components connected one after another in a single path.
In a series circuit, the same route passes through every component. If that route is broken at any point, there is no complete path, so current cannot flow anywhere in the circuit.
Parallel circuit
A parallel circuit has components connected in separate branches across the supply.
In a parallel circuit, each branch gives a separate route through part of the circuit. Opening one branch does not necessarily break the other branches, because they can still form complete paths with the supply.
[DIAGRAM: asset_name: Series and parallel circuits - diagram 01; asset_slug: p15_series_and_parallel_circuits__diagram_01; recommended_method: image_gen; description: NovaLearn pale-palette two-panel circuit schematic: Series is a complete one-path loop with a cell and two lamps; Parallel has a standard cell and exactly two closed lamp branches, with no rail stubs or bypass wire.]
Choosing the circuit
When Edexcel asks which circuit is more appropriate, it is usually looking for a reason linked to how the circuit behaves, not a vague answer such as "it is better".
| Need in the application | More suitable arrangement | Physics reason |
|---|---|---|
| One switch should turn the whole circuit on or off | Series switch with the load | The switch is in the only path, so opening it breaks the whole circuit. |
| Several lamps should be controlled independently | Parallel branches | Each lamp can have its own branch and switch. |
| One lamp failing should not turn off all the other lamps | Parallel branches | Other branches can still have complete paths. |
| All components should stop together when one part opens | Series arrangement | A break anywhere in the single path stops the current everywhere. |
Domestic lighting is a key example. A house does not use one long series chain of lamps. If it did, one failed lamp or one open switch could break the only path and turn off many lights. Also, changing the number of lamps in the chain would change the current through all of them.
Domestic lights are connected in parallel branches. This means a bedroom light, kitchen light and hallway light can be switched independently. If one lamp fails, the other branches can still work because they are not all relying on the same single path. Each branch is connected directly across the supply, so each lamp receives the full supply voltage and works at its normal brightness: adding another parallel lamp does not make the existing lamps share a smaller voltage (assuming the supply voltage stays constant).
Series circuits are still useful when the whole circuit should be controlled together. For example, a simple switch is placed in series with a lamp so that opening the switch breaks the only path to that lamp.
Current in a series circuit
In a series circuit, there are no junctions where current can split into different branches. The current has only one route, so the current is the same through each series component.
This does not mean the current is "used up" by the first lamp or resistor. Current is the rate at which charge passes a point in the circuit. In a complete series circuit, the same charge flow passes through each component in turn.
The brightness of lamps in series gives a useful clue. If two identical lamps are connected in series to one cell, the same current passes through both lamps. If a third identical lamp is added in series, there is still only one current, but it is smaller than before, so all three lamps are dimmer.
The important series idea is global: a change anywhere in the single path affects the current everywhere in that path.
What changes series current
The specification asks how current in a series circuit depends on three things: the applied voltage, the number of components and the nature of the components.
The applied voltage is the voltage supplied across the whole series circuit. If the same series circuit is connected to a larger applied voltage, for example by adding a second cell in series facing the same way, the current tends to increase. If the applied voltage is reduced, the current tends to decrease.
The number of resistive components matters because every series component is in the only path. At the same supply voltage, adding another lamp or resistor in series increases the total resistance, so the current decreases. Removing a series component, while still keeping a complete circuit, usually makes it easier for current to flow, so the current increases.
The nature of the components matters because different components affect the current differently. How strongly a component opposes the current is called its resistance: the greater the total resistance in the single path, the smaller the current. A short connecting wire has very little effect compared with a lamp or resistor. An open switch, broken lamp or disconnected wire is different again: it creates a gap, so there is no complete path and the current is zero.
| Change to the same basic series circuit | Expected effect on current | Why |
|---|---|---|
| Increase the applied voltage | Current increases | The supply pushes charge around the circuit more strongly. |
| Decrease the applied voltage | Current decreases | The supply pushes charge around the circuit less strongly. |
| Add another lamp in series | Current decreases | There is another component in the only path. |
| Replace a lamp with a component that opposes current less | Current increases | The single path is easier for current to pass through. |
| Open the switch | Current becomes zero | The circuit is incomplete. |
Changing the supply or the load
Adding a cell changes the supply; adding a lamp changes the load. These changes have different effects, even though both add a component to the same circuit.
For example, two 1.5 V cells connected in series in the same direction provide 3.0 V. Their voltages add because the positive terminal of one is connected to the negative terminal of the other. This larger applied voltage drives a larger current through an unchanged resistive load. If one of the two identical cells is reversed, their voltages oppose and the total is zero: adding a cell does not always increase the supply voltage.
By contrast, adding a second lamp in series with the first adds resistance while the supply voltage stays the same. The current falls everywhere in the single path. The first lamp does not receive less current because the second lamp uses charge up; the current is smaller through both.
Quick recap
- Series components share one path and the same current. Opening that path stops current throughout the circuit.
- Parallel branches allow independent operation; each lamp connected across the supply receives the full supply voltage.
- At fixed supply voltage, adding a lamp or resistor in series increases total resistance and reduces current everywhere in that path.
- Adding a cell increases supply voltage only when its voltage aids the existing cell. Reversed cells oppose each other.