Series and parallel circuits
In this topic you will:
- find out the difference between series and parallel circuits
- find out how current flows in a parallel circuit
- compare current and voltage in series and parallel circuits
- describe the effects of adding cells and lamps on current and voltage in circuits
Getting started
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A cell connected in series with a lamp and buzzer using straight lines for wires. All components are connected end-to-end in a single loop.
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An ammeter.
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The current is 2 A. In a series circuit, the current is the same at all points.
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The lamp will go out. If the buzzer breaks, the circuit is incomplete and current cannot flow.
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The buzzer will also stop working. A break in any part of a series circuit stops the current from flowing.
Key words
- branches
- connected in parallel
- connected in series
- parallel circuit
- battery
- mains
- rating
- supply
- voltage
- voltmeter
- volts
Series circuits
The circuits you used at Stage 7 were all series circuits. Series means all the components are connected end-to-end or one after the other. We sometimes say the components are connected in series.
In a series circuit, there is only one path for the current to flow. This means the current is the same all the way around a series circuit. In a series circuit, all of the current flowing out of one component flows into the next component.
The circuit in this diagram is a series circuit.
If the switch in this circuit is opened, both the lamp and the buzzer will stop operating. They both stop operating at the same time because the current in the whole circuit stops flowing when the switch is opened.
If we want to operate the lamp and the buzzer separately from the same cell, then we need a parallel circuit.
Parallel circuits
In a parallel circuit, there is more than one path for the current to flow. The paths where the current can flow are called branches.
The name parallel comes from the circuit diagram because the branches are drawn using parallel lines. The components in each of the branches are sometimes said to be connected in parallel.
Look at the parallel circuit in the diagram to the right. Current from the cell flows to the branch in the circuit. At the branch, the current is divided. If the two lamps are the same, the current will be divided equally between them.
When the current comes to the other side where the branches join again, the current combines (adds together) again.
The parallel circuit in this diagram below has ammeters to show how the current is shared between the branches.
Common Mistake
Students often assume that current is the same in all parts of a parallel circuit. However, the total current splits at the branches and adds up again when they rejoin.
Worked Example: Current in Parallel Circuits
The rule for parallel circuits is:
the current through the cell is equal to the total of the current in all the branches.
This circuit diagram can be used to summarise the rule for parallel circuits. In the first circuit on the right, the readings on ammeters A₁ and A₄ are equal. The readings on ammeters A₂ and A₃ will add up to the reading on A₁ (and A₄).
This rule still works even when the current in the branches is different. Look at this second circuit:
The circuit has three parallel branches, each with a different current. The current through the cell is equal to the total of the currents through each of the branches:
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1.0 A + 2.0 A + 0.5 A = 3.5 A
In the circuit below, the total current is 5.5 A. Three known branch currents are 3.0 A, 1.5 A, and one unknown (X). We can calculate X.
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X + 3.0 + 1.5 = 5.5
X + 4.5 = 5.5
X = 5.5 - 4.5
X = 1.0
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The missing current through the buzzer is 1.0 A.
As with all calculations, don’t forget to include the correct unit (A) in your final answer!
Advantages of parallel circuits
In a parallel circuit, the current through a branch can keep flowing, even if the current stops flowing in the other branches.
This means:
- components in the same circuit can be switched on and off independently
- if a component in one branch stops working, the other branches are not affected
Look at the parallel circuit in the diagram on the right. The circuit has two branches. Each branch has a lamp and a switch.
When switch S₁ is closed, then lamp L₁ will light. This will not affect lamp L₂ because L₂ is on a different branch.
When switch S₂ is closed, then lamp L₂ will light. This will not affect lamp L₁ because L₁ is on a different branch.
If both lamps are switched on and lamp L₁ stops working, then lamp L₂ will not be affected.
Components in a parallel circuit can be switched on and off separately by having switches on each branch. The components can also be all switched on or off together if the switch is between the cell and the branches.
Look at this second circuit diagram on the right. For any lamp to light in this circuit, switch S₄ must be closed.
Switches S₁, S₂ and S₃ can then be used to control each lamp separately.
If all the lamps are on, then opening S₄ will cause all of the lamps to go off.
If all the lamps are off, but switches S₁, S₂ and S₃ are closed, then closing S₄ will cause all lamps to light together.
Quick Fact
Parallel circuits are commonly used in household wiring. This allows devices to operate independently and safely, even if one part of the circuit fails.
Questions
- A The current through the cell is equal to the current in the first branch.
- B The current through the cell is equal to the current in the last branch.
- C The current through the cell is the total of the current in each branch.
- D The current through the cell does not depend on the current in the branches.
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C — The total current through the cell is the sum of the branch currents in a parallel circuit.
Write the letter or letters that are correct.
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C and D are parallel circuits — the components are on separate branches.
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One single loop with a cell and four lamps in a line, connected end-to-end.
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Four branches, each with a lamp, all connected across a cell in parallel layout.
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- Each lamp can be controlled independently
- If one lamp fails, the others will still work
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Current per lamp = total current ÷ number of lamps
Current = 2.0 A ÷ 4 = 0.5 A
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A: X = 2 A (since 1 A + X = 3 A)
B: X = 2 A (top and bottom add to middle)
C: X = 1.5 A (total 3 A - others)
D: X = 1.0 A (0 + 0.5 + 0.5 = 1 A)
5. Calculate the missing current, X, in each of these circuits.
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6 A = X + 3 A
X = 6 A − 3 A
X = 3 A
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1.0 A = 0.4 A + X
X = 1.0 A − 0.4 A
X = 0.6 A
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0.3 A = 0.2 A + X
X = 0.3 A − 0.2 A
X = 0.1 A
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0.25 A = 0.15 A + 0 A + X
X = 0.25 A − 0.15 A
X = 0.1 A
Think Like a Scientist
In this investigation, you will make predictions about current and then test your predictions. Work in groups of two or three.
You will need:
- three lamps
- one buzzer
- other electrical components (optional)
- correct number of cells to operate these components
- wires
- ammeter
Steps
- Set up a circuit as shown in the first diagram.
- When you know the circuit is working correctly, put the ammeter in the position shown in the second diagram.
- Remove the ammeter and put it in the position shown in the third diagram.
Follow-Up Questions
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Use a table to record A₁ and A₂ values for each test configuration.
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A₃ should be approximately equal to A₁ + A₂ in a parallel circuit.
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Compare the measured A₃ value with your prediction. If close, the prediction is validated.
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Total current = sum of branch currents if the branches are wired correctly.
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Use standard symbols and label all ammeter positions (e.g., A₁, A₂, A₃).
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Remaining branch current = total current − sum of known branch currents.
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Each diagram should clearly show ammeter positions and expected current paths.
Peer Assessment
For each of these statements, decide how confident you are. Give yourself 5 if you are very confident and 1 if you are not confident at all.
- I know how to build parallel circuits from circuit diagrams.
- I know how to measure the current at different positions in a parallel circuit.
- I know how to correctly predict a missing current value in a parallel circuit.
Connecting What You've Learned
In the previous lesson, you explored parallel circuits — systems where electric current can take multiple paths. You learned that in parallel circuits, the voltage across each component is the same, and adding more components doesn't reduce the brightness of the bulbs because the current divides across the branches.
Now, you're ready to dig deeper into what makes this happen: voltage. Understanding voltage is key to predicting how components behave in both series and parallel circuits. You'll see how changing the voltage affects things like brightness, current strength, and energy transfer.
This next section will help you answer questions like: What exactly is voltage? How does it drive current? And why does it behave differently in parallel vs. series circuits?
Quick Fact
Voltage is sometimes called “electric pressure” because it pushes electric charges through a circuit. More voltage means more push — which often means more current flowing.
What is voltage?
Voltage is linked to the electrical energy in a circuit. Voltage is measured in units called volts. The symbol for volts is V.
Voltage is related to the electrical energy supplied to a circuit by the cell, battery or power supply. Voltage is linked to energy, but it is not the same as energy.
Most cells supply 1.5 V. A battery is two or more cells connected in series. Batteries commonly supply 6 V, 9 V or 12 V. Each of the cells or batteries in this picture has a different voltage.
The sockets found on the walls of buildings supply a mains voltage. Mains in this context means an electrical supply that comes from a power station or generator of some kind. In most countries the mains voltage is between 220 and 240 V. In some countries, the mains voltage is 110 or 120 V. The next picture shows some mains sockets from different countries.
Sometimes, we refer to the source of energy in a circuit as the supply. The supply could be a cell, a battery, a power supply or the mains.
This diagram shows circuit symbols for a battery made from two cells, a battery made from many cells, and a 240 V mains supply.
Voltage is also linked to the energy changed by components in a circuit. For example, lamps change electrical energy into light and thermal energy. Most components have a voltage rating.
The lamps used in schools for electrical experiments are often rated at 3 V or 6 V. The rating tells us the maximum voltage that can be used.
Measuring voltage
Voltage is measured using a voltmeter.
This picture shows a digital voltmeter, an analogue voltmeter and the circuit symbol for a voltmeter.
A voltmeter is connected in a different way to an ammeter.
An ammeter measures the current flowing through a component, so the ammeter is connected in series with the component.
Common Mistake
Students often confuse voltage with current. Remember: voltage is the energy per unit charge, while current is the rate of flow of charge.
The voltmeter measures the energy difference either side of a component, so the voltmeter is connected in parallel with the component.
Look at the drawing of the circuit and circuit diagram. The ammeter is connected in series with the lamp and the voltmeter is connected in parallel with the lamp.
Voltage in a series circuit
Energy is always conserved, so the energy changed by the components in a circuit must be equal to the energy supplied by the cell, battery or power supply.
That means the voltages across each component in a series circuit must add up to the voltage of the supply.
Look at the circuit in this diagram. All three lamps are identical. They change the same quantity of energy, so have the same voltage. The voltage across all the lamps adds up to the voltage from the battery.
Look at this next circuit, where the components are not the same.
In this circuit, the lamp is changing more energy than the buzzer, so the voltage across the lamp is higher than the buzzer. The voltages across the lamp and buzzer add up to the voltage of the battery.
Important Concept
In a series circuit, the total voltage of the power supply is shared between the components. Identical components receive equal voltage, while different components receive voltages proportional to the energy they convert.
The voltages across all the components in a series circuit add up to the voltage of the supply.
We discovered at Stage 7 that the current is the same all the way around a series circuit. This is because the circuit has no branches and the current can only flow in one path. The voltage in a series circuit can be different across different components.
Adding more components in a series circuit
The voltage from the supply in a series circuit is shared between each of the components. That means adding components such as lamps or buzzers will cause each component to get a smaller share of the voltage.
Compare these two series circuits. Both have the same type of battery and both have identical lamps.
Adding more components in a series circuit will decrease the current. As components are added, it becomes more difficult for the power supply to push the electrons around the circuit.
Compare these two series circuits. The one with more components has a smaller current.
Common Mistake
Students sometimes assume the voltage stays the same for each component when more are added in series. In fact, the voltage is divided among them, so each gets a smaller share.
Adding more cells in a series circuit increases the voltage of the supply.
One 1.5 V cell gives a supply voltage of 1.5 V.
Two 1.5 V cells gives a supply voltage of 2 × 1.5 V = 3 V.
A 12 V battery contains eight cells each of 1.5 V making 8 × 1.5 V = 12 V.
Increasing the number of cells in the same series circuit will:
- increase the current in the circuit
- increase the voltage across each component
Voltage in a parallel circuit
Look carefully at the drawing of a parallel circuit.
Lamp 1 is connected directly across the terminals of the 1.5 V cell. The voltage across lamp 1 is 1.5 V.
If you follow the wires from lamp 2, you can get to the terminals of the cell without going through lamp 1. That means we can think of lamp 2 also being connected directly across the terminals of the cell. That means the voltage across lamp 2 is also 1.5 V.
The voltages across each of the branches of a parallel circuit are equal to the voltage of the supply.
The voltage of the battery in both circuits is 9 V.
In the left circuit, the voltage across each lamp is also 9 V.
Look at the right circuit and you will see that the voltage across the branches of a parallel circuit is the same whether or not the components are the same. The lamp and the buzzer are different, but the voltage across the branches is still the same (9 V).
Last topic explained that the current can be different in the branches of a parallel circuit. This is because the current can flow in different paths. The voltage in all branches of a parallel circuit is the same.
Quick Fact
In a parallel circuit, all components receive the full voltage of the supply regardless of their position. This allows multiple devices to operate at full power simultaneously.
Adding more components in a parallel circuit
Adding more branches to a parallel circuit gives more paths for the current to flow through. The more paths there are for current to flow through, the easier it becomes. That means the current through the cell increases.
Compare these two parallel circuits. Each has the same type of battery and the lamps are identical.
Adding more components to any one branch of a parallel circuit will decrease the current in that branch. Remember that the voltage across any branch will be the same, so adding more components in the branch makes it harder for current to flow in that branch.
Adding cells to a parallel circuit increases the supply voltage so it also:
- increases the voltage across each branch
- increases the current through the cell
- increases the current through each branch
Worked Example: Current in Parallel Branches
In a parallel circuit, if the total current from the battery is 2 A and there are four identical branches, the current in each branch is:
total current ÷ number of branches
2 A ÷ 4 = 0.5 A per branch
If one branch has more resistance (e.g., more components), the current in that branch will be less than in the others.
Questions
series, parallel, current, voltage
An ammeter is used to measure the ................. through a component.
An ammeter should be connected in ................. with the component.
A voltmeter is used to measure the ................. across a component.
A voltmeter should be connected in ................. with the component.
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An ammeter is used to measure the current through a component.
An ammeter should be connected in series with the component.
A voltmeter is used to measure the voltage across a component.
A voltmeter should be connected in parallel with the component.
Which of these shows the correct way to do this? Write one letter.
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D is the correct circuit. The ammeter is in series with the lamp and the voltmeter is in parallel across it.
Questions
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The voltage across one lamp is 220 V.
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The switch should be placed in series before the branches to control the entire parallel arrangement.
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Current per lamp = 2.4 A ÷ 6 = 0.4 A
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The total current will increase because more branches are added, each drawing additional current.
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Each lamp would receive 12 V, which is more than their rated 2.0 V. They could overheat or blow.
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12 V ÷ 2 V = 6 lamps
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The circuit should show 6 lamps in a single line connected to a 12 V battery.
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A voltmeter should be connected in parallel across one of the lamps.
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9 V – 2 V = 7 V across B1
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The voltage across L1 will decrease, as the total voltage is now shared among more components.
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The current will decrease because the total resistance increases in the series circuit.
Questions
The voltage across one buzzer is 1 V.
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Total voltage = 6 V
Voltage used by 2 buzzers = 2 × 1 V = 2 V
Remaining voltage = 6 V − 2 V = 4 V
Each lamp gets 2 V
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The total voltage is now 12 V. Since all components are identical and in series, each buzzer will now have 2 V across it.
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The current in the circuit increases because the total voltage increases while the resistance stays the same.
Think Like a Scientist
In this investigation, you will explore how the current through a battery depends on the number of identical branches in a parallel circuit.
You will need:
- four or six identical lamps
- battery or suitable power supply to light up to six lamps in parallel
- switch
- ammeter
- leads
- connectors
Steps
- Set up the circuit as shown in the first diagram.
- Measure and record the current with one lamp.
- Add another lamp in parallel with the first lamp as shown in the second diagram.
- Measure and record the current with two lamps.
- Continue adding lamps in parallel, one at a time. Do not change the position of the ammeter. Measure and record the current through the battery each time.
- Repeat each measurement two further times. Decide whether any more repeats are needed.
- Calculate the average result for each set of measurements.
Follow-Up Questions
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Create a table with columns for number of lamps and current through the battery (in A).
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The graph should show a straight line through the origin if the lamps are identical, indicating current increases linearly with the number of branches.
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The current increases as more lamps are added in parallel.
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The total current approximately doubles, assuming all lamps have equal resistance.
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In a parallel circuit, the total current is the sum of the current through each branch. Adding more identical branches increases total current linearly.
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Use your graph to estimate the current value for eight lamps based on the trend line.