What is the difference between a series and parallel circuit?

In a series circuit, components are connected one after another, so the same current flows through all of them, but the voltage is shared. In a parallel circuit, components are connected across the same two points, so the voltage is the same across each component, but the current splits between branches. A key difference: if one component fails in a series circuit, the whole circuit stops working; in a parallel circuit, other branches continue to work.

How do you calculate total resistance in a parallel circuit?

To calculate total resistance in a parallel circuit, use the formula: 1/R_total = 1/R1 + 1/R2 + 1/R3 + ... For example, if you have two 10 Ω resistors in parallel, 1/R_total = 1/10 + 1/10 = 2/10 = 0.2, so R_total = 1/0.2 = 5 Ω. Always remember to invert your answer at the end. The total resistance in parallel is always less than the smallest individual resistance.

Why do bulbs in a parallel circuit stay bright when you add more bulbs?

In a parallel circuit, each bulb is connected directly across the power supply, so each receives the full voltage. Adding more bulbs in parallel does not change the voltage across existing bulbs, so they remain at the same brightness. However, the total current from the supply increases because each new bulb provides an extra path for current, so the battery may drain faster.

What happens to current when you add a resistor in series?

Adding a resistor in series increases the total resistance of the circuit. Since the voltage from the supply is constant, Ohm's law (I = V/R) tells us that current decreases. For example, if you add a 5 Ω resistor to a circuit with a 10 V supply and a 10 Ω resistor, total resistance becomes 15 Ω, so current drops from 1 A to 0.67 A.

How do you measure current and voltage in a series circuit?

To measure current, an ammeter must be connected in series with the component, so the same current flows through it. To measure voltage (potential difference), a voltmeter must be connected in parallel across the component, because it measures the difference in electrical energy between two points. In a series circuit, the ammeter reading is the same everywhere, while voltmeter readings across different components add up to the supply voltage.

Why are household circuits wired in parallel?

Household circuits are wired in parallel so that each appliance (lamp, TV, kettle) receives the full mains voltage (230 V in the UK) and can be switched on or off independently without affecting others. If they were wired in series, all appliances would have to be on at the same time, and adding one would reduce the voltage across the others, making them dimmer or less efficient.

Series and parallel circuits

WJEC

GCSE

This topic explores the fundamental differences between series and parallel circuits, focusing on the behavior of current and potential difference in each configuration. It requires learners to calculate net resistance, current, and potential difference, while understanding the practical design and testing applications of these circuits.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

Topic Overview

Series and parallel circuits are fundamental to understanding how electrical components behave in a circuit. In a series circuit, components are connected end-to-end, so the same current flows through each component, but the voltage is shared across them. In a parallel circuit, components are connected across the same two points, so the voltage across each component is the same, but the current is split between branches. Mastering these concepts is essential for analysing real-world circuits, from simple household wiring to complex electronic devices.

This topic is a core part of the WJEC GCSE Physics specification, building on basic ideas of current, voltage, and resistance. You will learn how to calculate total resistance in both types of circuits, predict the effects of adding or removing components, and interpret circuit diagrams. Understanding series and parallel circuits also lays the groundwork for topics like electrical power, energy transfer, and the National Grid.

Why does this matter? Because every electrical system you use—from a torch to a car's headlights—relies on these principles. For example, Christmas lights are often wired in series (so if one bulb fails, the whole string goes out), while household sockets are wired in parallel (so each appliance works independently). By the end of this topic, you'll be able to explain these everyday phenomena and solve circuit problems with confidence.

Key Concepts

Core ideas you must understand for this topic

→In a series circuit, the current is the same at all points, and the total resistance is the sum of individual resistances: R_total = R1 + R2 + R3 + ...
→In a parallel circuit, the voltage across each branch is the same as the supply voltage, and the total resistance is found using: 1/R_total = 1/R1 + 1/R2 + 1/R3 + ...
→Adding more resistors in series increases total resistance and decreases current; adding more resistors in parallel decreases total resistance and increases current.
→In a series circuit, if one component fails (e.g., a bulb blows), the circuit is broken and all components stop working. In a parallel circuit, other branches continue to work independently.
→The current in a parallel circuit splits at junctions, and the sum of currents in each branch equals the total current from the supply (Kirchhoff's Current Law).

What You Need to Demonstrate

Key skills and knowledge for this topic

Current is the same at any point in a single closed loop (series circuit).
Potential difference is shared between components in a series circuit.
Total resistance in a series circuit is the sum of individual resistances.
Potential difference is the same across components in parallel.
Total current in a parallel circuit is the sum of currents in each branch.
Adding resistors in series increases total resistance.
Adding resistors in parallel decreases total resistance.
Correct use of circuit symbols including diodes, LDRs, and thermistors.

Marking Points

Key points examiners look for in your answers

Current is the same at any point in a single closed loop (series circuit).
Potential difference is shared between components in a series circuit.
Total resistance in a series circuit is the sum of individual resistances.
Potential difference is the same across components in parallel.
Total current in a parallel circuit is the sum of currents in each branch.
Adding resistors in series increases total resistance.
Adding resistors in parallel decreases total resistance.
Correct use of circuit symbols including diodes, LDRs, and thermistors.

Examiner Tips

Expert advice for maximising your marks

💡Always draw a circuit diagram if one is not provided to help visualize the path of the current.
💡Remember that for parallel circuits, the potential difference across each branch is equal to the source voltage.
💡Check if the question asks for total resistance or individual component values.
💡Ensure you can identify the difference between a series and parallel connection in a complex diagram.
💡Always label your circuit diagrams clearly with component symbols and values. When calculating total resistance in parallel, show the reciprocal step (1/R_total = ...) and then invert your answer. Marks are often awarded for method, not just the final number.
💡For series circuits, remember that the current is the same everywhere, so you can use Ohm's law (V = IR) for each component individually. For parallel circuits, the voltage is the same across each branch, so use Ohm's law branch by branch.
💡When answering 'explain' questions, use the correct terminology: 'current', 'voltage', 'resistance', 'potential difference'. Avoid vague terms like 'power' or 'electricity' when you mean current. Also, state whether the circuit is series or parallel before applying rules.

Common Mistakes

Pitfalls to avoid in your exam answers

Confusing the rules for current and potential difference between series and parallel circuits.
Incorrectly assuming that adding a resistor in parallel increases the total resistance.
Failing to recognize that current splits in parallel branches.
Misinterpreting circuit diagrams when identifying series vs parallel components.
Misconception: Current is 'used up' as it flows around a circuit. Correction: Current is the flow of charge; it is not consumed. In a series circuit, the same current flows through all components. In a parallel circuit, current splits but the total current is conserved.
Misconception: Voltage is the same everywhere in a series circuit. Correction: In a series circuit, voltage is shared between components. The sum of the voltages across each component equals the supply voltage. In a parallel circuit, voltage is the same across each branch.
Misconception: Adding more resistors in parallel always increases total resistance. Correction: Adding resistors in parallel provides more paths for current, so total resistance decreases. For example, two identical resistors in parallel have half the resistance of one resistor.

Frequently Asked Questions

Common questions students ask about this topic

Before You Start

Prior knowledge that will help with this topic

•Basic understanding of current, voltage, and resistance, including the units (amps, volts, ohms).
•Ability to use Ohm's law (V = IR) to calculate one quantity given the other two.
•Familiarity with circuit symbols (cell, bulb, resistor, switch, ammeter, voltmeter) and how to draw simple circuit diagrams.

Likely Command Words

How questions on this topic are typically asked

Describe

Explain

Calculate

Represent

Ready to test yourself?

Practice questions tailored to this topic

Series and parallel circuits

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

Before You Start

Likely Command Words

Ready to test yourself?

Related Topics in WJEC GCSE Physics

Absorption and emission of ionising radiations and of electrons and nuclear particles

Atomic structure

Black body radiation (qualitative only)

Colour and frequency; differential effects in transmission, absorption and diffuse reflection

Topic Synopsis

Key Concepts & Core Principles

Exam Tips & Revision Strategies

Common Misconceptions & Mistakes to Avoid

Examiner Marking Points

Series and parallel circuits

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

What is the difference between a series and parallel circuit?

How do you calculate total resistance in a parallel circuit?

Why do bulbs in a parallel circuit stay bright when you add more bulbs?

What happens to current when you add a resistor in series?

How do you measure current and voltage in a series circuit?

Why are household circuits wired in parallel?

Before You Start

Likely Command Words

Ready to test yourself?

Related Topics in WJEC GCSE Physics

Absorption and emission of ionising radiations and of electrons and nuclear particles

Atomic structure

Black body radiation (qualitative only)

Colour and frequency; differential effects in transmission, absorption and diffuse reflection