How do you combine resistors in series and parallel?

Resistors in series add up, so the total is larger than the biggest. Resistors in parallel combine by reciprocals: one over the total equals the sum of the reciprocals, so the total is smaller than the smallest, and for two equal resistors it is half. For two in parallel you can use product over sum.

What is the potential divider equation?

V out equals V in times R2 over (R1 plus R2), where R2 is the resistor the output is taken across. It tells you how two series resistors share a supply voltage, and rearranging it lets you design a divider that gives a required output voltage, then round to the nearest E24 preferred value.

How do an LDR and a thermistor behave?

An LDR has a high resistance in the dark and a low resistance in bright light. An NTC thermistor has a high resistance when cold and a low resistance when hot. Each is used in a potential divider with a fixed resistor so the change in resistance becomes a change in output voltage.

Why are pull-up, pull-down and current-limiting resistors used?

A pull-up or pull-down resistor gives a switch input a definite logic level instead of floating: a pull-down holds it low when the switch is open, a pull-up holds it high. A current-limiting resistor in series with an LED stops a destructive current, with R equal to the supply voltage minus the LED forward voltage, all divided by the LED current.

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WJEC GCSE Electronics: resistive components in circuits overview

Q: How is resistive components content examined?

It sits in Component 1 (Discovering Electronics), a 1 hour 30 minute written paper worth 40% of the GCSE. Expect series and parallel resistance calculations, colour-code and E24 questions, potential divider calculations and design, sensor-divider reasoning for LDRs and thermistors, and LED current-limiting calculations.

An overview of the resistive components content in Component 1 of WJEC Eduqas GCSE Electronics, covering combining resistors in series and parallel, resistor codes and the E24 series, potential dividers, the LDR and thermistor as input sensors, pull-up and pull-down resistors and current-limiting an LED.

Generated by Claude Opus 4.87 min readWJEC Eduqas GCSE Electronics, Component 1Updated 2026-06-15

Reviewed by: AI editorial process; not yet individually human-reviewed

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What the topic covers
How this content is examined
How to study it
For the official specification

The resistive components content of WJEC Eduqas GCSE Electronics builds on the circuit concepts and is part of Component 1 (Discovering Electronics). It covers how resistors combine, how to read them, the all-important potential divider, the LDR and thermistor as sensors, and the resistors that give switches a logic level and protect LEDs. This page maps the content and links to a focused answer page for each part.

What the topic covers

Resistors in series and parallel: Adding resistors in series, the reciprocal rule for parallel, and reading value, tolerance and power rating from colour codes and the E24 series. See Resistors in series and parallel.
Potential dividers: How two resistors share a supply voltage, the divider equation, and designing a divider for a required output. See Potential dividers.
Input sensors: LDR and thermistor: How an LDR and an NTC thermistor change resistance, and using them in a divider to produce a changing voltage. See Input sensors: LDR and thermistor.
Pull resistors and current-limiting: Pull-up and pull-down resistors for switch inputs, and the series resistor that protects an LED. See Pull resistors and current-limiting.

How this content is examined

This content sits in Component 1 (Discovering Electronics), a written paper of 1 hour 30 minutes worth 40% of the GCSE. Expect series and parallel resistance calculations, colour-code and E24 questions, potential divider calculations and design, sensor-divider reasoning, and LED current-limiting calculations. The potential divider is one of the most heavily examined ideas in the course.

How to study it

Master the two combining rules. Series adds; parallel uses reciprocals (and the total is smaller than the smallest).
Drill the divider equation. Practise both finding the output and designing for a required output, then rounding to E24.
Learn the sensor directions. LDR: high in dark, low in light. NTC thermistor: high when cold, low when hot.
Always include the LED resistor. Use $R = \dfrac{V_{\text{supply}} - V_{\text{LED}}}{I}$ and round up.
Explain why pull resistors are needed. A floating input is undefined; a pull resistor fixes the level when the switch is open.

For the official specification

WJEC Eduqas publishes the full GCSE Electronics specification, past papers and mark schemes at wjec.co.uk. Always revise from the current specification and the board's own past papers, because question style and the printed equation and symbol lists are board-specific.

Sources & how we know this

WJEC Eduqas GCSE Electronics specification (from 2017) — WJEC Eduqas (2017)