What does the Basic Circuit Concepts module of O-Level Electronics cover?

It covers the foundations every later topic relies on: what current, voltage (potential difference) and resistance are, together with their units (the ampere, the volt and the ohm); how to read and draw a circuit diagram using standard symbols and conventional current; Ohm's law in the form V equals IR and its rearrangements; electrical power and energy, including the cost of running a device; and the rules for how current, voltage and resistance behave in series and in parallel circuits.

How do current, voltage and resistance behave differently in series and in parallel?

In a series circuit the current is the same everywhere, the supply voltage is shared between the components, and the total resistance is the sum of the individual resistances. In a parallel circuit the voltage across each branch is the same as the supply, the current divides between the branches, and the total resistance is found from the reciprocal rule, so the combined resistance is always smaller than the smallest single branch.

How is Ohm's law used in O-Level Electronics calculations?

Ohm's law states that for a metal conductor at constant temperature the current is directly proportional to the voltage, written V equals IR. Rearranged, it gives I equals V over R and R equals V over I, so any one of the three quantities can be found from the other two. A common trap is forgetting to convert milliamperes to amperes or kilohms to ohms before substituting, which throws the answer out by a factor of a thousand.

Why is the Basic Circuit Concepts module so important for the rest of the course?

Every later module reuses these ideas. Potential dividers, transistor switches and amplifier biasing all rest on Ohm's law and the series and parallel rules. Sizing an LED series resistor, working out the power a component dissipates, and predicting the voltage at a point in a sensor circuit are all direct applications of this module, so a shaky foundation here makes the analogue and systems topics much harder.

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Basic Circuit Concepts: O-Level Electronics module overview of current, voltage, resistance, Ohm's law, power, and series and parallel circuits

An O-Level Electronics overview of the Basic Circuit Concepts module. Current, voltage and resistance and their units, standard circuit symbols and conventions, Ohm's law, electrical power and energy, and the rules for series and parallel circuits, with links to every dot point.

Generated by Claude Opus 4.87 min readSEAB O-Level Electronics: Basic Circuit ConceptsUpdated 2026-06-13

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

Jump to a section

What this module is about
The three quantities and their units
Reading and drawing a circuit
Ohm's law: the central tool
Power and energy
Series and parallel circuits
A worked calculation tying the module together
How this module is examined
Check your knowledge

What this module is about

Basic Circuit Concepts is the foundation of O-Level Electronics. Before you can analyse a sensor circuit, bias a transistor or design a logic system, you need a secure grip on three quantities and how they relate: current, voltage (potential difference) and resistance. This module sets those definitions, fixes the units, teaches you to read and draw a circuit, gives you Ohm's law as the central tool, adds power and energy so you can size components safely, and finishes with the series and parallel rules that govern how real networks behave.

This overview ties the module together and links to every dot point, each with its own worked answers and practice questions. See the full set at /sg-o-level/electronics/syllabus/basic-circuit-concepts.

The three quantities and their units

Everything begins with current, voltage and resistance. Current is the rate of flow of charge, measured in amperes; voltage is the energy transferred per unit charge between two points, measured in volts; and resistance opposes the flow of current, measured in ohms. Conventional current is taken to flow from the positive terminal of the supply to the negative terminal, the opposite direction to the electron flow, and this convention is used throughout the course.

Reading and drawing a circuit

A circuit must be communicated on paper so anyone can build it. Circuit diagrams and conventions covers the standard symbols for cells, resistors, lamps, switches and meters, and the rules for connecting instruments: an ammeter goes in series so the same current passes through it, while a voltmeter goes in parallel across the component whose potential difference is being measured.

Ohm's law: the central tool

Ohm's law states that for a metal at constant temperature the current is directly proportional to the voltage, so $V = IR$ . Rearranged, $I = V/R$ and $R = V/I$ let you find any quantity from the other two. An ohmic component gives a straight voltage-current graph through the origin; non-ohmic components such as a filament lamp or a diode give curves. This single equation appears in almost every later calculation in the course.

Power and energy

A component does not just carry current; it converts electrical energy at a rate called power. Electrical power and energy gives the three forms of the power equation, $P = VI$ , $P = I^2 R$ and $P = V^2/R$ , and treats energy as power multiplied by time. This lets you check that a resistor will not overheat and work out the cost of running a device from its power rating and the price of electricity.

Series and parallel circuits

Real circuits combine components, and the rules differ for the two ways of connecting them. Series and parallel circuits sets out that in series the current is the same everywhere, the voltage is shared, and resistances add; while in parallel the voltage is shared across each branch equally, the current divides, and the combined resistance is found from the reciprocal rule and is always smaller than the smallest branch.

A worked calculation tying the module together

Two resistors across a supply

Two resistors, $R_1 = 200\ \Omega$ and $R_2 = 400\ \Omega$ , are connected in series across a $6.0\ \text{V}$ supply. Find the current drawn from the supply, the voltage across each resistor, and the total power dissipated.

Step 1: Find the total resistance

In series the resistances add: $R_\text{total} = R_1 + R_2 = 200 + 400 = 600\ \Omega$ .

Step 2: Find the current with Ohm's law

The same current flows through both resistors in a series circuit. Using $I = \dfrac{V}{R_\text{total}} = \dfrac{6.0}{600} = 0.010\ \text{A} = 10\ \text{mA}$ .

Step 3: Find the voltage across each resistor

The current is $0.010\ \text{A}$ through each, so $V_1 = IR_1 = 0.010 \times 200 = 2.0\ \text{V}$ and $V_2 = IR_2 = 0.010 \times 400 = 4.0\ \text{V}$ . The two add to $6.0\ \text{V}$ , as the supply voltage must be shared in series.

Step 4: Find the total power

Using $P = VI = 6.0 \times 0.010 = 0.060\ \text{W} = 60\ \text{mW}$ . As a check, $P = I^2 R_\text{total} = (0.010)^2 \times 600 = 0.060\ \text{W}$ , which agrees.

How this module is examined

Convert units first. Almost every numerical question hides a prefix; converting to amperes, volts and ohms before substituting protects the easy marks.
State the right rule for the connection. Decide whether the components are in series or in parallel, then apply the matching current, voltage and resistance rule rather than mixing them.
Show the working. Markers reward the correct rearrangement of $V = IR$ and the inclusion of units in the final answer, not just the number.

Check your knowledge

Work through the quiz for this module to test current, voltage and resistance, Ohm's law, power and the series and parallel rules under timed conditions, then review the worked explanations.

Sources & how we know this

Singapore Examinations and Assessment Board (SEAB) — Singapore Examinations and Assessment Board (2026)
Subjects and syllabuses, Ministry of Education Singapore — Ministry of Education, Singapore (2026)