Ch 6 · 4.3.1 Circuit Diagrams and Circuit Components · O Level Physics 5054

Topic 6 of 17 in Electricity and Magnetism — O Level Physics 5054 — 59 lessons.

Introduction

Electricians and engineers all over the world read the same circuit diagrams, because every component has a standard symbol. Learning these symbols, and what each component does, lets you read any circuit in an exam. (Syllabus section 4.3.1.)

Key ideas — Drawing circuit diagrams

🧑‍🏫 Teacher's explanation

Think of a circuit diagram like a map of a city's water pipes. You don't draw every single molecule of water or the texture of the metal; instead, you use standardized symbols to show where the pumps (batteries), the valves (switches), and the narrow pipes (resistors) are located. If the lines don't connect, the water stops flowing. If they cross without a dot, it's like a bridge over a road—the water doesn't jump across.

📖 Definitions

🔍 Worked example — Easy

A student draws a circuit with a single cell, a switch, and a lamp. If the switch is open, what is the state of the circuit?

🔍 Worked example — Standard

A circuit contains a 12V power supply, a fixed resistor of 4Omega, and a variable resistor. If the ammeter reads 2.0 A, what is the resistance of the variable resistor?

🔍 Worked example — Exam-style

A circuit consists of a 6V battery connected in series to a lamp and a resistor. The lamp has a resistance of 3Omega and the resistor has a resistance of 3Omega. Calculate the power dissipated by the lamp.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Components: sources, switches and resistors

🧑‍🏫 Teacher's explanation

Think of a circuit like a water system in a house. The cells or batteries are the water pumps that provide the pressure (e.m.f.), while the wires are the pipes. If you want to control how much water flows, you might use a valve (a switch) or a narrow pipe (a resistor) to create resistance.

📖 Definitions

🔍 Worked example — Easy

A circuit contains two cells connected in series. Each cell has an e.m.f. of 1.5 V. Calculate the total e.m.f. of the battery.

🔍 Worked example — Standard

A 12 V power supply is connected to a fixed resistor of 4Omega and a variable resistor. The current in the circuit is measured to be 2 A. Calculate the resistance of the variable resistor.

🔍 Worked example — Exam-style

A potential divider consists of a 100Omega fixed resistor R_1 and a 400Omega variable resistor R_2 in series. A 10 V supply is connected across the pair. Calculate the potential difference across the variable resistor V_2.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Components: output devices, sensors and meters

🧑‍🏫 Teacher's explanation

Think of an electrical circuit like a water pipe system. The components are the different parts of the plumbing: a pump (battery) pushes water, a narrow pipe (resistor) slows it down, and a valve (switch) can stop the flow entirely. Some components are 'sensors'—they change how much they resist the flow based on the environment, like a valve that automatically closes if the water gets too hot.

📖 Definitions

🔍 Worked example — Easy

A simple circuit consists of a 12 V battery, a switch, and a lamp. If the ammeter reads 0.5 A when the switch is closed, calculate the resistance of the lamp.

🔍 Worked example — Standard

A thermistor (NTC) is used in a circuit. At 20^circC, its resistance is 5000 Omega. If the temperature rises to 40^circC and its resistance drops to 1000 Omega, calculate the change in resistance.

🔍 Worked example — Exam-style

A motor is connected to a 24 V power supply. The motor draws a current of 3.0 A. Calculate the power delivered to the motor.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Components: magnetic, protective and one-way devices

(Your exam paper uses the symbols shown in the syllabus; practise drawing each one neatly.)

🧑‍🏫 Teacher's explanation

Think of a circuit like a water plumbing system. Most components are just like pipes or valves, but some are specialized. A diode is like a 'one-way valve' that prevents water from flowing backward, which is vital for protecting sensitive electronics. A fuse is like a 'weak link' in a pipe; if the pressure (current) gets too high, the link breaks to prevent the whole system from exploding.

📖 Definitions

🔍 Worked example — Easy

A circuit contains a 12 V battery and a diode. If the diode is placed in the circuit, what happens to the current if the battery is flipped so the positive terminal faces the flat side of the diode symbol?

🔍 Worked example — Standard

A fuse is rated at 5 A. A circuit draws a current of I = P/V. If a heater with power P = 110 W is connected to a 220 V supply, will the fuse blow?

🔍 Worked example — Exam-style

A magnetising coil is used to activate a relay. The coil has a resistance of 10Omega and is connected to a 5 V source. Calculate the current flowing through the coil.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Reading a circuit

When interpreting a circuit:

🧑‍🏫 Teacher's explanation

Think of a circuit like a water pipe system. The battery is the pump, and the wires are the pipes. Current is the flow of water. If you have one pipe after another, that is a series circuit; the water has only one path to take, so the flow rate (current) is the same everywhere. If the pipe splits into two branches and then joins back together, that is a parallel circuit; the water can split up, but the total flow is the sum of the flows in each branch.

📖 Definitions

🔍 Worked example — Easy

A simple circuit consists of a 12 V battery, a closed switch, and two identical resistors of 4Omega each connected in series. Calculate the current flowing through the circuit.

🔍 Worked example — Standard

A 6 V power supply is connected to two resistors in parallel. One resistor is 10Omega and the other is 15Omega. Calculate the total current leaving the power supply.

🔍 Worked example — Exam-style

A circuit contains a 12 V battery, a switch, and a lamp. An ammeter is placed in series with the lamp, and a voltmeter is connected across the lamp. If the ammeter reads 2.0 A and the voltmeter reads 12 V, what is the resistance of the lamp?

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Worked example

A student draws a circuit: a cell, a switch, a lamp and an ammeter in a single loop, with a voltmeter across the lamp. A diode is added in the loop, pointing against the conventional current. What happens when the switch is closed?

🔍 Worked example — Easy

A 6.0 V battery is connected to a circuit containing a diode and a lamp. If the diode is placed in reverse-bias, what is the current reading on an ammeter placed in series with the lamp?

🔍 Worked example — Standard

A circuit consists of a 12 V power supply, a switch, a diode (forward-biased), and a 4.0 \Omega resistor. Calculate the current in the circuit when the switch is closed.

🔍 Worked example — Exam-style

A student uses a 9.0 V battery to power a circuit with a diode and a lamp. When the diode is forward-biased, the ammeter reads 0.50 A. Calculate the potential difference across the lamp.

⚠️ Common errors

Common mistakes

Practice

Answers: (1) (a) a rectangle with a diagonal arrow through it; (b) a triangle pointing at a bar; (c) a rectangle in a circle with two arrows pointing in. (2) An NTC thermistor. (3) It uses a small current through a coil (an electromagnet) to close or open a switch in a second circuit, often one carrying a large current. (4) Ammeter in series; voltmeter in parallel across the component.

Ch 6 · 4.3.1 Circuit Diagrams and Circuit Components · O Level Physics 5054

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