Ch 3 · 4.2.2 Electrical Current · O Level Physics 5054

Topic 3 of 17 in Electricity and Magnetism — O Level Physics 5054 — 59 lessons.

Introduction

When you switch on a torch, the lamp lights almost instantly. Charge flows round the circuit, carrying energy from the battery to the lamp. The rate of that flow is the current. (Syllabus section 4.2.2.)

Key ideas — What current is

Electric current is the charge passing a point per unit time:

🧑‍🏫 Teacher's explanation

Imagine a busy hallway in a school. If you want to measure the 'flow' of students, you don't just count the total number of students in the building; you count how many students pass a specific line on the floor every minute. In electricity, the 'students' are tiny particles of charge, and the 'line' is a specific point in the wire. The rate at which these charges pass that point is what we call electric current.

📖 Definitions

🔍 Worked example — Easy

A steady current of 0.5 A flows through a component. Calculate the total charge that passes a point in the component in 2 minutes.

🔍 Worked example — Standard

A charge of 15 C passes through a point in a circuit in 0.5 s. Calculate the current.

🔍 Worked example — Exam-style

A small sensor detects a current of 250 mA. Convert this value into Amperes (A).

⚠️ Common errors

🧠 Quick check 1

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🎯 Exam question

Key ideas — Current in metals

A metal contains free electrons. When a cell is connected, they drift through the metal from the negative terminal towards the positive terminal. This movement of charge is the current.

🧑‍🏫 Teacher's explanation

Imagine a crowded hallway in a school where everyone is walking in the same direction. If you stand at a single doorway and count how many students pass through you every second, you are measuring the 'flow rate' of the crowd. In a metal wire, the 'students' are actually tiny particles called free electrons. Even though the metal atoms stay mostly in place, these electrons are free to wander around.

📖 Definitions

🔍 Worked example — Easy

A steady current of 0.5 A flows through a wire for 2 minutes. Calculate the total charge Q that passes a point in the wire.

🔍 Worked example — Standard

A charge of 15 C flows through a lamp in 3 seconds. Calculate the current I.

🔍 Worked example — Exam-style

An ammeter measures a current of 1.2 A in a circuit. If the total charge passing through the circuit in 5 minutes is Q, calculate Q.

⚠️ Common errors

🧠 Quick check 1

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🎯 Exam question

(a) State the direction of the flow of conventional current. [1] (b) A charge of 4.0 C passes through a point in the wire in 0.5 s. Calculate the current in the wire. [2] [3 marks]

Key ideas — Conventional current and electron flow

Long before electrons were discovered, scientists chose to say current flows from positive to negative. This is conventional current, and it is the direction shown by arrows on circuit diagrams.

🧑‍🏫 Teacher's explanation

Imagine a crowded hallway where everyone is moving in one direction. If you count how many people pass a specific doorway every minute, you are measuring a 'flow rate'. In electricity, we don't count people; we count electric charge. The 'flow rate' of this charge is what we call electric current.

📖 Definitions

🔍 Worked example — Easy

A steady current of 0.5 A flows through a wire. Calculate the total charge passing a point in the wire in 20 s.

🔍 Worked example — Standard

A charge of 12 C passes through a lamp in 3 minutes. Calculate the current in amperes.

🔍 Worked example — Exam-style

An ammeter is used to measure a current. If the current is 2.5 A and the ammeter has ranges of 1 A, 3 A, and 10 A, which range should be selected, and what is the reading if the needle is halfway between 2 A and 3 A on that range?

⚠️ Common errors

🧠 Quick check 1

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🎯 Exam question

Key ideas — Direct and alternating current

On a graph of current against time, d.c. is a flat line on one side of zero; a.c. is a wave that goes above and below zero.

🧑‍🏫 Teacher's explanation

Think of electric current like the flow of water through a pipe. If you have a pump pushing water steadily in one direction, that is like Direct Current (d.c.). If you have a piston moving back and forth, pushing water forward and then pulling it back repeatedly, that is like Alternating Current (a.c.).

📖 Definitions

🔍 Worked example — Easy

A charge of 12.0 C passes a point in a circuit in 4.0 s. Calculate the current.

🔍 Worked example — Standard

An ammeter shows a current of 0.045 A. Convert this value to milliamperes (mA).

🔍 Worked example — Exam-style

A battery provides a constant current of 2.5 A. A second circuit uses an alternating current that reaches a peak charge flow of 0.5 C every 0.1 s in one direction. Compare the magnitude of the current in both circuits.

⚠️ Common errors

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🎯 Exam question

Key ideas — Measuring current: ammeters

🧑‍🏫 Teacher's explanation

Imagine a busy highway where cars are passing a specific toll booth. If you want to know how many cars pass per hour, you count them over a set time. In a wire, the 'cars' are electric charges and the 'toll booth' is a point in the circuit. The rate at which these charges pass that point is what we call electric current.\n\nThink of current like water flowing through a pipe. If you want to measure how much water is moving, you'd place a flow meter directly in the path of the water. You can't put it 'next' to the pipe; it must be part of the loop so the water actually flows through it. This is why we connect ammeters in series. If you tried to connect it in parallel (like a side-track), the electricity would take the easiest path through the meter, which has almost zero resistance, causing a short circuit.\n\nWe also distinguish between the direction of 'conventional current' and 'electron flow'. Historically, scientists thought current flowed from positive to negative. We now know that in metals, it is actually the tiny, negative electrons moving from the negative terminal to the positive terminal. It's like a crowd where the 'negative' people move one way, but we track the 'positive' flow in the opposite direction.

📖 Definitions

🔍 Worked example — Easy

A charge of 15 C passes a point in a circuit in 3.0 s. Calculate the current.

🔍 Worked example — Standard

A constant current of 0.45 A flows through a bulb for 2 minutes. Calculate the total charge that passes through the bulb.

🔍 Worked example — Exam-style

An ammeter is used to measure a current. The reading is 0.045 A. Express this current in milliamperes (mA).

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Worked example

Example 1. A charge of 120 C flows through a lamp in 1 minute. Find the current.

🔍 Worked example — Easy

A charge of 120 C flows through a lamp in 1 minute. Calculate the current in amperes.

🔍 Worked example — Standard

A constant current of 0.25 A flows through a resistor for 8.0 minutes. Calculate the total charge that passes through the resistor.

🔍 Worked example — Exam-style

An ammeter is used to measure the current in a circuit. If 4.5 C of charge passes a point in 0.5 s, calculate the current. State whether the direction of electron flow is the same as the conventional current.

⚠️ Common errors

Common mistakes

Practice

Answers: (1) The charge passing a point per unit time. (2) Q = 3.0 × 20 = 60 C. (3) From the negative terminal round the circuit to the positive terminal. (4) d.c. flows in one direction (from a cell or battery); a.c. keeps reversing direction (from the mains). (5) 0–1 A: it is just above 0.4 A and gives the most precise reading without going off scale.

Ch 3 · 4.2.2 Electrical Current · O Level Physics 5054

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