Ch 2 · 4.2.1 Electrical Charge · O Level Physics 5054

Topic 2 of 17 in Electricity and Magnetism — O Level Physics 5054 — 49 lessons.

Introduction

Rub a balloon on your hair and it sticks to the wall. Pull off a jumper in the dark and you may see tiny sparks. Both are static electricity: charge that has built up on an object instead of flowing. (Syllabus section 4.2.1.)

Key ideas — Two kinds of charge

There are two kinds of electric charge: positive (+) and negative (−). Charge is measured in coulombs (C).

🧑‍🏫 Teacher's explanation

Think of electric charge like a social rule for tiny particles. Some particles are 'positive' and some are 'negative'. If two particles are the same (both positive or both negative), they want to stay away from each other—this is 'repulsion'. If they are different, they are drawn together—this is 'attraction'.

📖 Definitions

🔍 Worked example — Easy

An object has a charge of +4.0 C. A second object has a charge of -2.0 C. State whether they will attract or repel each other.

🔍 Worked example — Standard

A glass rod is rubbed with a silk cloth. The rod becomes positively charged. Explain, in terms of electrons, what has happened to the rod.

🔍 Worked example — Exam-style

A point charge Q is placed in an electric field. A second positive test charge q is brought near it. If the force on q is directed towards the right, what is the direction of the electric field lines at that point?

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

(a) Explain why the rod becomes charged. [2] (b) The rod is placed between two parallel metal plates, one positive and one negative. Describe the pattern of the electric field lines between the plates. [1] [3 marks]

Key ideas — Charging by friction: only electrons move

When two insulators are rubbed together, electrons (negative charge) are transferred from one to the other. The protons stay put: they are locked in the nuclei.

🧑‍🏫 Teacher's explanation

Imagine a crowd of people in a room. Some people are wearing red hats (protons) and some are wearing blue hats (electrons). The people in red hats are bolted to their chairs—they cannot move. The people in blue hats, however, are very energetic and can run around the room. When two groups of people rub against each other, the 'blue hat' people might jump from one group to the other. Because the 'red hat' people can't move, the group that loses blue hats ends up with more red hats than blue hats, making them 'positive'. The group that gains blue hats now has more blue than red, making them 'negative'.

📖 Definitions

🔍 Worked example — Easy

A glass rod is rubbed with a silk cloth. The rod loses 3.0 × 10^-17 C of negative charge to the cloth. What is the final charge on the glass rod?

🔍 Worked example — Standard

An insulator is rubbed with a cloth, transferring 5.0 × 10^-9 C of electrons from the cloth to the insulator. State the charge on the cloth.

🔍 Worked example — Exam-style

A polythene rod is rubbed with a woollen cloth. The rod gains 1.2 × 10^-16 C of electrons. Calculate the total charge on the woollen cloth.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

(a) State the type of charge that is transferred during this process. [1] (b) Explain why the glass rod becomes positively charged. [2] (c) The student then brings the charged glass rod near a small piece of paper. The paper is attracted to the rod. Explain this observation using the concept of charge. [2] [5 marks]

Key ideas — Electric fields

An electric field is a region in which an electric charge experiences a force.

🧑‍🏫 Teacher's explanation

Imagine you are standing in a room filled with invisible fans. If you throw a tiny paper ball (a charge) into the room, the wind (the electric field) will push it in a certain direction. The 'wind' is the electric field, and the 'push' is the force.

📖 Definitions

🔍 Worked example — Easy

A single positive point charge is placed in space. Describe the direction of the electric field lines at a point 5 cm away from the charge.

🔍 Worked example — Standard

Two parallel metal plates are placed 0.05 m apart. Plate A is charged positively and Plate B is charged negatively. Describe the electric field pattern between the plates.

🔍 Worked example — Exam-style

A glass rod is rubbed with a silk cloth. The rod becomes positively charged. Explain, using the electron model, why the rod is now positive.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

(a) Describe how the student could use a battery and a small bulb to distinguish between the two. [2] (b) Using the electron model, explain why the plastic rod does not allow current to flow. [2] [4 marks]

Key ideas — Conductors and insulators

The electron model. In a conductor (a metal), some electrons are free (delocalised): they are not tied to any one atom and can move through the material, carrying charge. In an insulator, all the electrons are held tightly to their atoms, so charge cannot flow.

🧑‍🏫 Teacher's explanation

Think of electricity like a crowd of people in a room. In a 'conductor', the room is filled with people who are free to run around anywhere; if you push one person, the movement spreads quickly through the crowd. In an 'insulator', everyone is strapped into a chair; even if you push one person, they can't move to help anyone else, so the movement stays stuck in one spot.

📖 Definitions

🔍 Worked example — Easy

A student rubs a polythene rod with a woollen cloth. The rod becomes negatively charged. State the charge of the rod and explain why the charge moved from the cloth to the rod.

🔍 Worked example — Standard

A point charge of +2.0 × 10^-6 C is placed in an electric field. If the force experienced by a second charge of +4.0 × 10^-6 C is 0.80 N, calculate the electric field strength E at that point.

🔍 Worked example — Exam-style

Two parallel metal plates are separated by a distance of 0.05 m. One plate is charged positively and the other negatively. Describe the electric field pattern between the plates and state the direction of the field lines.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Practical notes

Other demonstrations: a rubbed balloon picks up small pieces of paper; a charged rod bends a thin stream of water from a tap.

Worked example

A polythene rod is rubbed with a woollen cloth and becomes negatively charged. Explain what has happened, and state the charge on the cloth.

🔍 Worked example — Easy

A polythene rod is rubbed with a woollen cloth. The rod becomes negatively charged. State the charge on the cloth and explain why.

🔍 Worked example — Exam-style

A point charge is placed in an electric field. The electric field line at a specific point is directed vertically upwards. Describe the force experienced by a positive charge placed at that point and draw/describe the field direction for a negative charge at that same point.

⚠️ Common errors

Common mistakes

Practice

Answers: (1) They have the same type of charge (both positive or both negative). (2) Electrons are transferred from the rod to the cloth; the rod is left with fewer electrons than protons. (3) A region in which an electric charge experiences a force. (4) Straight, parallel, evenly spaced lines from the positive plate to the negative plate; a uniform field. (5) Copper has free electrons that can move through it and carry charge; in plastic, the electrons are held tightly to their atoms and cannot move.

Ch 2 · 4.2.1 Electrical Charge · O Level Physics 5054

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