Ch 1 · Simple Magnetism and Electrical Quantities · IGCSE Physics 0625
Topic 1 of 3 in Electricity and Magnetism — IGCSE Physics 0625 — 56 lessons.
Introduction
Before you can analyse circuits, motors and generators you need the basic vocabulary: magnetic poles and fields, electric charge, current, potential difference, resistance and power. These quantities appear in almost every electricity question, and definitions in words are worth easy marks on the Extended papers.
Key ideas — Simple magnetism
1. Magnetic poles. A magnet has a north (N) and a south (S) pole. Like poles repel; unlike poles attract. A magnet attracts magnetic materials — iron, steel, nickel and cobalt — whichever pole is brought near. Non-magnetic materials (copper, aluminium, wood, plastic) are not attracted.
🧑🏫 Teacher's explanation
Imagine you are walking through a crowd where everyone is trying to push you away or pull you closer. Magnetism works similarly; every magnet is surrounded by an invisible 'influence zone' called a magnetic field. If you place another magnet or a magnetic piece of metal in this zone, it feels a push or a pull.
📖 Definitions
🔍 Worked example — Easy
A student uses a bar magnet to test several materials. Material A is attracted to both poles of the magnet. Material B is repelled by the North pole of the magnet. Identify which material is a permanent magnet and explain why.
🔍 Worked example — Standard
A soft iron core is placed inside a coil of wire. When current flows, the core becomes a magnet. If the current is increased, what happens to the magnetic field strength at a point 2 cm from the core?
🔍 Worked example — Exam-style
A magnetic field is mapped using iron filings. In region X, the filings are very densely packed. In region Y, the filings are spread far apart. Compare the relative strength of the magnetic field in region X and region Y.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) State whether material X is a permanent magnet or a magnetic material. [1] (b) Explain why material Y must be a magnet. [2] [3 marks]
Key ideas — Electric charge
6. Two kinds of charge. Charge is positive or negative. Like charges repel; unlike charges attract. When a polythene rod is rubbed with a cloth, electrons (negative) are transferred from the cloth to the rod, so the rod becomes negative and the cloth equally positive. Only negative charge (electrons) moves when solids are charged by friction; positive charges stay in the nuclei.
🧑🏫 Teacher's explanation
Think of electric charge like a type of 'social personality' that particles have. Some particles are 'positive' and some are 'negative'. The golden rule of electricity is that people with the same personality want to stay away from each other (repulsion), while people with different personalities are drawn together (attraction).
📖 Definitions
🔍 Worked example — Easy
A polythene rod is rubbed with a dry cloth. If 5.0 × 10^-7 C of negative charge is transferred from the cloth to the rod, what is the final charge on the cloth?
🔍 Worked example — Standard
A student uses a gold-leaf electroscope to detect charge. The gold leaf rises by 2.5 cm when a charged rod is brought near. If the leaf rises by 5.0 cm when a different rod is used, what can be concluded about the second rod's charge relative to the first?
🔍 Worked example — Exam-style
Determine the direction of the electric field lines for a system consisting of a sphere with charge +4.0 × 10^-9 C and a sphere with charge -4.0 × 10^-9 C placed 0.10 m apart.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) State the observation that would indicate the material is a conductor. [1] (b) The student finds that a piece of dry wood does not make the lamp light up. Explain this result in terms of the movement of charge. [2] [3 marks]
Key ideas — Current, e.m.f. and p.d.
8. Current. Electric current is the flow of charge, measured in amperes (A) with an ammeter connected in series. Conventional current flows from the positive terminal of the supply to the negative; the electrons in a metal wire actually flow the opposite way (negative to positive). In metals, current is a flow of free electrons. (S) The direction of conventional current (+ to −) and of electron flow (− to +) is Supplement content.
🧑🏫 Teacher's explanation
Think of electricity like water flowing through a plumbing system. The current (I) is the rate at which the water flows (liters per second). The charge (Q) is the total amount of water that has passed a point. The electromotive force (e.m.f.) is like the pressure provided by a pump that pushes the water through the entire loop, while the potential difference (p.d.) is the pressure drop across a specific part of the pipe, like a narrow nozzle, where energy is used up.
📖 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 through the wire.
🔍 Worked example — Standard
A battery has an e.m.f. of 12.0 V. If 4.0 C of charge is moved around a complete circuit by this battery, calculate the total work done (W).
🔍 Worked example — Exam-style
A light bulb has a potential difference of 3.5 V across it. If 150 C of charge passes through the bulb in 5 minutes, calculate the current flowing through the bulb.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Resistance
10. Resistance opposes current:
🧑🏫 Teacher's explanation
Imagine water flowing through a pipe. The current is the flow rate of the water, and the voltage (p.d.) is the water pressure pushing it through. Resistance is like the narrowness or roughness of that pipe. If the pipe is very long or very thin, it is much harder for the water to get through, meaning the resistance is high.
📖 Definitions
🔍 Worked example — Easy
A 12 V battery is connected to a resistor. An ammeter measures a current of 3.0 A. Calculate the resistance of the resistor.
🔍 Worked example — Standard
A wire has a resistance of 10 Omega. If the length of the wire is doubled while the cross-sectional area remains the same, what is the new resistance?
🔍 Worked example — Exam-style
A filament lamp is connected to a 6.0 V source. The current is 1.5 A. As the lamp gets hotter, the resistance increases to 5.0 Omega. Calculate the new current flowing through the lamp.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Calculate the resistance of the wire. (b) The student replaces the wire with one that is twice as long but has the same thickness. Calculate the new resistance. [3 marks]
Key ideas — Electrical energy and power
11. Power and energy. Circuits transfer energy from the source to components and then to the surroundings.
🧑🏫 Teacher's explanation
Think of electricity like water flowing through a pipe. The voltage (V) is the water pressure, and the current (I) is the flow rate (how much water passes per second). Power (P) is how much work that water can do—like turning a water wheel. If you have high pressure and a high flow rate, you can turn a much larger wheel very quickly.
📖 Definitions
🔍 Worked example — Easy
A lightbulb is connected to a 12 V supply and draws a current of 0.5 A. Calculate the power of the bulb.
🔍 Worked example — Standard
An electric heater has a power rating of 2.5 kW. How much energy, in kilowatt-hours (kWh), does it use if it is left on for 3 hours?
🔍 Worked example — Exam-style
A television has a power rating of 150 W. It is used for 4 hours every day. Calculate the total energy used in one day in Joules (J).
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Practical notes
Worked example
Example 1 — resistance. A lamp has a p.d. of 6.0 V across it and a current of 0.25 A through it.
Common mistakes and examiner tips
Practice
Answers: (1) No. Attraction happens with any magnetic material; only repulsion would prove it is a magnet. (2) Friction transfers electrons from the cloth to the rod; the rod gains extra electrons, so it has an overall negative charge (the cloth becomes positive). (3) R = 4.5/0.30 = 15\ Omega; P = 0.30 × 4.5 = 1.35\ W ≈ 1.4\ W. (4) E = Pt = 1500 × 240 = 3.6 × 10^5\ J. (5) 0.150 × 6.0 × 30 = 27\ kW h; cost = 27 × 20 = 540 cents. (6) Q = It = 0.40 × 300 = 120\ C. (7) Quarter of the length, so R = 3.0/4 = 0.75\ Omega. (8) A curve through the origin whose gradient decreases as V increases (current rises less and less). As the current increases the filament gets hotter, so its resistance increases. (9) Radial straight lines, equally spaced round the sphere, with arrows pointing inwards towards the sphere.