Ch 2 · Electric Circuits and Electrical Safety · IGCSE Physics 0625
Topic 2 of 3 in Electricity and Magnetism — IGCSE Physics 0625 — 60 lessons.
Introduction
With current, p.d. and resistance defined, you can now analyse whole circuits: how current divides, how p.d. is shared, and how sensors such as thermistors and light-dependent resistors switch things on automatically. The lesson ends with the safety features of mains electricity — fuses, trip switches, earth wires and double insulation — which appear regularly on both Core and Extended papers.
Key ideas — Circuit components
1. Symbols you must know. Cells, batteries, power supplies, generators, switches, fixed and variable resistors, heaters, lamps, motors, ammeters, voltmeters, fuses, relays, magnetising coils, transformers, potential dividers, and two sensors:
🧑🏫 Teacher's explanation
Think of an electric circuit like a water system in a house. The battery is the pump, the wires are the pipes, and the resistors are narrow sections of pipe that slow the flow. In a series circuit, there is only one path, so the water flow (current) must be the same everywhere. In a parallel circuit, the pipes split into branches; the total flow from the pump is the sum of the flows in each branch. \n\nTo control how much 'pressure' (potential difference) reaches a specific component, we use a potential divider. This is like having two different sized pipes in a row; the narrow one will cause a larger drop in pressure than the wide one. For safety, we use fuses and earth wires—think of these as emergency overflow valves that divert dangerous excess flow away from you to prevent a fire or shock.
📖 Definitions
🔍 Worked example — Easy
A circuit consists of a 12 V battery connected in series with a 4Omega resistor and a 2Omega resistor. Calculate the total resistance and the current in the circuit.
🔍 Worked example — Standard
A potential divider consists of a 100Omega fixed resistor (R_1) and a variable resistor (R_2) in series. If the total voltage is 6 V and R_2 = 200Omega, calculate the potential difference across R_1.
🔍 Worked example — Exam-style
A 12 V supply is connected to two resistors in parallel: R_1 = 4Omega and R_2 = 6Omega. Calculate the total current drawn from the supply.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Series circuits
🧑🏫 Teacher's explanation
Imagine a single-lane circular track for cars. If only one car can travel at a time, the number of cars passing any specific point per minute must be exactly the same, regardless of whether they are on a straight or a curve. This is exactly how current works in a series circuit; there is only one path, so the flow of charge is identical everywhere.
📖 Definitions
🔍 Worked example — Easy
A circuit consists of a 6.0 V battery connected in series with two resistors of 4.0 Omega and 2.0 Omega. Calculate the total resistance and the current in the circuit.
🔍 Worked example — Standard
Three cells, each providing 1.5 V, are connected in series. They are connected to a single resistor of 5.0 Omega. Calculate the total e.m.f. and the current flowing through the circuit.
🔍 Worked example — Exam-style
A 12 V power supply is connected to three resistors in series with resistances of 2.0 Omega, 3.0 Omega, and 5.0 Omega. Calculate the potential difference across the 3.0 Omega resistor.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Parallel circuits
🧑🏫 Teacher's explanation
Imagine a large river that suddenly splits into two smaller streams around an island, before merging back into one river downstream. The total amount of water flowing through the main river (the source current) must be the sum of the water in both streams. If one stream is blocked or narrow, the water just flows more through the other one, but the total volume remains the same. This is exactly how current behaves at a junction.
📖 Definitions
🔍 Worked example — Easy
A circuit has two resistors, R_1 = 10 Omega and R_2 = 40 Omega, connected in parallel. Calculate the total resistance R_T.
🔍 Worked example — Standard
A 12 V battery is connected to two lamps in parallel. Lamp A has a resistance of 4 Omega and Lamp B has a resistance of 6 Omega. Calculate the total current I_total leaving the battery.
🔍 Worked example — Exam-style
In a parallel circuit, a 0.5 A current enters a junction. It splits into two branches. If the current in the first branch is 0.2 A, what is the current in the second branch?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Potential dividers
4. Sharing the p.d. For a constant current, the p.d. across a conductor increases as its resistance increases. Two resistors in series across a supply therefore divide the supply p.d., with the larger resistor getting the larger share. (S) A variable potential divider (a resistor with a sliding contact, or a fixed resistor with a variable resistor) gives an output p.d. that can be adjusted between zero and the supply p.d.
🧑🏫 Teacher's explanation
Imagine a water pipe system where the water pressure (potential difference) is split between two different sized pipes. If you have two pipes in a row, the one that is narrower and harder to push water through (higher resistance) will cause a much larger drop in pressure (potential difference) than the wide, easy-to-flow pipe.
📖 Definitions
🔍 Worked example — Easy
A 12 V battery is connected to two resistors in series: R_1 = 4 Omega and R_2 = 8 Omega. Calculate the potential difference across R_2.
🔍 Worked example — Standard
A potential divider uses a fixed resistor R_1 = 100 Omega and a variable resistor R_2. The input voltage is 6 V. If the output voltage across R_2 is 4 V, calculate the resistance of R_2.
🔍 Worked example — Exam-style
An LDR is used in a potential divider with a 10 V supply. The fixed resistor is 2 k\Omega. In the dark, the LDR resistance is 8 k\Omega. Calculate the potential difference across the LDR in the dark.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Electrical safety
6. Mains wiring. A mains circuit has a live (line) wire at a high alternating voltage, a neutral wire near 0 V that completes the circuit, and an earth wire for safety. Switches and fuses must be in the live wire, so that when a switch is off or a fuse has blown the appliance is disconnected from the high voltage. With a switch in the neutral wire, the appliance would still be live even when switched off.
🧑🏫 Teacher's explanation
Think of electricity like water flowing through pipes. A fuse is like a very thin, fragile section of pipe designed to burst if the water pressure (current) gets too high, preventing the main pipes from exploding. A circuit breaker is like a smart valve that snaps shut when it senses too much flow, and can be reopened once the problem is fixed.
📖 Definitions
🔍 Worked example — Easy
An electric heater is rated at 2 ext A. Which of the following fuses is most appropriate: 1 ext A, 3 ext A, or 10 ext A?
🔍 Worked example — Standard
A metal-cased lamp has a fault where the live wire touches the metal case. The resistance between the live wire and the earth wire is 0.5 Omega. If the mains voltage is 230 V, calculate the current flowing through the earth wire.
🔍 Worked example — Exam-style
A circuit has a 13 A fuse. An appliance draws 12 A during normal operation. If a fault causes the current to rise to 15 A, explain why the fuse will blow and why the switch must be in the live wire.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Explain why the earth wire is necessary in this situation. [2] (b) Describe how the fuse or circuit breaker responds to this fault. [2] [4 marks]
Practical notes
Worked example
Example 1 — series circuit. In the diagram, the 12 V supply is connected to resistors of 2.0 Ω, 4.0 Ω and 6.0 Ω in series.
🔍 Worked example — Easy
A 12 V battery is connected to three resistors of 2.0 Omega, 4.0 Omega, and 6.0 Omega in series. Calculate the total resistance and the current flowing through the circuit.
🔍 Worked example — Standard
Two resistors, 6.0 Omega and 3.0 Omega, are connected in parallel across a 12 V supply. Calculate the total resistance and the total current from the source.
🔍 Worked example — Exam-style
A 9.0 V supply is connected to a 1.0 kOmega fixed resistor and a thermistor in series. The thermistor has a resistance of 2.0 kOmega. Calculate the potential difference across the thermistor (V_th) and the fixed resistor (V_fixed).
⚠️ Common errors
Common mistakes and examiner tips
Practice
Answers: (1) 3 × 1.5 = 4.5\ V. (2) R = 12\ Omega, I = 6.0/12 = 0.50\ A, V = 0.50 × 7.0 = 3.5\ V. (3) Any two: each lamp gets the full supply p.d. (full brightness); lamps can be switched on and off independently; if one lamp breaks the others stay lit. (4) I = 1200/230 = 5.2\ A, so use the 13 A fuse (5 A would blow). (5) With the switch in the live wire, turning it off disconnects the lamp from the high voltage, so it is safe to touch or change the bulb; in the neutral wire the lamp would still be at a high voltage. (6) 1/R = 1/4.0 + 1/12 = 3/12 + 1/12 = 4/12, so R = 3.0\ Omega; currents 6.0/4.0 = 1.5\ A and 6.0/12 = 0.50\ A; total 2.0\ A. (7) 2.5 - 0.90 = 1.6\ A; conservation of charge (current into a junction = current out). (8) V = 1.0/4.0 × 12 = 3.0\ V. (9) It increases: the LDR's resistance rises in the dark, so it takes a larger share of the supply p.d.