A circuit fault changes the readings. You are asked to say what the fault is from the readings, or to predict the readings for a named fault. The circuits in questions are low-voltage ideal ones: a cell or battery, resistors or lamps, and meters.
This lesson is about explaining and reasoning on paper and screen. It gives no instructions for working on mains wiring. If a real appliance seems faulty, stop using it and ask a qualified electrician.
What do the two main faults look like?
An open circuit has a break. No complete path means zero current. A voltmeter across the break reads the full supply voltage, and a voltmeter across a working component in series reads 0 V.
A short circuit puts a low-resistance wire across a component. The resistance of the circuit drops, so the current rises. The component that is shorted has about 0 V across it.
For the calculations, use the series rules from potential difference, resistance and circuits: total resistance is the sum, and the same current flows through each part. You can test a circuit like this safely in the circuit reasoning simulator.
Worked example
An invented circuit: a 6 V cell in series with a 2 Ω resistor R₁ and a 4 Ω resistor R₂.
Normal working: R = 2 + 4 = 6 Ω. I = V ÷ R = 6 ÷ 6 = 1 A. Across R₁: 1 × 2 = 2 V. Across R₂: 1 × 4 = 4 V. The two add to 6 V.
Fault A, R₂ shorted by a wire. Now only R₁ remains: R = 2 Ω, so I = 6 ÷ 2 = 3 A. Across R₁: 3 × 2 = 6 V. Across R₂: 0 V.
Fault B, a break in the wire at R₂ (open circuit). I = 0 A. Across R₁: 0 × 2 = 0 V. A voltmeter across the break reads 6 V.
Explanation in words for Fault A: “The short circuit removes the resistance of R₂, so the total resistance falls from 6 Ω to 2 Ω. The supply voltage is unchanged, so the current rises from 1 A to 3 A.”
The mistake to watch for
A common slip is to think a voltmeter reading 0 V across a component means there is no current in the circuit.
Mistaken answer: “The voltmeter across R₂ reads 0 V, so the circuit is broken.”
A reading of 0 V across R₂ is also what a short circuit gives, and then the current is larger, not zero.
The correction is to check the ammeter as well.
Zero current plus full voltage across the gap means an open circuit. Large current plus zero voltage across a component means that component is shorted. Always say which reading you use as evidence.
Check yourself
1. A 12 V battery is in series with a 4 Ω and an 8 Ω resistor. Find the normal current, then the current if the 8 Ω resistor is shorted.
Show answer
Normal: R = 12 Ω, I = 12 ÷ 12 = 1 A. Shorted 8 Ω: only 4 Ω remains, so I = 12 ÷ 4 = 3 A.
2. In a series circuit with one lamp, the ammeter reads 0 A and a voltmeter across the lamp reads 6 V from a 6 V cell. What is the fault?
Show answer
There is a break in the lamp (filament broken), an open circuit. No current flows, and the whole 6 V appears across the gap, which here is the lamp.
3. Why can a short circuit be dangerous in a real appliance?
Show answer
It lowers the resistance, so the current becomes very large. A large current heats the wires, which can cause overheating. A fuse is used to break the circuit first.
Where this leads next
Put fault reasoning together with energy in checking a unit conversion in an energy-use calculation, and return to the overview at electrical energy and safe interpretation.
Students who can calculate the readings but find the written explanation harder often gain from online one-to-one Physics tuition, where a teacher checks each claim against the numbers.