IGCSE Physics (0625)

Calculating quantities from readings

What this topic tests, the mistakes that cost marks, and one exam-style question worked through step by step.

Key ideas and common mistakes

When calculating resistance using R = V/I, the answer should be rounded to match the least number of significant figures in the measured values, and the correct unit (ohms) must be included.
Common mistake: Students often forget to round or omit the unit.
To calculate resistance using R = V/I, substitute the given voltage and current values directly and express the answer with the correct unit (ohms).
Common mistake: A common mistake is to omit the unit or invert the formula (I/V instead of V/I).
To find the mass of an object added to a container, subtract the mass of the empty container from the combined mass.
Common mistake: A common mistake is to add or use the wrong order, giving a negative or incorrect value.
To find the thickness of one identical object from a stack, divide the total measured height by the number of objects.
Common mistake: Students who forget this may mistakenly use the total height as the answer or divide incorrectly.
To find the volume of liquid removed from a container, subtract the final volume from the initial volume.
Common mistake: A common mistake is to subtract in the wrong order or use the remaining volume instead of the amount taken out.
To find the resistance of a component, use only the voltage measured across that component and the current through it.

Worked example

A student investigates the resistance of a wire. The voltmeter reads V = 2.1 V across an 80.0 cm length of resistance wire and the ammeter reads I = 0.32 A. Calculate the resistance R of this length of wire using R = V / I. Give the unit and a suitable number of significant figures.

Try it first, then show the working

R = V / I = 2.1 / 0.32
= 6.5625 Ω
Quote to 2-3 sig figs: 6.6 Ω

Answer: 6.6 Ω

Try one yourself

For the full 100.0 cm length of the resistance wire, the voltmeter reads V = 2.5 V and the ammeter reads I = 0.32 A. Calculate the resistance R of…Answer it and get it marked → A student measures the mass of an empty dish as mD = 102.5 g. She places a ball in the dish and the balance reads a combined mass mC = 114.1 g.…Answer it and get it marked → Calculate the thickness t of one washer. Show your working.Answer it and get it marked →

61 exam-style questions on Calculating quantities from readings, each marked to the scheme.

Calculating quantities from readings

Key ideas and common mistakes

Worked example

Try one yourself

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