IGCSE Physics (0625)

Agreement within experimental accuracy

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

Key ideas and common mistakes

To check if two measured values agree within a given percentage, calculate the absolute difference as a percentage of the accepted value and compare it to the allowed limit.
Common mistake: Students often forget to use the accepted value as the reference or miscalculate the percentage.
Compare the percentage difference between measurements to the stated accuracy limit.
Common mistake: Students often confuse absolute and percentage differences or overlook the specified threshold for agreement.
To justify direct proportionality between two variables, compare their ratios from different data points and check if the percentage difference is within the given tolerance.
Common mistake: Students often forget to express the difference as a percentage of the smaller value or misapply the criterion.
To decide if two measured values agree within experimental limits, calculate their numerical difference and compare it to the stated tolerance.
Common mistake: Students often forget to check the actual difference or misapply the given threshold.
To judge agreement using a percentage difference criterion, calculate the absolute difference between the two values, divide by the larger value, and compare the result to the given percentage threshold.
Common mistake: Students often forget to use the larger value as the denominator or omit converting to a percentage.
A valid conclusion must explicitly compare the measured rates using their numerical values to support which surface absorbs thermal radiation better.

Worked example

A student measures the period of a pendulum and uses it to calculate a value of g = 9.9 m/s². The accepted value of g is 9.8 m/s². Two values are considered to agree within the limits of experimental accuracy if they are within 5% of each other. State whether the student's value agrees with the accepted value, and support your answer with a calculation.

Try it first, then show the working

Any 2 of these:

States YES / the values agree (must match the calculation) · Calculates the difference 9.9 - 9.8 = 0.1 m/s² · Expresses the difference as a percentage, approximately 1%, well within the 5% limit, so the values agree

Answer: Yes, the values agree. The difference is 9.9 - 9.8 = 0.1 m/s². As a percentage of 9.8 this is (0.1 / 9.8) x 100 = about 1%, which is well below the 5% limit. The difference is very small compared with the values, so the measured value agrees with the accepted value within the limits of experimental accuracy.

Try one yourself

State the quantity a student should calculate to decide whether two measured values agree within the limits of experimental accuracy.Answer it and get it marked → State whether the two measurements agree within the limits of experimental accuracy.Answer it and get it marked → A student measures the focal length of a converging lens twice using two different object-to-screen distances. The first measurement gives f1 = 15.2…Answer it and get it marked →

15 exam-style questions on Agreement within experimental accuracy, each marked to the scheme.

Agreement within experimental accuracy

Key ideas and common mistakes

Worked example

Try one yourself

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