CIE A Level Physics (9702) 2019-2021

Revision Notes

2.1.3 Calculating Uncertainty

Calculating Uncertainty

  • There is always a degree of uncertainty when measurements are taken; the uncertainty can be thought of as the difference between the actual reading taken (caused by the equipment or techniques used) and the true value
  • Uncertainties are not the same as errors
    • Errors can be thought of as issues with equipment or methodology that cause a reading to be different from the true value
    • The uncertainty is a range of values around a measurement within which the true value is expected to lie, and is an estimate
  • For example, if the true value of the mass of a box is 950 g, but a systematic error with a balance gives an actual reading of 952 g, the uncertainty is ±2 g
  • These uncertainties can be represented in a number of ways:
    • Absolute Uncertainty: where uncertainty is given as a fixed quantity
    • Fractional Uncertainty: where uncertainty is given as a fraction of the measurement
    • Percentage Uncertainty: where uncertainty is given as a percentage of the measurement

  • To find uncertainties in different situations:
  • The uncertainty in a reading: ± half the smallest division
  • The uncertainty in a measurement: at least ±1 smallest division
  • The uncertainty in repeated data: half the range i.e. ± ½ (largest – smallest value)
  • The uncertainty in digital readings: ± the last significant digit unless otherwise quoted

Calculating Uncertainties, downloadable AS & A Level Physics revision notes

How to calculate absolute, fractional and percentage uncertainty

 

Combining Uncertainties

  • The rules to follow
  • Adding / subtracting data – add the absolute uncertainties

Combining Uncertainties (1), downloadable AS & A Level Physics revision notes

  • Multiplying / dividing data – add the percentage uncertainties

Combining Uncertainties (2), downloadable AS & A Level Physics revision notes

  • Raising to a power – multiply the uncertainty by the power

Combining Uncertainties (3), downloadable AS & A Level Physics revision notes

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