Sources of Uncertainty (OCR A Level Physics)

Random & Systematic Errors

• Measurements of quantities are made with the aim of finding the true value of that quantity

• In reality, it is impossible to obtain the true value of any quantity as there will always be a degree of uncertainty

• The uncertainty is an estimate of the difference between a measurement reading and the true value

• Random and systematic errors are two types of measurement errors that lead to uncertainty

Random error

• Random errors cause unpredictable fluctuations in an instrument’s readings as a result of uncontrollable factors, such as environmental conditions

• This affects the precision of the measurements taken, causing a wider spread of results about the mean value

• To reduce random error:

  • Repeat measurements several times and calculate an average from them

Systematic error

• Systematic errors arise from the use of faulty instruments used or from flaws in the experimental method

• This type of error is repeated consistently every time the instrument is used or the method is followed, which affects the accuracy of all readings obtained

• To reduce systematic errors:

  • Instruments should be recalibrated, or different instruments should be used

  • Corrections or adjustments should be made to the technique

Systematic errors on graphs are shown by the offset of the line from the origin

Zero error

• This is a type of systematic error which occurs when an instrument gives a reading when the true reading is zero

  • For example, a top-ban balance that starts at 2 g instead of 0 g

• Zero errors can be removed by taking the difference of the offset from each value

  • Eg. If a scale starts at 2 g instead of 0 g, a measurement of 50 g would actually be 50 - 2 = 48 g

  • The offset could be positive or negative

Precision & Accuracy

Precision

• Precise measurements are ones in which there is very little spread about the mean value, in other words, how close the measured values are to each other

• If a measurement is repeated several times, it can be described as precise when the values are very similar to, or the same as, each other

• The precision of a measurement is reflected in the values recorded - measurements to a greater number of decimal places are said to be more precise than those to a whole number

Accuracy

• A measurement is considered accurate if it is close to the true value

• The accuracy can be increased by repeating measurements and finding a mean of the results

• Repeating measurements also helps to identify anomalies that can be omitted from the final results

The difference between precise and accurate results

Representing precision and accuracy on a graph