In measurements made in physics, the terms precision and accuracy are frequently used. They should be distinguished clearly. The precision of a measurement is determined by the instrument or device being used and the accuracy of a measurement depends on the fractional or percentage uncertainty in that measurement.

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devices-to-measure-physical-quantitiesWe use many devices to measure physical quantities, such as length. Time. And temperature. They all have some limit of precision

For example, When the length of an object is recorded as 25.5 cm by using a metre rod having smallest division in millimeter, it is the difference of two readings of the initial and final positions. The uncertainty in the single reading as discussed before is taken as ± 0.05 cm which is now doubled and is called absolute uncertainty equal to ± 0.1 cm. absolute uncertainty, in fact, is equal to the least count of the measuring instrument.

Precision or absolute uncertainty (least count) = ± 0.1 cm

uncertainty-equation

Another measurement taken by vernier callipers with least count as 0.01 cm is recorded as 0.45 cm. it has

precision or absolute uncertainty (least count) = ± 0.01 cm

uncertainty-equation-1

Thus the reading 25.5 cm taken by metre rule is although less precise but is more accurate having less percentage uncertainty or error.

Whereas the reading 0.45 cm taken by vernier callipers is more precise but is less accurate. In fact, it is the relative measurement which is important. The smaller a physical quantity, the more precise instrument should be used. Here the measurement 0.45 cm demands that a more precise instrument, such as micrometer screw gauge, with least count 0.001 cm, should have been used. Hence, we can conclude that:

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Colour printing uses just four colours – cyan, magenta, yellow and black to produce the entire range or colours. All the colours in this book have been made from just these four colours.

 A precise measurement is the one which has less absolute uncertainty and an accurate measurement is the one which has less fractional or percentage uncertainty or error.

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