Making measurement by usisng some instrument or other is inevitable in physic.Hence, ensuring that careful and accurate reading are obtained for any measurement is of utmost importance.Table below shows some most commonly used instrument for making measurement in physics experiments.
|Physical quantity |Instrument | |Length |Meter rule | |Thickness,diameter |Vernier callipers,micrometer screw gauge | |Mass |Electronic balance,beam balance, spring balance | |Time |Stop watch | |Temperature |Thermometer | |Volume |Measuring cyinder | |Current |Ammeter |
Uncertainties,Accuracies and Error in measurements
When we take measurement there are always uncertainties in the values.For example the diameter of a pencil is 7.5mm if we use a ruler with mm scale.The rulercan measure up to the nearest 0.5mm.But if we use amicrometer screw gauge it is 7.55mm which measures up to the nearest 0.01mm. the difference between the two values is in their uncertainties.The measurement using the micrometer screw gauge is better because it has a smalle uncertaintily.it is said to be more accurate.Thus less uncertainty means more accuracy.
The accuracy of a value indicates how close it is to the true value, when we write our measurement as 7.55+/- 0.01mm we mean the value is between 7.54mm and 7.56mm the first number is the uncertainity of the measurement. The accuracy of a measurement depends on the limits of the measurement made in the experiment.The lmit,instead,depends on various factor:
i ) the type and quality of the instrument
ii)the skill of the person taking the reading
iii)the number of trials made in the measurement
ERROR OR UNCERTAINTY
In any measurement,error is unavoidable .Thus, measurement made are accurate up to a cfretain degree depending on the accuracy of the reading.The reading in turn depends on the accuracy of the instrument and also on the physical constraints of the observer.Hencd all reading taken using any instrument have an inaccuracy or error..The eror of the reading expressed together with the reading. For example, a student may measure the time taken for a pendulum to make 20 complete oscillation as 40.8 by using a stop watch.
The reading 40/8s indicates that the time is accurate up to 0.1s. Thus the measurement of the time maybe expressed as (40.8+/-0.1) where 0.1s is specifically referred to as the absolute error the measurement of time.Absolute error of the instrument is usually obtain by taking the smallest division of the instrument.In using a meter rule to measure length, suppose a student express his reding as 50.2cm the absolute error will then be 0.1cm and the result may be expressed together with its absolute error as(50.2+/- 0.1)cm.
Greater accuracy obtained with vernier callipers the smallest division of the instrument is 0.01cm which known as the absolute error of the vernier callipers.Thus readings taken using a pair of vernier callipers or micrometer screw gauge.In the case vernier callipers or micrometer screw gauge. In the case vernier callipers the smallest division of the instrument is 0.01cm which known as the absolute error of the vernier callipers.
Thus,reading taken using a pair of vernier callipers maybe expressed up to an accuracy of 0.01cm.An even greater accuracy is obtanained when...