Experimental Skills

1. Experimental Skills http://www.physics.unc.edu/~deardorf/uncertainty/UNCguide.html http://phys.columbia.edu/~tutorial/index.html http://cnx.org/content/m15029/latest/

2. Errors (Uncertainty) • An error in a measurement is defined as the difference between the true, or accepted value of a quantity and its measured value.

3. Types of Errors (1) • Random errors • Random errors arise from unknown and unpredictable variations in condition. • It fluctuates from one measurement to the next. • to estimate the reading of a scale • changes in temperature or wind, while the experiment is in progress. • Random error can be reduced by repeating the measurement several times and find the average value of the readings.

4. Types of Errors (2) • Parallax error • Parallax error is an error in reading an instrument due to the eye of the observer and pointer are not in a line perpendicular to the plane of the scale.

5. Types of Errors (3) • Systematic Errors • Systematic errors cause all measurements to be shifted systematically in one direction either larger or smaller than it should be. • Systematic errors associated with particular instruments or techniques. • e.g. improperly calibrated instruments • zero error, which cause by an incorrect position of the zero point, • Systematic errors are difficult to detect. • Systematic errors are ‘one-sided’ errors. • Systematic errors cannot be reduced by increasing the number of measurements. • Systematic errors can be compensated if the errors are known.

6. Types of Errors (4) • Zero error • A zero error arises when the measuring instrument does not start from exactly zero. • Zero errors are consistently present in every reading of a measurement. • The zero error can be positive or negative.

7. Personal errors (or Mistakes) • Personal errors arise from the mistakes of the experimenter. • Observational mistakes may be due to the personal bias or carelessness of the experimenter while reading the scale of an instrument. • Arithmetic mistakes usually occur while performing the needed calculations. • This class of errors can be completely eliminated if the experimenter exercises utmost caution and skepticism while performing the experiment. • If the scales are read incorrectly or if the calculations are wrongly carried out, the entire result will be wrong! • Therefore, the experimenter is strongly encouraged to cross-check the data and calculations. • In a lab group, each partner should independently read the data and check any calculations for accuracy.

8. Accuracy and Precision • Accuracy is the closeness of agreement between a measurement value and a true value or accepted value. • Precision means how fine the divisions or segments (Sensitivity) are and how repeatable the results are (Consistency). 

9. Precision

10. Precise or Accurate? Accurate, Not Precise Precise, Not Accurate Precise and Accurate

11. Below is a data table produced by three groups of students who were measuring the mass of a paper clip which had a known mass of 1.0003 g. The last row is the average of their measurements.

12. Sensitivity • The sensitivity of an instrument is its ability to detect small changes in the quantity that is being measured. • Thus, a sensitive instrument can quickly detect a small change in measurement. • Measuring instruments that have smaller scale parts are more sensitive. • Sensitive instruments need not necessarily be accurate.

13. Vernier Slide Calipers http://www.vjc.moe.edu.sg/fasttrack/physics/vernier13.htm http://www.phy.uct.ac.za/courses/c1lab/vernier1.html http://physics401.one-school.net/2009/01/142-vernier-caliper.html

14. Micrometer Screw Gauge http://www.vjc.moe.edu.sg/fasttrack/physics/macrometer_ya_v6.htm http://physics401.one-school.net/search/label/micrometer