P 251 Laboratory Activity 1 Measurement

All measurements have two parts: A numerical part that gives the result of the comparison between the chosen unit and the thing being measured. A unit part which tells what standard unit was used to make the measurement. Both parts must be recorded if the measurement is to be useful. In this activity we will examine the rules related to the numerical part of a measurement.

Working with the Numerical Part of a Measurement Accuracy, Precision and Uncertainty in Measurements

Accuracy, Precision and Uncertainty in Measurement There is no such thing as a perfect measurement. Each measurement contains a degree of uncertainty due to the limits of instruments and the methods employed in using them. Uncertainty arises from three sources: 1. Mistakes 2. Systematic Errors 3. Random Errors Mistakes can be traced to carelessness, inattention, improper training, bad habits, lack of innate ability, poor judgment, adverse measuring or observing conditions, and various negative attitudes, emotions and perceptions. Mistakes can not be predicted or quantified but can be reduced if not eliminated. Systematic errors can usually be traced to instrument maladjustment, lack of calibration, or the environment. They can be quantified and corrected.

Random errors are caused by human and instrument imperfections. Personal errors. Since humans are directly involved with all measurements, and since humans are imperfect, errors are inevitable in measurements. Instrumental errors. All measurements employ instruments, from the simple meter stick to the most sophisticated electronic device. Some error is always present in the measurements due to imperfection in manufacture, adjustment or basic characteristics of the instrument. Calculation errors. Unless sufficient digits are recorded and carried through all computation steps, and unless conversion factors and constants contain sufficient digits, round-off errors occur. While random errors can not be predicted or eliminated they can be estimated using the mathematical laws of statistics and probability.

Two concepts used to express the uncertainty in measurements are accuracy and precision.

The accuracy of a measurement refers to how close the measured value is to the true or accepted value. For example, if you used a balance to find the mass of a known standard 100.00 g mass, and you got a reading of 78.55 g, your measurement would not be very accurate. Precision refers to how close together a group of measurements actually are to each other. Precision has nothing to do with the true or accepted value of a measurement, so it is quite possible to be very precise and totally inaccurate. In many cases, when precision is high and accuracy is low, the fault can lie with the instrument. If a balance or a thermometer is not working correctly, they might consistently give inaccurate answers, resulting in high precision and low accuracy.

A dartboard analogy can demonstrate the difference between accuracy and precision. Imagine a person throwing darts, trying to hit the bullseye. The closer the dart hits to the bullseye, the more accurate his or her tosses are. If the person misses the dartboard with every throw, but all of their shots land close together, they can still be very precise. Low Accuracy Low Precision Low Accuracy High Precision High Accuracy High Precision Students must strive for both accuracy and precision in all of their laboratory activities. Make sure that you understand the workings of each instrument, take each measurement carefully, and recheck to make sure that you have precision. Without accurate and precise measurement your calculations, even if done correctly, are quite useless.

In laboratory exercises, students are expected to follow the same procedure that scientists follow when they make measurements. Each measurement should be reported with all of the digits that are certain plus one digit with a value that has been estimated. For example, if a student is reading the level of water in a graduated cylinder that has lines to mark each 10 milliliters of water, then he or she should report the volume of the water to the nearest whole mL. V=722mL

Measuring Liquids When you put a sample of a liquid into a graduated cylinder you will notice a curve at the surface of the liquid. This curve, which is called a meniscus, may be concave (curved downward) or convex (curved upward). Hold the graduated cylinder so that the meniscus is at eye level. Read the volume of the liquid at the bottom of a concave meniscus or at the top of a convex meniscus.

Significant Figures Values read from the measuring instrument are expressed with numbers known as significant figures. For each measurement made it is important to consider significant figures and to keep in mind the uncertainties involved in measurement. When scientists report the results of their measurements it is important that they also communicate how 'close' those measurements are likely to be. This helps others to duplicate the experiment, but also shows how much 'room for error' there was. Significant figures are digits read from the measuring instrument plus one uncertain digit estimated by the observer. Example: a measurement of 123.74 centimeters was made with a meter stick having millimeter marks. The figures 1, 2, 3, and 7 are certain while the 4 is estimated.

107.02m 0.0002700kg 55000s 0.000175g Rules For Significant Digits Digits from 1-9 are always significant. Zeros between two other significant digits are always significant. 107.02m One or more additional zeros to the right of both the decimal place and another significant digit are significant. 0.0002700kg Zeros used solely for spacing the decimal point (placeholders) are not significant. 55000s 0.000175g Terminal zeros in a whole number may or may not be significant.

Rules For Rounding Decide how many significant figures will be kept; look at the first digit to be rejected. If it is less than five (5) drop all rejected digits 27.084kg (4) 27.08kg If it is greater than five (5) or equal to five (5) followed by other nonzero digits drop all rejected digits and increase the last retained significant digit by one (1). 0.52600s (2) 0.53s 1,535,400km (3) 1,540,000km If it is equal to five (5) drop all rejected digits and increase the last retained significant digit by one (1) only to make it even. 27.500m (2) 28m 0.00165s (2) 0.0016s

Examples How many significant digits are in each of the following measurements? 1.75m 3 302.00g 5 5.0003s 5 2000kg 1 0.00053cm 2 1.00053cm 6 Round each of the following measurements to the indicated number of significant digits: 3.08546cm (3) 3.09cm The first zero (0) after the 2 is significant, the other zeros are not significant but only used to place the decimal point. 20304kg (2) 20000kg 0.00000515s (2) 0.0000052s 10.0000m (3) 10.0m

Measuring Uncertainty in Measurements When an accepted or standard value of the physical quantity is known, the percent error is calculated to compare an experimental measurement with the standard. Percent error is a measure of accuracy. When no standard exists, or when it is desired to measure the precision of an experiment, percent difference is calculated. Percent difference measures how much two or more measurements of the same quantity differ from each other.

Examples: An experiment designed to measure the density of copper obtained a value of 8.37g/cm3. If the value listed in a reference table was 8.92g/cm3, what was the percent error in the experimental value? Note: % error is usually rounded to one or two significant digits.

Five students in a lab group each measured the length of a wooden block. The results are listed below. Student #1- 25.32cm Student #2- 25.15cm Student #3- 25.44cm Student #4- 24.89cm Student #5- 25.71cm What is the % difference of their measurements? The group should use the average value of 25.30cm in future calculations.

Performing Arithmetic Operations with Measurements When multiplying or dividing, your answer may only show as many significant digits as the multiplied or divided measurement showing the least number of significant digits. Example: 22.37 cm x 3.10 cm x 85.75 cm = 5946.50525 cm3 22.37cm has 4 significant digits. 3.10cm has 3 significant digits. 85.75cm has 4 significant digits. The product answer can only show 3 significant digits because that is the least number of significant digits in the original problem. 22.37 cm x 3.10 cm x 85.75 cm = 5950 cm3

When adding or subtracting your answer can only show as many decimal places as the measurement having the fewest number of decimal places. Example: 3.76 g + 14.83 g + 2.1 g = 20.69 g 3.76g and 14.83g each have two decimal places 2.1g has only one decimal place The sum can only have one (1) decimal place. 3.76 g + 14.83 g + 2.1 g = 20.7 g Example:

Example: The length and width of a rectangle are measured to be 12.7cm and 8.48cm respectively. What is the perimeter of the rectangle? Rounding to one decimal place:

What is the area of the rectangle? Rounding to three (3) significant digits: