The Basic Units of Measurement

The abbreviation SI comes from the French le Systems International. 

By themselves, numbers have limited meaning. Read this sentence When my son was 7 he walked 3, and when he was 4 he threw his baseball 8 and said his school was 5 away. The sentence is confusing because we don t know what the numbers mean— the units are missing. The meaning becomes clear, however, when we add the missing units to the numbers When my son was 7 months old he walked 3 steps, and when he was 4 years old he threw his baseball 8 feet and said his school was 5 minutes away. Units make all the difference, hi chemistry, units are critical. Never write a number by itself always use its associated units—otherwise your work will be as confusing as the initial sentence. 

The two most common unit systems are the English system, used in the United States, and the metric system, used in most of the rest of the world. The English system uses imits such as inches, yards, and pounds, while the metric system uses centimeters, meters, and kilograms. The most convenient system for science measmements is based on the metric system and is called the International System of units or SI units. SI units are a set of standard units agreed on by scientists throughout the world. 

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It is not unreasonable to expect the detector manufacturer to specify their products in units that are most useful to their customers. It is therefore recommended that detector sensitivities be given not only in the basic units of measurement but also in g/ml of a readily available solute. The solute chosen should be one that often occurs in mixtures with which the detector will be frequently used for analysis. Unfortunately, instrument manufacturers are not reputed to listen favorably to such simple suggestions and it is likely the analyst will need to measure the detector sensitivity experimentally. A simple procedure for measuring detector sensitivity will be given later in this chapter. 

The basic unit of measure for radioactivity is the number of atomic decays per unit time. In the SI system, this unit is the becquerel (Bq), dehned as one decay per second. An older, widely used measure of activity is the curie (Ci). Originally... 

We can only speculate on the true size of the total timber resource of the United States. To date, all of the inventories and surveys on a national scale have been based on volume measurements of the merchantable parts of trees. Tables I, II, and III reflect this. Merchantable volume is a vague term, particularly since merchantability limits are rapidly changing. The concept of whole-tree utilization has reinforced this confusion. With the development of whole-tree harvesting methods, previously non-merchantable parts of the tree are chipped and used for pulp and paper, composite products, and fuel. These new concepts of utilization make the whole tree the basic unit of measurement. Since accurate volume determination is difficult on irregular shaped objects, weight of biomass is the new standard of measure for all tree components. 

Endurance is how long an activity can be performed at the required load and rate level. Thus, the basic unit of measure is time. Time is the only measure of how long it takes to complete a task. If the focus is on a given variable (e.g., strength, speed, or endurance), it is necessary to either control or measure the others. When the focus is endurance, the other factors of force or torque, speed, and joint angle, can be described as conditions under which endurance is measured. Because of the interactions of endurance and load or endurance and time (as e.g.), a number of endurance-rdatedmeasures have evolved. These endurance-related measures have clouded endurance testing. 

Scientific Notation Writing Large and Small Numbers 12 2.5 The Basic Units of Measurement 22 2.10 Numerical Problem-Solving... 

In most problems, the concentration of contaminant is so small that there is virtually no difference between the concentration based on the mass flowrate of water and the mass flowrate of the mixture. However, it is important to be consistent and follow the convention given in Equation 26.1. The other point to note is regarding the units. It is convenient to define the flowrate in terms of metric tons (typically tons per hour or tons per day). It is also convenient to define the concentration in terms of parts per million (ppm). If the basic unit of flowrate is taken to be tons and concentration to be parts per million, then the mass load is measured in grams (typically grams per hour or grams per day). 

The basic unit of radioactivity is the curie, Ci. One curie is the amount of radioactive material that emits particles at a rate of 3.7 X 1010 disintegrations per second (dps), or 2.2 X 1012 min-1 (dpm). Amounts that large are seldom used in experimentation, so subdivisions are convenient. The milli-curie (mCi, 2.2 X 109 min-1) and microcurie (yu,Ci, 2.2 X 106 min-1) are standard units for radioactive measurements (see Table 6.2). The radioactivity unit of the meter-kilogram-seconds (MKS) system is the becquerel (Bq). A becquerel, named in honor of Antoine Becquerel, who studied uranium radiation, represents one disintegration per second. The two systems of measurement are related by the definition 1 curie = 3.70 X 1010 becquerels. Since the becquerel is such a small unit, radioactive units are sometimes reported in MBq (mega, 106) or TBq (tera, 1012). Both unit systems are in common use today, and radioisotopes received through commercial sources are labeled in curies and bequerels. 

Although they differ in detail, it may be accepted that the basic unit of the cluster is a tetrahedron with one interstitial iron (most likely Fe3+ [52, 53] surrounded by four vacancies on the nearest octahedral site, which is found locally in the magnetite structure. The wiistite structure is then understood to have these unit tetrahedra arranged in some ordered manner. From this point of view, the measurements suggesting three phases separated by second- or higher-order transitions within the wiistite phase [22, 22a, 78] can be interpreted as successive loss of different types of order as the temperature is raised or the number of the unit tetrahedra decreases (the reduction proceeds). However, no definite conclusions have yet been drawn and indeed, the existence of these three subphases is still disputed [19, 20, 23, 24, 28]. 

The effect of THC, 7.5 mg and 15 mg, on auditory functioning has been investigated in eight men in a doubleblind, randomized, placebo-controlled, crossover trial (97). Blood concentrations of THC were measured for up to 48 hours after ingestion, and audiometric tests were carried out at 2 hours. There were no significant differences across treatments, suggesting that cannabis does not affect the basic unit of auditory perception. 

The use of the ending -on for the names of subatomic particles began with electron, a word that was coined in 1891 by the Irish physicist George J. Stoney (1826-1911) by modifying the word electric to come up with a name for the basic unit of electricity. This was six years before J. J. Thomson (1856-1940) actually measured the electron as a particle. 

In the next section, we introduce the standards for basic units of measurement. These standards were selected because they allow us to make precise measurements and because they are reproducible and unchanging. The values of fundamental units are arbitrary. In... 

Energy is measured in terms of its ability to perform work or to transfer heat. Mechanical work is done when a force / displaces an object by a distance d w =/ x d. The basic unit of energy is the joule. One joule is the amount of work done when a force of 1 newton acts over a distance of 1 m thus 1 J = 1 N-m. The newton is the amount of force required to accelerate a 1-kg mass by 1 m/sec2, so the basic dimensions of the joule are kg m2 s 2,Thc other two units in wide use, the calorie and the BTU (British thermal unit), are defined in terms of the heating effect on water. Because of the many forms that energy can take, there are a correspondingly large number of units in which it can be expressed. A few of these are summarized in Table 1. 

The basic unit of time is the second. Longer periods of time are measured in minutes and hours (instead of kiloseconds, etc.), but shorter periods use the regular metric prefixes. Thus a millisecond is 0.001 second. We should not have any trouble with time because we are so familiar with the longer periods and now have become familiar with the subunits. However, watch out for times stated in two units, such as "an hour and 15 minutes."... 

The English system is a collection of functionally unrelated units. In the English system of measurement the standard pound (lb) is the basic unit of weight. The fundamental unit of length is the standard yard (yd), and the basic unit of volume is the standard gallon (gal). The English system is used in the United States in business and industry. However, it is not used in scientific work, primarily because it is difficult to convert from one unit to another. Eor example. 

Replication means that the basic experimental measurement is repeated. For example, if one is measuring the CO2 concentration of blood, those measurements would be repeated several times under controlled circumstances. Replication serves several important functions. First, it allows the investigator to estimate the variance of the experimental or random error through the sample standard deviation (s) or sample variance (i ). This estimate becomes a basic unit of measurement for determining whether observed differences in the data are statistically significant. Second, because the sample mean (x) is used to estimate the true population mean (/a), replication enables an investigator to obtain a more precise estimate of the treatment effect s value. If s is the sample variance of the data for n replicates, then the variance of the sample mean is = s /n. 

The basic representation of a measurement itself in terms of the actual units of measure is often referred to as the raw form. For measures of performance, the term raw score is frequently applied. Generally, some form of assessment (i.e., judgment or interpretation) is typically required. Assessments may be applied to (or, viewed from a different perspective, may require) either a single measure of groups of them. Subjective assessments are frequently made that are based on the practitioner s familiarity with values for a given parameter in a particular context. However, due to the large number of parameters and the amount of experience that would be required to gain a sufficient level of familiarity, a more formal and objective realization of the process that takes place in subjective assessments is often employed. This process combines the measured value with objectively determined reference values to obtain new metrics, or scores, that facilitate one or more steps in the assessment process. 

In Chapter 2 of this text, you were introduced to the International System (SI) of measiuements. What are the basic units of this system for mass, distance, time, and temperature What are some of the prefixes used to indicate common multiples and subdivisions of these basic units Give three examples of the use of such prefixes, and explain why the prefix is appropriate to the quantity or measurement being indicated. 

SECTION 1.4 Measurements in chemistry are made using the metric system. Special emphasis is placed on SI units, which are based on the meter, the kilogram, and the second as the basic units of length, mass, and time, respectively. SI units use prefixes to indicate fractions or multiples of base units. The SI temperature scale is the Kelvin scale, although the Celsius scale is frequently used as well. Density is an important property that equals mass divided by volume. 

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