Fundamental SI units

Measurements usually consist of a unit and a number expressing the quantity of that unit. Unfortunately, many different units may be used to express the same physical measurement. For example, the mass of a sample weighing 1.5 g also may be expressed as 0.0033 lb or 0.053 oz. For consistency, and to avoid confusion, scientists use a common set of fundamental units, several of which are listed in Table 2.1. These units are called SI units after the Systeme International d Unites. Other measurements are defined using these fundamental SI units. For example, we measure the quantity of heat produced during a chemical reaction in joules, (J), where... 

Electrical units. The fundamental SI unit is the unit of current which is called the ampere (A), and which is defined as the constant current which, if maintained in two parallel rectilinear conductors of negligible cross-section and of infinite length and placed one metre apart in a vacuum, would produce between these conductors a force equal to 2 x 10 7 newton per metre length. 

Newton s law states that force = mass X acceleration. You also know that energy = force X distance and pressure = force/ area. From these relations, derive the dimensions of newtons, joules, and pascals in terms of the fundamental SI units in Table 1-1. Check your answers in Table 1-2. 

Look back at the seven fundamental SI units given in Table 1.3 and you ll find that measures for such familiar quantities as area, volume, density, speed, and pressure are missing. All are examples of derived quantities rather than fundamental quantities because they can be expressed using one or more of the seven base units (Table 1.5). 

Accurate measurement is crucial to scientific experimentation. The units used are those of the Systeme Internationale (SI units). There are seven fundamental SI units, together with other derived units Mass, the amount of matter an object contains, is measured in kilograms (kg) length is measured in meters (m) temperature is measured in kelvins (K) and volume is measured in cubic meters (m3). The more familiar metric liter (L) and milliliter (mL) are also still used for measuring volume, and the Celsius degree (°C) is still used for measuring temperature. Density is an intensive physical property that relates mass to volume. 

What is the difference between a derived SI unit and a fundamental SI unit Give an example of each. 

The unconventional dimension of kg/m3 is the result of our consistent application of the SI rather than the older CGS system of units. The fundamental SI units are meter, kilogram (mass), second, ampere, Kelvin (K) an Candela, while force, weight, pressure etc. are derived magnitudes. For conversion tables see the back flyleaf of this volume. 

A fundamental characteristic of the SI system is the fact that each defined quantity has only one unit. Thus, the fundamental SI unit of energy is the joule and the fundamental SI unit of power is the watt. While a joule is defined as a newton meter, it refers to a unit force moving through a unit distance. The... 

TABLE A2.1 The Fundamental SI Units Physical Quantity Name of Unit Abbreviation... 

For many years, most scientists worldwide have used the metric system. In 1960 an international agreement established a system of units called the International System (le Systeme International in French), abbreviated SI. This system is based on the metric system and the units derived from the metric system. The fundamental SI units are listed in Table A2.1. 

One physical quantity that is very important in chemistry is volume, which is not a fundamental SI unit it is derived from length. A cube with dimensions of 1 m on each edge has a volume of (1 m)3 = 1 m3. Then, recognizing that there are 10 decimeters (dm) in a meter, the volume of the cube is (10 dm)3 = 1000 dm3. A cubic decimeter, dm3, is commonly called a liter (L), which is a unit of volume slightly larger than a quart. Similarly, since 1 dm equals 10 centimeters (cm), the liter (1 dm)3 contains 1000 cm3, or 1000 milliliters (mL). 

Electric current is tlie fundamental electrical dimension in SI its unit is the ampere (A). Detennine units for tlie following quantities, as combinations of fundamental SI units. 

The fundamental SI unit of activity is the Becquerel (Bq). One Bq is equal to one disintegration per second (dps). Because this is a very small unit, it is more often expressed in kilobequerels or kBq. However, the older historical unit of activity Q is normally used for radiopharmaceuticals. The Curie was defined in terms of the number of disintegrations per second of 1 g of Ra and is equal to 3.7 x 10 ° dps. Other commonly used imits are millicurie and microcurie (mCi and pCi). The unit of Ci represents absolute activity (A). However, relative activity R is proportional to the efficiency of the counting device. The device reports in counts per minute. 

Ampere The unit of measurement of current. 1 ampere = 1 coulomb/second. This is a fundamental SI unit from which many other electrical units are developed. If two infinitely long wires are placed parallel to each other, 1 meter apart, and an electrical current is passed through them such that 1 meter of length contains a magnetic force of 2 x 10 Newtons between the two wires, then 1 ampere of current is flowing in each wire. 

The fundamental SI unit of length is the meter, which is a little longer than a yard (1 meter = 39.37 inches). In the metric system fractions of a meter or multiples of a meter can be expressed by powers of 10, as summarized in Table 5.3. 

Another important measurable quantity is mass, which can be defined as the quantity of matter present in an object. The fundamental SI unit of mass is the kilogram. Because the metric system, which existed before the SI system, used the gram as the fundamental unit, the prefixes for the various mass units are based on the gram, as shown in Table 5.5. 

Express each of the following units in terms of the fundamental units (cm, g, s) of the Gaussian system (a) dyne (b) erg (c) statcoulomb. Express each of the following units in terms of fundamental SI units (m, kg, s) (d) newton (e) joule. 

Figure 1-1 Of the fundamental SI units in Table 1-1, only the kilogram is defined by an artifact, rather than by a reprodudble physical measurement. The international kilogram in France, made from a Pt-lr alloy in 1885, has been removed from its protective enclosure to be weighed against working copies only in 1890,1948, and 1992. Its mass could change from reaction with the atmosphere or from wear, so there is ongoing research to define a standard for mass based on measurements that should not change over time. [Bureau International des Folds et Mesures.]...

The low-suspension concentration will allow use of the properties of the suspending medium in calculations. Reasonable values for density and viscosity of water at approximate room temperature are 1000 kg/m and 0.001 kg/m s, respec-tiveiy. The feed rate f(Q) is 0.001 mVs and the pressure drop 2 x lO kg/s m, both expressed in fundamental SI units. The characteristic velocity within the hydrocyclone can be calculated from Equation 10.38 ... 

Express each of the following units in terms of fundamental SI units (m, kg, s) (a) newton (b) joule. 

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