{Physics} Physical quantities, standards and unit- topic for students and competative examination (USA UK India others)

Physical quantities standards and units

1. Laws of physics and expressed in terms of physical quantities such as time, force, temperature, density and numerous others. Physical quantities are often divided into fundamental and derived quantities. Derived quantities are those whose definitions are based on other physical quantities , e.g. speed area density etc. Fundamental quantities are not defined in terms of other physical quantities e.g. length, mass and time. 

Math problem on Average topic

Physical quantities may, in general, be divided in two classes: 

(1)  Scalar quantities and (2) vector quantities. Is killer quantity is one which has only magnitude. A vector quantity has both magnitude and direction. Thus, when we say that the height of a tree is 20 m or there is 5 l of water in a bucket, we are dealing with scalar quantities. On the other, when we say that a force of two Newton's ( newton is a unit of force) is acting on a body, the information is incomplete unless we state the direction of the force, for instance two Newton's vertically upwards. Force is there for a vector quantity. Mass, length, time, volume, speed, energy ,work are examples of scalar quantities. Velocity ,momentum ,force acceleration are examples of vector quantities. 

  The measurement of physical quantities involves two steps: (1) the choice of a standard unit and (2) the comparison of the standard to the quantity to the measured. Does a number and unit determine the measure of a quantity. For example, when we say that the mass of a person is 35 kilograms, it means that he is mass is 35 times the unit of mass, kilogram. 

  Does all measurement in physics required standard units. Earlier workers in various countries used different system of units. In 1960, the General Conference of Weight and Measures recommended that a metric system of measurement called the International System of Units, abbreviated as SI units, be used.

  1.1. UNIT of LENGTH:

      The SI unit of length is the metre (m) , originally divined as the distance between two lines on a particular platinum iridium rod at 0°C  kept at the International Office of Weight  and Measures at Severs near Paris.

 In 1983, the General Conference of Weight and Measures redewined the metre as a length of the path travelled by a light in vacuum during a time interval of 1/299792458 of a second.

 Various other matric unit used for measuring length are related to the metre by either multiples or submersibles of 10. Thus 

1 kilometre (km) =1000 m

1 centimetre(cm) = 10-² or 1/100 m

1 millimetre (mm) = 1/1000 m

Very small distance are measured in micrometers or microns, angstroms, nanometers and femtometers.

 

 In practice large distance are expressed in kilometres, wavelength of light in angstroms and nuclear size in femtometres. The astronomical distances, e.g. the distance of a star from the Earth, are expressed in light years. The light year is a unit of length and is equal to the distance travelled by light in one year.

 1 light year = 9.46 x 10¹⁵ meter.

1.2 Unit of Mass: 

       The SI units of mass is the kilogram. The standard kilogram is the mass of a particular solid cylinder of platinum-Iridium alloy kept at Sevres. 

1.3 Unit of Time: 

       The SI unit of time is the second. The second was formally the 1/ 86400 th part of a mean solar day. The mean solar day is the average period between successive transit of the sun across the meridian, taken over 12 months, at any part of the Earth's  surface. Since the speed of rotation of the Earth is slowing down with time, the second so defined was not considered reliable for very accurate measurements. The second world's redefined in 1967 by using an atomic clock who is works on energy changes in gaseous caesium atoms. The caesium atoms in the atomic clock act like a pendulum in a pendulum clock.