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# Expressing larger and smaller physical quantities

Once the fundamental units are defined, it is easier to express larger and smaller units of the same physical quantity. In the metric (SI) system these are related to the fundamental unit in multiples of 10 or 1/10. Thus 1 km is 1000 m and 1 mm is 1/1000 metre. Table lists the standard SI prefixes, their meanings and abbreviations.

Expressing larger and smaller physical quantities

Once the fundamental units are defined, it is easier to express larger and smaller units of the same physical quantity. In the metric (SI) system these are related to the fundamental unit in multiples of 10 or 1/10. Thus 1 km is 1000 m and 1 mm is 1/1000 metre. Table  lists the standard SI prefixes, their meanings and abbreviations.

In order to measure very large distances, the following units are used.

(i) Light year

Light year is the distance travelled by light in one year in vacuum.

List: Prefixes for power of ten

Power of ten        Prefix         Abbreviation

10�'15  femto F

10�'12  pico   p

10�'9   nano  n

10�'6   micro m

10�'3   milli   m

10�'2   centi  c

10�'1   deci   d

101    deca   da

102    hecto h

103    kilo    k

106    mega M

109    giga   G

1012   tera    T

1015   peta   P

Distance travelled = velocity of light x 1 year

1 light year    = 3 x 108 m s�'1 x 1 year (in seconds)

= x 108 x 365.25 x 24 x 60 x 60

=     9.467 x 1015 m

light year  = 9.467 x 1015 m

(ii) Astronomical unit

Astronomical unit is the mean distance of the centre of the Sun from the centre of the Earth.

1 Astronomical unit (AU) = 1.496 x 1011 m

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11th 12th std standard Class Physics sciense Higher secondary school College Notes : Expressing larger and smaller physical quantities |