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**

10âˆ'15 femto F

10âˆ'12 pico p

10âˆ'9 nano n

10âˆ'6 micro

10âˆ'3 milli m

10âˆ'2 centi c

10âˆ'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 sâˆ'1 x 1 year (in seconds)

*=* 3 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* 10*11* m*

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

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