Summary physics units and measurements

1
Units and
Measurement
The compa rison of an
y physical qu an ti ty
called m ea su re m en w it h it s st an d~ rd un
t. it is

Physical Quantities
All the quantities w
hich ca n be measure
terms of which th e la d directly or indire
ws of Physics ar e de ctly in
AUantities. scribed ar e called ph
ysical


standard amount of
a physical quantity
yeical quantity of th chosen to measure
e same kind is calle th e
· y reproducible, in d a physical unit. It
ternationally accept should be
ed.
ndamental Quantiti
es and Their Units
ph ysical qu an ti ti es w
hich ar e independen
d fundamental q u t to each other are
an ti ti es an d th ei r un
amental units. it s ar e called
----------
!lftllllllltllltal --
Fundamental units
Name Symbol Definition
Metre m The metre is the len
gth of the path
travelled by light in va
cuum during a
time interval of l/29
9,792.458 of a
-- second . ll 98 3)
Kilogram kg The kilogram is equa
l to the mass of
the lntemat1011al prot
kq ra m (a plat1num otype of ttle
-,n
cylinder) kept at Inte dlum all<.: y
rnational B11r"'d-.. 1
W9iWlts and Measure
s at Se~,e~ n ... d,
Pa s fr a r,e 1188
9\

, 2 Han dbook of Physics
OE
fun dam ent al units
Definition 'fh
Fundamental - -c-d-·uratlon of - --.
· - .- th qu
Name Symbol __ - · - - -d d
quantities
-- -- - - - - The scc on ,77 15 . f ti
iod s o _1e ra 1::it,r,,
de
5
1 0 per
Second 9 , 192 ' 63nd,· ng to the tran ~1t,on bctwcu S•l
corr two hyp erfin e levels of lh c grounr1
esp o
the A
. (1967)
sta te of the.
cesium __ -13 .. 3at om
The ampere is t~at c~nstant ~ur~ent
de
A h. h if maintained ,n two s.rJ1gl1 t
u11
Electric current Ampere
;ar~ll~I conduc~ors of inf1~1te l~ngth,
of negligible circular _cros -;-sect,on
and placed 1 m apart ,n vacuum,
would produce a force betw en the5e 7
conductors equal to 2 x 1a· Nim of
length . (19 48) _
K The kel-vin is the fraction 1/~73. 16 of
Thermo dynamic Kelvin the thermodynamic temrerature of the
temperature triple point of water. (1967)
mol The mole is the amour)t of substance
c
Amount of Mole a system which contains as many
substance elementary entities as there are ato ms
in 0.012 kg _of ~.9r~on - 12. (19 71) _
The candela is the luminous intensi ty
i
Can dela cd its
Luminous a given direction of a source that em
intensity monochromatic radiation of frequency
540 x 1d Hz and that has a rad iant
2

intensity in that direction of 1/683 wa
P~~ -~~era9ian . (1979)
__ __ __ __ __ __ __ __
tit ies an d Th eir Un its
Supplementary Fundamental Qu an
n and ste rad ian are two sup ple me nta ry fun dam ent al un its . It
Ra dia
pectively.
measures plane angle and solid angle res
- - -- ---- --- - - - - - Supple mentary units
· Supplementary ·-- -- - - -- ·
-- - - - - - - - - - - -- -
fundamental - -
Name Symbol Definition
quantities
-- - --- - - ·- ·- -- the
PIJne angle Radian . rad One radian is the angle subtended at
gth to
centre of a circle by an arc equal in len
the radius of the circle. i.e. d8 = ds
r
Sol,,j angle
·- ~-- - ~ -.
Steradian -- =- -- =- -- -- -- -- --
solid
-le subt~~ded ·a
ang
sr One steradian is the
of the
the centre of a sphere, by that surface
~quJrf
sph_ere , which is equal in areJ , to the
rad,us of the si:;here .
1.e. dA