Competitive Exams: SI Derived units

Examples of SI derived units whose names and symbols include SI derived units with special names and symbols

Derived quantity

Name

Symbol

dynamic viscosity

pascal second

Pa x s

moment of force

newton meter

N x m

surface tension

newton per meter

N/m

angular velocity

radian per second

rad/s

angular acceleration

radian per second squared

rad/s2

heat flux density, irradiance

watt per square meter

W/m2

heat capacity, entropy

joule per kelvin

J/K

specific heat capacity, specific entropy

joule per kilogram kelvin

J/(kg x K)

specific energy

joule per kilogram

J/kg

thermal conductivity

watt per meter kelvin

W/(m x K)

energy density

joule per cubic meter

J/m3

electric field strength

volt per meter

V/m

electric charge density

coulomb per cubic meter

C/m3

electric flux density

coulomb per square meter

C/m2

permittivity

farad per meter

F/m

permeability

henry per meter

H/m

molar energy

joule per mole

J/mol

molar entropy, molar heat capacity

joule per mole kelvin

J/(mol x K)

exposure (x and rays)

coulomb per kilogram

C/kg

absorbed dose rate

gray per second

Gy/s

radiant intensity

watt per steradian

W/sr

radiance

watt per square meter steradian

W/(m2 x sr)

catalytic (activity) concentration

katal per cubic meter

kat/m3

Examples of SI derived units whose names and symbols include SI derived units with special names and symbols

Units outside the SI that are accepted for use with the SI

Name

Symbol

Value in SI units

minute (time)

min

1 min = 60 s

hour

h

1 h = 60 min = 3600 s

day

d

1 d = 24 h = 86 400 s

degree (angle)

°

1° = (i/180) rad

minute (angle)

1 = (1/60)° = (i/10 800)

rad

second (angle)

1inch = (1/60) = (i/648 000) rad

inch

liter

L

1 L = 1 dm3 = 10 − 3 m3

metric ton (a)

t

1 t = 103 kg

neper

Np

1 Np = 1

bel (b)

B

1 B = (½) ln 10 Np (c)

electronvolt (d)

eV

1 eV = 1.602 18 × 10 − 19 J, approximately

unified atomic mass unit (e)

u

1 u = 1.660 54 × 10 − 27 kg, approximately

astronomical unit (f)

ua

1 ua = 1.495 98 × 1011 m, approximately

  1. In many countries, this unit is called tonne.

  2. The bel is most commonly used with the SI prefix deci: 1 dB = 0.1 B.

  3. Although the neper is coherent with SI units and is accepted by the CIPM, it has not been adopted by the General Conference on Weights and Measures (CGPM, Conference Generale des Poids et Mesures) and is thus not an SI unit.

  4. The electronvolt is the kinetic energy acquired by an electron passing through a potential difference of 1 V in vacuum. The value must be obtained by experiment, and is therefore not known exactly.

  5. The unified atomic mass unit is equal to 1/12 of the mass of an unbound atom of the nuclide 12C, at rest and in its ground state. The value must be obtained by experiment, and is therefore not known exactly.

  6. The astronomical unit is a unit of length. Its value is such that, when used to describe the motion of bodies in the solar system, the heliocentric gravitation constant is (0.017 202 098 95) 2 ua3 × d − 2. The value must be obtained by experiment, and is therefore not known exactly.

The liter in Table given in the previous page deserves comment. This unit and its symbol l were adopted by the CIPM in 1879. The alternative symbol for the liter, L, was adopted by the CGPM in 1979 in order to avoid the risk of confusion between the letter l and the number 1. Thus, although both l and L are internationally accepted symbols for the liter, to avoid this risk the preferred symbol for use in the United States is L. Neither a lowercase script letter l nor an uppercase script letter L are approved symbols for the liter.

Other units outside the SI that are currently accepted for use with the SI by NIST are given in Table 7. These units, which are subject to future review, should be defined in relation to the SI in every document in which they are used; their continued use is not encouraged. The CIPM currently accepts the use of all of the units given in Table 7 with the SI except for the curie, roentgen, rad, and rem. Because of the continued wide use of these units in the United States, NIST still accepts their use with the SI.

Other units outside the SI that are currently accepted for use with the SI, subject to further review

Name

Symbol

Value in SI units

1 nautical mile = 1852 m

knot

1 nautical mile per hour = (1852/3600) m/s

are

a

1 a = 1 dam2 = 102 m2

hectare

ha

1 ha = 1 hm2 = 104 m2

bar

bar

1 bar = 0.1 MPa = 100 kPa = 1000 hPa = 105 Pa

Angstrom

Å

1 Å = 0.1 nm = 10 − 10 m

barn

b

1 b = 100 fm2 = 10 − 28 m2

curie

Ci

1 Ci = 3.7 × 1010 Bq

roentgen

R

1 R = 2.58 × 10 − 4 C/kg

rad

rad

1 rad = 1 cGy = 10 − 2 Gy

rem

rem

1 rem = 1 cSv = 10 − 2 Sv