SI units Flashcards
What are SI units?
(Publication date, where used)
A metric system of measurement comprising seven base units, derived units and prefixes. These can be used to produce fractions or multiples of units by a factors of 10.
Published in 1960
Adopted throughout the world, except in the USA
What are the seven fundamental SI units?
Acronym SMMACKK:
Second (s)
Mole (mol)
Metre (m)
Ampere (A)
Candela (cd)
Kelvin (K)
Kilogram (kg)
All other units in the SI system can be derived from these seven base units
Second
(symbol, quantity of, description)
s
Time
The duration of a certain number of oscillations of a caesium-133 atom
Mole
(symbol, quantity of, description)
mol
Amount of substance
The amount of substance that contains the same number of particles as there are atoms in 0.012 (12g) of carbon-12
Metre
(symbol, quantity of, description)
m
Length or distance
Length travelled by light in a vacuum in 1/300,000,000 of a second
Ampere
(symbol, quantity of, description)
A
Electric current
The current in two parallel conductors of infinite length, placed 1m apart in a vacuum that would produce force of 2x10-7 N/m
Candela
(symbol, quantity of, description)
cd
Intensity of illumination
Luminous intensity of monochromatic light
Kelvin
(symbol, quantity of, description)
K
Thermodynamic temperature
1/273.16 of the thermodynamic triple point of water
Kilogram
(symbol, quantity of, description)
kg
Mass
The mass of the prototype kilogram held in Sevres, France. NB this is the only physical quantity within the base units. However there is debate about whether to change this as the prototype is slowly degrading
What is the base unit for time?
Second
=the duration of a certain number of oscillations of a caesium-133 atom
What is the base unit for amount of substance
Mole (mol)
=the amount of substance that contains the same number of particles as there are atoms in 0.012kg (12g) of carbon-12
What is the base unit for length or distance
Metre (m)
=length travelled by light in a vacuum in 1/300,000,000 of a second
What is the base unit for electric current
Ampere (A)
=the current in two parallel conductors of infinite length, placed 1m apart in a vacuum that would produce force of 2x10-7 N/m
What is the base unit for intensity of illumination
Candela (cd)
=Luminous intensity of monochromatic light
What is the base unit for temperature
Kelvin (K)
=1/273.16 of the thermodynamic triple point of water
What is the base unit for mass?
Kilogram (kg)
=the mass of the prototype kilogram held in Sevres, France
How do prefixes to SI units work?
Any SI unit can have a prefix which gives it a fraction or multiple of 10. When used it is still classified as an SI unit
e.g. a centimeter is 0.01 of a metre, and therefore STILL classified as an SI unit.
For example, although pascal is the SI unit for pressure, kilopascal is also classed as an SI unit as it uses the kilo-prefix
Note that while the base unit for mass is the kilogram, prefixes to mass work relative to a gram.
State the symbol and multiple of 10 of the following SI prefixes:
Pico
Nano
Micro
Milli
Centi
Deci
Pico (p): 10-12
Nano (n): 10-9
Micro (µ): 10-6
Milli (m): 10-3
Centi (c): 10-2
Deci (d): 10-1
State the symbol and multiple of 10 of the following SI prefixes:
Deca
Hecto
Kilo
Mega
Giga
Tera
Deca (da): 10^1
Hecto (h): 10^2
Kilo (k): 10^3
Mega (M): 10^6
Giga (G): 10^9
Tera (T): 10^12
Area
(Derived unit of, symbol)
Square metre
m^2
Volume
(Derived unit of, symbol)
Cubic metre
m^3
Speed
(Derived unit of, symbol)
metres per second
m.s^-1
Velocity
(Derived unit of, symbol)
metres per second, with direction
m.s^-1
Acceleration
(Derived unit of, symbol)
metres per second squared
m.s^-2
Current density
(Derived unit of, symbol)
Ampere per square metre
A.m^-2
Hertz
(symbol, quantity of, equivalents, SI base unit)
Hz
Frequency
No equivalents
s^-1
Newton
(symbol, quantity of, equivalents, SI base unit)
N
Force
No equivalents
kg.m.s^-2 aka kg x m/s2
Note: newtons are the product of mass (kg) multiplied with acceleration (metres per second per second squared)
Pascal
(symbol, quantity of, equivalents, SI base unit)
Pa
Pressure, stress
N/m^2
kg.m^-1.s^-2 aka kg/m/s2
Joule
(symbol, quantity of, equivalents, SI base unit)
J
Energy, work, heat
N⋅m, C⋅V, W⋅s
kg⋅m^2⋅s^-2 aka kg x m2/s2
Watt
(symbol, quantity of, equivalents, SI base unit)
W
Power
J/s, V⋅A
kg⋅m^2⋅s^-3
Coulomb
(symbol, quantity of, equivalents, SI base unit)
C
Electrical charge or quantity
No equivalents
s.A
Volt
(symbol, quantity of, equivalents, SI base unit)
V
Voltage, electrical potential
W/A, J/C
kg⋅m^2⋅s^-3⋅A^-1
Farad
(symbol, quantity of, equivalents, SI base unit)
F
Capacitance
C/V, s/Ω
kg^-1⋅m^-2⋅s^4⋅A^2
Ohm
(symbol, quantity of, equivalents, SI base unit)
Ω
Resistance, impedance
V/A
kg⋅m^2⋅s^-3⋅A^-2 aka kg x m2/s3/A2
Henry
(symbol, quantity of, equivalents, SI base unit)
H
Inductance
V.s/A
kg⋅m^2⋅s^-2⋅A^-2
Weber
(symbol, quantity of, equivalents, SI base unit)
Wb
Magnetic flux
T.m^2
kg⋅m^2⋅s^-2⋅A^-1
Tesla
(symbol, quantity of, equivalents, SI base unit)
T
magnetic flux density
Wb/m^2
kg⋅s^-2⋅A^-1
Define one day in the SI system
Time is NOT in the SI system
1 day is 86,400 seconds i.e. 86.4 kiloseconds in the SI system
Define Litre in the SI system
Litres (and derivatives e.g. ml) are NOT in the SI system
1L = 1000 cm^3
Define degrees celsius in the SI system
Degrees celsius is NOT in the SI system
Convert degrees celsius to kelvin by adding 273.15
i.e. 1 degree celsius = 274.15K
What is the conversion between atmosphere, bar kPa, mmHg and cmH20
1 atmosphere = 1.013 Bar = 101.3 kPa = 760mmHg = 1020 cmH20
What is the conversion between kPa and mmHg?
1kPa = 7.6 mmHg
What is the conversion between between cmH2O and mmHg?
10cmH2O = 7.6mmHg
Define weight
Weight is force acting upon mass
i.e. people on the moon have less force i.e. gravity acting upon them so they weigh less, even though their mass is the same
What is the conversion between celsius and Kelvin?
Kelvin = celsius - 273.15
Do derived units always have to be expressed as base units?
No
Derived units can be expressed as base units, a combination of base and derived units, or two derived units.
The Pascal, for example, can be expressed as base units (kg/m-1/s-2) or a combination of base and derived units (N/m2).
