Units in Physics (mechanical, electricity, magnetism, light and optics) including Si units.

Posted on Tuesday the 27th of February, 2007 at 8:11 pm in Physics

This is a reference list with notes of all SI and derived units in physics. The notes provide a brief explanation of some of the more confusing elements, but be warned that the full explanation could take many pages, and may be explained elsewhere on this website.

Physics has only 5 base units. (Plus the SI units Mole and Candela, but these are rarely used in Physics.)

Name Abbreviation (Symbol) Standard Unit Notes
Name Abbreviation (Symbol) Standard Unit Notes
Length l, x (for distances) Meter (m) A meter is defined as the distance light travels in a vacumm in \frac{1}{299 792 458} of a second (in physics it is customary to use metric measurements although the basic principles apply if you to use feet instead of meters)
Mass m, M (when used with measurements in meters) Kilogram (kg) A kilogram is defined as the weight of a specific platinum-iridium cylinder
Time t Second (s) Seconds are defined as 9,192,631,770 vibrations of radiation from a cesium atom
Temparature T Kelvin (K) A degree kelvin is defined as \frac{1}{273.16} of the distance between absolute 0 and the triple point of water
Electric Current I Ampere (A) An ampere is the amount of charge (C) passing through a surface per second, and is defined as the current which produces a force of 2*10^{-7} newtons per meter of length between two infinitely long, perfectly straight and parallel conductors with an infinitely small cross section separated by one meter in a vacuum..

Each of these base units is defined on fundamental constants, and all other units are based on these five units. At times it useful to break longer equations down to their most basic units to determine if the equation makes sense. The most common combinations of these basic units are given their own symbols and names. These common units are as follows.

Name (alphabetically) Abbreviation (Symbol) Unit – Derivation Notes
Name (alphabetically) Abbreviation (Symbol) Unit – Derivation Notes
Acceleration a \frac{\text{m}}{\text{s}^2} (meters per second squared) Acceleration is literally the rate of change of the rate of change of an object’s position.
Angle \theta,\varphi radian A radian is defined as the angle an arc length, equal to the circle’s radius, makes with the center of the circle.
Capacitance C Farad (F) \frac{\text{C}}{\text{V}} (Charge over the Potential)
Charge Q ), q ,e (of elementary particles) Coulomb (C) \text{A*s} (Amperes times seconds) Literally the charge is the amount of current that flows over the entire time period.
Density \rho \frac{\text{kg}}{\text{m}^3} Density is the amount of mass in every cubed unit length
Displacement s, d (distance), h (height) meters – m Displacement is the total change in length in any single direction. Sometimes it is used as the absolute change in distance –if you were walk all the way around the earth less one meter, your displacement would be one meter
Electric Field E \frac{\text{V}}{\text{m}} (Electric potential per meter) An electric field is the amount of electric potential over any given area.
Electric Flux \Phi_e \text{V*M}(Electric Potential time meters) Electric flux is the electric field through some area.
Electromotive Force (emf) \scr{E} or \epsilon Volt (V) Electromotive force is a potential difference in volts.
Electron Volt eV \text{e*J} The Electron Volt is the amount of energy change of a charge-field system when a charge of magnitude e is moved through a potential difference of 1V. It is used in place of the Joule (energy).
Energy E (total),U (potential),K (kinetic) Joule (J) kg * \frac{\text{m}^2}{\text{s}^2}
Entropy S \frac{\text{J}}{\text{K}}
Force F Newton (N) kg \frac{\text{m}}{\text{s}^2}, \frac{\text{J}}{\text{m}}
Frequency f,v Hertz (Hz) \frac{\text{cycles}}{\text{s}}
Heat Q Joule (J) Joule is used for work, heat and energy, but remember that Joule is a unit of energy not of energy transfer. This means that heat can have a Joule of energy, but can’t be measured in Newtons.
Magnetic Field B Telsa (T) \frac{\text{Wb}}{\text{m}^2}
Magnetic Flux \Phi_m Weber (Wb) \text{kg} \frac{\text{m}^2}{\text{A}} \text{s}^2
Momentum p \text{kg} \frac{\text{m}}{\text{s}}
Potential (Electric) V or \delta V Voltage (V) \frac{\text{J}}{\text{C}} Potential is the amount of excess charge. It can be compared with potential energy.
Power P, \scrP \frac{\text{j}}{\text{s}} Power is the amount of work done in any given time.
Pressure P Pascal (Pa) \frac{\text{N}}{\text{m}^2}
Resistance R Ohm \frac{\text{V}}{\text{A}} Resistance is the amount of energy that is lost in the transfer of energy through an object.
Torque \tau \text{N*m} Torque is usually shown with units N*m even though it is technically joules.
Velocity v \frac{\text{m}}{\text{s}} Velocity is the speed and direction of an object.
Wavelength \lambda Meter (m)
Work W Joule (J) \text{N*m} Work is the amount of force outputted over some distance.

For of the derived units such as electric flux there are multiple possible unit definitions. The two standard definitions for electric flux are Vm and \frac{Nm^2}{C} as the following derivation from the former to the latter shows they are both correct.

V*m = \frac{J}{C}m = \frac{\frac{kg*m^2}{s^2}}{As}*m = \frac{kg*m^3}{As^3} = \frac{Nm^2}{As} = \frac{Nm^2}{C}

This derivation only used units given in the above table, but there are other ways.

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  1. Holbrook chick posted the following on May 20, 2008 at 10:49 am.

    you need more about the units used for the formulas such as what units mass can be in ect,it would be very helpful!!!!!
    —-Thanks—-

    Reply to Holbrook chick
  2. ghhh posted the following on July 9, 2008 at 2:30 am.

    :twisted: :oops: :mrgreen: :idea: :| :arrow: :idea: :?: :!:

    Reply to ghhh
    1. samir ghosh posted the following on March 18, 2009 at 8:49 am.

      can you tell me please what is the SI unit of stress?

      Reply to samir ghosh
      1. archana posted the following on July 16, 2009 at 7:49 am.

        unit of stress is NEWTON PER METER SQAURE

        Reply to archana
      2. mohamed sulaiman posted the following on February 3, 2010 at 3:31 am.

        can you please tell / explain to me about physics unit called victor the formula application and how it could be drived

        Reply to mohamed sulaiman
      3. zulaikha posted the following on September 26, 2010 at 8:03 am.

        the si unit of stree is mega pascal

        Reply to zulaikha
        1. jamesavery posted the following on June 1, 2011 at 11:31 pm.

          ummmmm idk…geez :oops:

          Reply to jamesavery
    2. memi posted the following on September 12, 2011 at 11:38 am.

      :D yeah..gudddddddddddddddddddddddddd

      Reply to memi
  3. sameer khan posted the following on December 6, 2009 at 12:20 pm.

    the unit of stress if pascal and newton per meter sqared both ..

    the unit of magnetic field strength is ampere per meter and physical units dont any any strength

    Reply to sameer khan
  4. ksudhakar posted the following on October 6, 2010 at 2:25 pm.

    :idea:

    Reply to ksudhakar
  5. S.HARSHITHA REDDY posted the following on November 15, 2010 at 12:17 am.

    IS IT POSSIBLE TO LEARN ALL THE DIMENSIONS AND UNITS AT THE AGE OF 8YEARS

    Reply to S.HARSHITHA REDDY
    1. subhajit posted the following on May 16, 2011 at 2:30 am.

      :mrgreen: yes itsnot impossible at all !! 8O 8O :idea: :idea:

      Reply to subhajit
  6. sainath posted the following on December 13, 2010 at 12:03 pm.

    :D

    Reply to sainath
  7. Barnamay Samanta posted the following on March 24, 2011 at 10:59 am.

    :) :) :) :) :) :) :D :o 8O :lol: :oops: :roll: gggggggoooooooooooooooooooooooooooooooddddd

    Reply to Barnamay Samanta
  8. M.vignesh posted the following on March 28, 2011 at 11:06 am.

    I expect something more from you……..

    Reply to M.vignesh
  9. HOSLAMA BEN LARDEN posted the following on May 2, 2011 at 4:40 pm.

    ROW ROW FIGHT DA POWER

    Reply to HOSLAMA BEN LARDEN
  10. kashif posted the following on May 5, 2011 at 3:54 am.

    i like these unit. it is very helpful i am very thank ful to all

    Reply to kashif
  11. riza arenal posted the following on June 26, 2011 at 9:47 pm.

    its not really well xplained…. :(
    not so clear :o :D :lol:

    Reply to riza arenal
  12. cayleen posted the following on June 27, 2011 at 7:05 am.

    nice ..
    it really works to me thanks a lot!!
    more power and goDblESS :o :D :) ;)

    Reply to cayleen
  13. bibhas posted the following on August 20, 2011 at 7:17 pm.

    Thank you.This is really very helpful to me.

    Reply to bibhas
  14. oriscia clarle posted the following on November 23, 2011 at 6:11 pm.

    i told my physics teacher that you have to give the symbol for the quanity…. i was right all a long thank :D :!: :!: :!:

    Reply to oriscia clarle
  15. viji posted the following on December 17, 2011 at 5:58 am.

    its very useful thanks :lol: :oops: :roll: :evil:

    Reply to viji
  16. physics_hata posted the following on January 19, 2012 at 10:39 am.

    I have my physics semester 1 exam 2day :cry:

    Reply to physics_hata
  17. physics_hata posted the following on January 19, 2012 at 10:41 am.

    I have my physics semester 1 exam 2day :evil: :cry: :| :x :? :(

    Reply to physics_hata

Leave a reply

:) :D :( :o 8O :? 8) :lol: :x :P :oops: :cry: :evil: :twisted: :roll: ;) :!: :?: :idea: :arrow: :| :mrgreen: