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Units and Prefixes

Updated on October 4, 2010

Units and Prefixes

In Physics we are often concerned with measuring. There is an international system of units that is used by scientists worldwide. This is known as the SI system, ( Systmem Internationale).

The base units in this system are:

Mass:- measured in Kilograms (kg)


Length:- measured in metres (m)


Time:- measured in seconds (s)


Current:- measured in amperes (amps.A)


Temperature:- measured in Kelvin (K)


Other quantities are also measured in particular units derived from the above.


Examples of these derived units are:


Speed:- measured in metres per second (m/s)


Force:- measured in newtons (N)


Energy:- measured in joules ( j )


Charge:- measured in coulombs (C)


Voltage:- measured in volts (V)


There are more units which I will introduce within other hubs as I go along.


Sometimes the size of the quantity being measured is much too large or small for the appropiate unit. In this situation we use multiples of the unit denoted by standard prefixes.


The common prefixes used at GCSE are:



Prefix       nano       micro       milli             kilo       mega       giga

Factor       10-9           10-6           10-3         I     103            106            109

Symbol       n            μ             m                k            M            G




In these cases the unit is multiplied by the factor,

for example:


5 milliamps (5mA) = 5 x 10-3 A


Now try the following:-


1. Convert 20km to m


2. How many amps is 30 μA?


3. What is 5000m in km?


4. Give 40000joules in MJ


Workings of examples

1. Convert 20km to m

On the calculator:


   20 x the factor of the symbol km

= 20 x 103

= 20000m

If the case is that your calculator isn't handy it can be done by this alternative method:



20 = 2 x 101

   = 2 x 101 x 103 (remember when multiplying you add the indices)

   = 2 x 104 (104 means you have four zeros, therefore the answer is 20000)

   = 20000m


2. How many amps is 30 μA ?

On the calculator:


   30 x the factor of the symbol μ

= 30 x 10-6

= 3 x 10-5

To do it manually without the calculator:


30 = 3 x 101

    = 3 x 101 x 10-6 (remember when multiplying you add the indices)

    = 1 + (-6) = -5 (therefore)

    = 3 x 10-5A


3. What is 5000m in km?


Remember there is 1000m in a km.


Quite simply you could divide by 1000 and get the answer as 5km.


On the Calculator:




    5000 x the factor by the symbol m

= 5000 x 10-3

= 5km


To do it manually without the calculator:


5000 = 5 x 103

         = 5 x 103 x 10-3

         = 3 - 3 = 0 (therefore)

         = 5km



4. Give 40000joules in MJ


1 megajoule = 1 000 000 joules


On the Calculator:


           40000 x the factor of the symbol j


In this case we don't have it in the above table, so we are going to divide by the factor MJ, which means our answer will be a fractional one, so it will have a negative power.




          40000 x the factor of the symbol M

             = 40000 ÷ 106

             = 4 x 104 ÷ 106

             = 0.04 MJ or 4 x 10-2 MJ

To do it manually without the calculator:


40000 = 4 x 104

          = 4 x 104 ÷ 106 ( when dividing remember to subtract the indices)

          = 4 - 6 = -2

          = 4 x 10-2 MJ


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    • t.elia profile imageAUTHOR


      5 years ago from Northern Ireland

    • profile image


      6 years ago

      Thank you t.elia

      i have learnt good staff and i solved all the question that you asked

      give us more please


    • t.elia profile imageAUTHOR


      8 years ago from Northern Ireland

      Cheers Carolina Muscle thanks

    • carolina muscle profile image

      carolina muscle 

      8 years ago from Charlotte, North Carolina

      I'm gonna bookmark this hub, so I can have these formulas handy. I'm an idiot when it comes to metric.


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