• A quantity is an “order of magnitude” larger than another quantity if it is about ten times larger
  • Similarly, two orders of magnitude would be 100 times larger, or 102

   

• When estimating values, it’s best to give the estimate of an order of magnitude to the nearest power of 10
• For example, the diameter of the Milky Way is approximately 1,000,000,000,000,000,000,000 m
• It is inconvenient to write this many zeros, so it’s best to use scientific notation as follows:

1,000,000,000,000,000,000,000 = 1 × 1021?m

• The order of magnitude is the part that indicates the number of zeros after the 1 ie. 1021
• Orders of magnitude make it easier to compare the relative sizes of objects, for example, a quantity with an order of magnitude of 106?is 10 000 times larger than a quantity with a magnitude of 102

1. The temperature of the surface of the Sun in Kelvin

• • The temperature of the surface of the Sun is about 6000 K
  • This is an order of magnitude of ~ 104?K

   

2. The power of a standard lightbulb

• • The power of a standard lightbulb is about 60 W
  • This is an order of magnitude of ~ 102?W

   

3. The volume of the room you are in now

• • This depends on the room you are in
  • The shape should roughly be cubic or (rectangular) cuboid
  • Volume = length × width × height
  • For a cubic room with length 3 m, volume = 33?= 27 m3
  • This is an order of magnitude of ~ 10 m3

   

• There are important physical quantities to learn in physics
• It is useful to know these physical quantities, they are particularly useful when making estimates
• A few examples of useful quantities to memorise are given in the table below (this is by no means an exhaustive list)

Estimating Physical Quantities Table