Physics - Units and Prefixes
OCR A-Level Physics 2022
Units and Prefixes
Units and Prefixes are a simple topic but very important and foundational for the rest of physics as a whole. It’s one of those things that are important learning because it improves fluency in thought involving physics problems and having a good conceptual understanding of what the units actually represent can also be helpful.
Units
Base and compound units
What is a base unit?
Standard units of measurement from which all other SI units are derived.
What is a compound unit?
An SI unit created from a combination of base units.
How do base units relate to base quantities?
Base units are standard units of measurement for the base quantities.
What are the 6 base quantities?
- length
- mass
- time
- electric current
- absolute temperature
- amount of substance
Base and SI units of specific quantities
Converting between unit powers
Q: There are $10^{24}$ free charge carriers per $m^3$ of material. How many are there per $mm^3$?
$10^{15}$
Q: $10^6$ particles strike each $mm^2$ of a surface per second. How many strike $1m^2$ of the surface per second?
$10^{12}$
Q: If there is 1 particle per $mm^2$, how many are there per $m^3$?
$10^9$
Q: Water has a density of $1000kgm^-3$. What is this in $kgcm^{-3}$?
$10^{-3}$
Q: Water has a density of $1000kgm^-3$. What is this in $gcm^{-3}$?
$1$
Units
Prefix values
Applying prefixes to variables
What is the first step in using a variable with a prefix?
- Replacing the prefix symbol with its equivalent multiplier.
- e.g. $0.16\muA = 0.16 \times 10^{-6}A$