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SI UNITS!
- Skyler Siu
- Apr 2
- 3 min read
I still remember how my physics teacher kept saying “be mindful of your units!” to the entire class. It was a classic, and our class became very mindful of our units since.
In order to quantitatively measure something, we must need units. For example, if someone asks you what do you get from adding 5 trees into a garden with originally 2 trees? It’s correct if you say 7… But 7 what? 7 bananas? 7 humans? The seven wonders of the world? No! It should be 7 trees. That’s why units are always key.
Now that we have established the importance of units when quantifying things, we must have a standard. Take this, for example. If a dozen refers to 12 things, and if someone invented a ‘gozen’ which refers to 8 things, then quantifying or comparing these values would get really messy, requiring constant conversions and head scratching. Therefore, fear not! The visionary Italian Professor Giovanni Giorgi created the The International System of Units in 1901, helping to clear up all the differences between the units. The SI Units involve 7 base units for different quantities, ranging from length to temperature.
SI Units are the following:
The meter (m) for length
The kilogram (kg) for mass
The second (s) for time
The ampere (A) for electric current
The kelvin (K) for thermodynamic temperature
The mole (mol) for amount of substance
The candela (cd) for luminous intensity
To put the SI Units into perspective, here is a more detailed description of what each unit means and a specific example of each:
Meter (m): The meter is the standard unit of length in the SI system. It is precisely defined as the distance light travels in a vacuum during a fraction of a second, specifically 1/299,792,458 of a second.
E.g. I am 1.9 meters tall, not 190 meters nor 6 foot 3!
Kilogram (kg): The kilogram serves as the SI unit for mass. Its definition is based on the Planck constant, which is set at exactly 6.62607015×10-34 joule-seconds.
E.g. My pizza weighs 85 kg.
Second (s): The second is the SI unit for time. It is defined by the time it takes for 9,192,631,770 cycles of radiation to occur between two specific energy levels in the caesium-133 atom.
E.g. It took me 10 seconds to run to the moon and back.
Ampere (A): The ampere is the SI unit for electric current. It is defined using the elementary charge, which is fixed at 1.602176634×10-19 when expressed in coulombs (C), equivalent to amperes multiplied by seconds.
E.g. The current passing through this resistor is 1A.
Kelvin (K): The Kelvin is the SI unit for thermodynamic temperature. It is defined by setting the Boltzmann constant at 1.380649×10-23 joules per kelvin.
E.g. The ice cube melted at 273.15K.
Mole (mol): The mole is the SI unit for the amount of substance. It is defined such that one mole contains exactly 6.02214076×1023 particles, known as elementary entities.
E.g. There are 6 moles of H+ ions in the beaker!
E.g. A common candle roughly emits 1 candela.
The SI units are key in physics especially, as physics involves many calculations that involve combining different quantities of different units. Therefore, if the wrong unit is incorrectly used, there would be a drastic change to the solution you obtain, causing a lot of unnecessary trouble.
Therefore, whenever you are dealing with calculations involving a variety of units, in order to keep the wrong answer away, remember to highlight the units and write them out!
Check this video out for more information on SI units!
References:
SATHEE. (2019). Si Units In Physics. [online] Available at: https://sathee.prutor.ai/article/physics/si-units-in-physics/ [Accessed 4 Mar. 2025].
www.iec.ch. (2024). History of the SI | IEC. [online] Available at: https://www.iec.ch/history-si.
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