Confusion regarding SI base units

In summary, the conversation discusses the concept of fundamental quantities and their units. It is noted that in SI, base units are defined in terms of physical experiments or prototypes, while derived units are defined in terms of combinations of base units. The example of current and charge is given to demonstrate that the selection of base units is based on experimental convenience rather than theoretical considerations. The conversation also raises the question of whether some quantities may be considered fundamental or derived depending on our ability to measure them accurately.
  • #1
I_am_learning
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What qualifies for being called a fundamental quantity and having its own fundamental unit? For example length is considered fundamental quantity and it has a unit of meter. But Area isn't considered fundamental. Is it because we know area can be CALCULATED by multiplying the length of the two sides of a rectangle? Suppose they didn't know the formula. Then, would they have called area fundamental and defined a unit, let's say Ar, as the area of a square whose length is 1m? And then measured areas of figures by comparing to the area of the square? My question is "do some quantities fail to be fundamental because we know how to calculate them from other fundamental quantity?" If yes, is there any chance that few of today's fundamental quantity be called derived in future?
I feel like I am missing something very fundamental and I am feeling quite ashamed for asking these questions.
 
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  • #2
big question in which I'm having a "discussion" with a UIUC physics prof emeritus right now (on Wikipedia).

i am convinced that the mole and the candela in SI are not fundamental at all and do not measure physical quantities that cannot be measured or described with other units. i am also convinced of nearly the same regarding temperature. temperature is really just another way or expressing energy.

but i really do think that electric charge is a fundamentally different physical quantity than length, time, and mass. i think that there are four dimensions of physical stuff, which need four base units, and all other physical quantity is described and measured from those four.

if you believe that temperature is a fundamental physical quantity, then it's five.
 
  • #3
rbj said:
big question in which I'm having a "discussion" with a UIUC physics prof emeritus right now (on Wikipedia).

i am convinced that the mole and the candela in SI are not fundamental at all and do not measure physical quantities that cannot be measured or described with other units. i am also convinced of nearly the same regarding temperature. temperature is really just another way or expressing energy.

but i really do think that electric charge is a fundamentally different physical quantity than length, time, and mass. i think that there are four dimensions of physical stuff, which need four base units, and all other physical quantity is described and measured from those four.

if you believe that temperature is a fundamental physical quantity, then it's five.

Really what it comes down to is how you want to measure things. Units are defined in a way that some experimental procedure can be carried out on some standard set-up and accurately reproduce the quantities involved which define the unit.

For example, in SI the unit of current is defined in such a way that two long parrallel wires carrying a current of 1A each in opposite directions, placed a meter apart will experience a force of 2e-7N. The unit of electrical charge is then defined in terms of current, as 1A*s.

In CGS though, current and charge are derived units, and the the unit of charge is based on the force between two point charges 1cm apart.

Whether something is a "base" unit or not is really a matter of how you measure things. It seems like you need at least three base units to reproduce all the constants you encounter in nature, but which dimensions are base or not is really totally aribitrary and comes down to a matter of experimental convience.
 
  • #4
yes, good comprehensive reply .. and:

all i would add is that no definition of any such entity that involves 'length' can not possibly be 'fundamental' .. There is nothing sacred about a 'meter' or 'cm', now, is there ... The same logic would apply to 'time', too, no? (yes i know there exists this thing called 'quantum time' but so far it's just a 'thing' ;)
 
  • #5
I_am_learning said:
What qualifies for being called a fundamental quantity and having its own fundamental unit? For example length is considered fundamental quantity and it has a unit of meter. But Area isn't considered fundamental. Is it because we know area can be CALCULATED by multiplying the length of the two sides of a rectangle? Suppose they didn't know the formula. Then, would they have called area fundamental and defined a unit, let's say Ar, as the area of a square whose length is 1m? And then measured areas of figures by comparing to the area of the square? My question is "do some quantities fail to be fundamental because we know how to calculate them from other fundamental quantity?" If yes, is there any chance that few of today's fundamental quantity be called derived in future?
I feel like I am missing something very fundamental and I am feeling quite ashamed for asking these questions.
First, as dipole mentioned, the SI system does not make a distinction between fundamental and non fundamental untis, it distinguishes between base and derived units. Base units are defined in terms of some physical experiment which can be performed or in terms of some prototype object. Derived units are defined in terms of combinations of base units.

A unit is selected as a base unit for the ease and reliability of measuring it, not for any theoretical considerations. From theory, you would expect charge to be a base unit and current to be derived. But it is easier to accurately measure current, so in SI current is the base unit and charge is derived. If some new experimental technique were developed which could measure charge more accurately, then the SI would switch which was the base unit.
 
  • #6
yes, thanks

tnx for your crystal-clear explanation
 

1. What are the seven base units of the International System of Units (SI)?

The seven base units of the SI are the meter (m) for length, kilogram (kg) for mass, second (s) for time, ampere (A) for electric current, kelvin (K) for temperature, mole (mol) for amount of substance, and candela (cd) for luminous intensity.

2. How are the SI base units defined?

The SI base units are defined by specific physical constants or phenomena. For example, the meter is defined as the distance traveled by light in a vacuum in 1/299,792,458 of a second. The kilogram is defined by the mass of a specific platinum-iridium alloy cylinder kept at the International Bureau of Weights and Measures in France.

3. Are there any other units that are derived from the SI base units?

Yes, there are many other units that are derived from the SI base units. These include units for area (square meter), volume (cubic meter), velocity (meters per second), and many others.

4. Can the SI base units be modified or changed?

No, the SI base units are universally accepted and cannot be modified or changed. However, their definitions may be updated or refined as new scientific discoveries are made.

5. Why is it important to use SI base units in scientific measurements?

Using SI base units allows for consistency and accuracy in scientific measurements, as they are based on standardized definitions and are universally accepted. This allows for easier communication and comparison of data between scientists and across different countries and disciplines.

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