# B The fundamental units in Physics

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1. Apr 4, 2017

### Vengo

I have been in my physics crash course for my entrance exam. I was looking at the chapter "Units and Measurement". I got a doubt in this chapter and I didn't get it cleared by my teacher. So I am posting this in this forum. We have 7 fundamental quantities and other quantities are derived quantities.In those fundamental quantities,We have (CURRENT)and its unit is A. but according to the definition. CURRENT is defined a rate of charge flowing across a cross sectional area of a conductor. and the formula is I=q/t. This looks like a derived quantity because t is a fundamental quantity. It is used in the formula. MY QUESTION IS WHETHER CURRENT IS A FUNDAMENTAL OR DERIVED QUANTITY???????????

2. Apr 4, 2017

### Mastermind01

Firstly, you don't need to shout (Capital letters) or bold or italicise your text. We get your question without all that.

Current is a fundamental quantity and ampere is the one the seven SI base units.

To elaborate on your answer, yes current is $\frac{dq}{dt}$ but in SI units current's unit the ampere is defined as

"The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one metre apart in vacuum, would produce between these conductors a force equal to 2×10−7 newtons per metre of length"

The fact that ampere is a base unit has to do with historical reasons and the fact that current is easier to measure accurately than charge.

Last edited: Apr 4, 2017
3. Apr 4, 2017

### Vengo

Thank you for clearing my doubt, Master

4. Apr 4, 2017

### f95toli

The definition of the ampere will actually change next year. After 2018 it will be defined via the electron charge which will then be a defined (exact) value giving a more "natural" definition. This also gives a direct connection to the second which is nice.

Note also that the realization of the Ampere described above is never used in practice. In fact, the Ampere is really never really realized directly; it it is always done using Ohm's law using the realizations of the voltage and ohm which we can measure with high precision and accuracy.

Note also that there are some good reasons for why the Ampere (and not the voltage or Ohm) is a base unit; the reasons are nor straightforward but it basically comes down to wanting the SI to be internally self-consistent.

5. Apr 4, 2017

### Staff: Mentor

Note that the distinction between fundamental units and derived units is purely a matter of convention. The SI convention this year is that current is fundamental and charge is derived. Other systems of units take the opposite approach.

It does not matter what convention you use, only that you use it consistently.

6. Apr 4, 2017

### Vengo

Thank you f95toli and Dale
But Dale, I cannot get you precisely

7. Apr 5, 2017

### Staff: Mentor

There are different systems of units: SI units, English units, cgs units, Lorenz Heaviside units, Planck units, etc. Different systems of units will make different choices about which are fundamental and which are derived. For example in SI units current is fundamental and charge is derived, but in cgs units both current and charge are derived

8. Apr 5, 2017

### Vengo

OH! thank you, Dale