# Questions tagged [si-units]

A set of internationally accepted units to aid in communication of measurements.

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### Can all physical quantities and units be derived from the seven base SI units? [closed]

I understood that from cesium that we derived the unit for second and procedurally with other universal constants we derived the other SI units. Can all other “physical” units be derived from the ...
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### Complete unit of vectors and their scalar counterparts [closed]

I'm aware that we use m/s as the SI unit for both speed and velocity. My question then is, we use units to define the magnitude of the vector (velocity), why not add units for the direction too? For a ...
588 views

### How to convert units when calculating a dimensionless quantity?

For instance, consider calculating this quantity: $$P= \frac{1}{H^4~~ } \times \left(\frac{1}{k_0}\right)^{0.2} ~~(1),$$ where $H$ and $k_0$ are constants in units of GeV and Mpc, respectively. How ...
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### Why did we choose Coulomb's constant $k$ as $9\cdot10^9 \rm Nm^2/C^2$ while define unit of charge instead of any other number?

I understand that while defining charge, Coulomb had to choose any arbitrary value of $k$ to describe unit of charge. But, why did we chose $9\cdot10^9 \rm Nm^2/C^2$ as the value of $k$, but not any ...
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### Unit problem: $\rm N/cm$ or $\rm N/m$

Is that really wrong to solve a simple Hooke’s law equation which gave Force in N and displacement in cm to find out the spring constant in N/cm? The problem I was attempting to solve did not specify ...
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1 vote
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### Mass of the Prototype $\rm kg$

With the new SI kg based upon Planck's constant $h$, is there any plan to eventually measure the former IPK using a Watt Balance, and determine how far its mass deviates from the new definition of 1 ...
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### What is the key difference between SI and MKS units?

In MKS as well SI system, the length, mass and time are measured in the units meter, kilogram and second, respectively. Then how the SI units are different from MKS units? What are the key ...
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### What is the value of the ideal gas constant in J/(mol*°C)? [duplicate]

I've seen in some sites that it is the same as in J/(mol*K), is that true? $8.314 \frac{J}{mol×K}=8.314 \frac{J}{mol×°C}$
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### Applicability of the $\frac{1}{4\pi}$ pre-factor for Coulomb Attraction in Nanocrystals

I am currently working on a thesis and stumbled upon a seemingly formal issue which I was not able to resolve myself. In short The classical coulomb energy between two charges (let's say, electrons ...
282 views

### What are natural units?

I need to make a presentation on natural units. My professor asked me to visualize a world where $c$ and $\hbar$ are actually equal to unity. Like, what are the consequences? I also want to know the ...
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### Convert Coulomb's law in CGS units to SI units

I recently translated the appendix to an electromagnetics text from 1945 into English. Now the client is asking me to update the formulas (I studied electrical engineering). The formulas use CGS units ...
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1 vote
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### Fundamental and derived physical quantities

I read that fundamental physical quantities are independent of each other but, if we write length = velocity x time, then length depends on the time interval so how come it is independent?
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### Why is the general unit for energy (in terms of energy bills) $\rm kWh$? [closed]

Why is the unit for energy (in terms of energy bills) expressed as $$\text{time} \cdot \frac{\text{energy}}{\text{time}}$$ rather than just energy? Wouldn't it be better to express it in megajoules (...
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### Where does $4\pi$ come from in electromagnetism?

We know that in Coulomb's law, the constant $k_e$ is $\frac{1}{4\pi \epsilon_0}$. Where does $4\pi$ come from? Why is it related to the force between two charges?
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### Are there any non-SI base units? [closed]

Are there any base units that are not in the SI system? If so why aren't they? Edit: By this I don’t mean other systems that you can convert to (ie. Feet, Ounces, Hours, etc) I mean new base units ...
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### Can unit prefixes be interpreted as algebraic stand-ins?

I am currently a little confused about whether unit prefixes can be interpreted algebraically For example: $$4km = 4\times 10^{3} m$$ Is it incorrect to say that $k$ is simply an algebraic stand in ...
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### Units for the Calculus of Variations [duplicate]

Just a quick question regarding the units for a quantity. I just started reading a QFT textbook, and it starts out with a little bit of Calculus of Variations. Specifically, there is a result that ...
326 views

### Radian unit mystery (damped oscillator)

I would be extremely grateful for any help that anyone could offer here. I am interested in solving the optical bloch equations for the excited state population Rabi oscillations with damping due to ...
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### The SI-unit of the cosmological constant (vacuum energy) is $\frac{1}{m^2}$. What does that have to do with Energy?

I just don't get how Energy is measured in $\frac{1}{m^2}$. Wasn't it measured in Joules? (source is https://en.wikipedia.org/wiki/Cosmological_constant#Equation)
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### Melting point of water [duplicate]

Wikipedia says about melting point that: The melting point of ice at 1 atmosphere of pressure is very close to 0 °C (32 °F; 273 K); this is also known as the ice point. But why it is very close and ...
1 vote
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### Is Coulomb written/found on any object/product as its unit?

Today I had physics class and the unit Coulomb made me wonder as to why it was even invented when I don't see it being used anywhere. When I asked where it may be shown just like bulbs have x Watts ...
### Why isn't the charge of an electron taken to be "$-1.6 \times 10^{-19}$ coulombs" [closed]
If we look at what the Wikipedia says about gravitomagnetic fields $B$ generated by rotating planets / stars we have: https://en.wikipedia.org/wiki/Lense%E2%80%93Thirring_precession#...