# Tagged Questions

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

7k views

### Coulomb force in SI and cgs

Coulomb force in SI is $F = \frac{Q1*Q2}{4\pi\varepsilon R^{2}}$ while in CGS $F = \frac{Q1*Q2}{R^{2}}$ why is it? I mean doesn't it any make difference in dimension? since $\varepsilon$ ...
14k views

### What is the logarithm of a kilometer? Is it a dimensionless number?

In log-plots a quantity is plotted on a logarithmic scale. This got me thinking about what the logarithm of a unit actually is. Suppose I have something with length $L = 1 km$. $\log L = \log km$ ...
8k views

### Why is the Ampere a base unit and not the Coulomb?

I always thought of current as the time derivative of charge, $\frac{dq}{dt}$. However, I found out recently that it is the Ampere that is the base unit and not the Coulomb. Why is this? It seems to ...
7k views

### Is there a symbol for “unitless”?

I'm making a table where columns are labelled with the property and the units it's measured in: Length (m) |||| Force (N) |||| Safety Factor (unitless) ||| etc... I'd like not to write "unitless" ...
1k views

### Why do we still not have an exact definition for a kilogram?

I read that there is an effort to define a kilogram in terms that can exactly be reproduced in a lab. Why has it taken so long to get this done? It seems this would be fairly important. Edit Today I ...
76 views

### If two ends were a certain “length” apart were they therefore at rest (or at least rigid) to each other? [closed]

Considering the definition of the SI unit of "length" [1] and [2 (" method a.")] I'm missing any requirements about the two "ends" of the required "path travelled by light" being "at rest to each ...
3k views

### Relating milliampere-hours to watt-hours for batteries

I've seen many batteries that are measured in milliampere hours (mAh), while others are measured in watt hours (wh). How can I convert them between each other so that I can actually compare them? It's ...
197 views

### Messed up units!

In the article, Environment-assisted quantum transport, $\gamma$ is a constant equal to $2\pi kT/\hbar*E_{R}/(\hbar\omega_{c})$ where $T$ is the temperature, $k$ is the Boltzmann constant. ...
219 views

### CGS Units for Magnetism

Why does the formula for magnetic field force include the speed of light in the denominator in cgs units? Where does the extra $c$ go in SI units?
6k views

### Why is the prospective new kilogram standard a sphere?

I can understand the choice of material, silicon 28, but why is it a sphere rather than (say) a cube? Article here I would have thought that a sphere would have been the hardest shape to machine ...
672 views

### What are the proposed realizations in the New SI for the kilogram, ampere, kelvin and mole?

The metrology world is currently in the middle of overhauling the definitions of the SI units to reflect the recent technological advances that enable us to get much more precise values for the ...
776 views

### Why were the SI base quantities chosen as such?

The reasons for choosing length, mass, time, temperature, and amount as base quantities look (at least to me) obvious. What I'm puzzling about is why current (as opposed to resistance, electromotive ...
2k views

### Why is the mole/“amount of substance” a dimensional quantity?

According to the BIPM and Wikipedia, "amount of substance" (as measured in moles) is one of the base quantities in our system of weights and measures. Why? I get why the mole is useful as a unit. In ...
6k views

### Do all equations have identical units on the left- and right-hand sides?

Do all equations have $$\text{left hand side unit} = \text{right hand side unit}$$ for example, $$\text{velocity (m/s)} = \text{distance (m) / time (s)},$$ or is there an equation that has different ...
2k views

### What's keeping us from simply redefining Avogadro's Number / the Mole as a definite integer?

This might be a question to ask in a Chemistry site, but because there is a lot of talk about redefining many units of measurements in terms of Avogadro's Number / the Mole, I was wondering why we ...
5k views

### Why is energy not an SI base unit?

According to a textbook I have begun to read, there are seven base SI units: Length Mass Time Temperature Amount of a substance Electric current Luminous intensity What I do not understand is, why ...
1k views

### The origin of femto, atto and zepto SI prefixes

Do you know why the SI prefixes: femto, atto, zepto have been accepted by Scientific Community, if this triad of metric units, is neither greek nor latin?
434 views

### There are plans to develop a better definition of a “second”. How does the current definition fall short?

The current definition of a second is stated here and I found a presentation on the BIPM site which discusses plans to change to a "better" definition of a second. You can find the presentation here. ...
43 views

### Can a value of “length, in meters” be attributed to a pair of ends which are rigid (but not at rest) to each other? [duplicate]

The definition of the SI base unit "metre" [1] doesn't seem to rule out explicitly that a certain value of "length, in meters" could be attributed to a pair of ends which are rigid to each other, but ...
2k views

### Is anything actually 1 meter long (or 1kg of weight)?

I believe that no real objects are actually (exactly) 1 meter long, since for something to be 1.00000000... meters long, we would have to have the ability to measure with infinite precision. ...
935 views

### “Natural units” of mass

Gravitational attraction is given by $\frac{GMm}{r^2}$ while attraction due to electric charge is given by $\frac{q_1 q_2}{r^2}$. Why does gravity need a constant while electric charge doesn't? ...
358 views

### Why is the candela a base unit of the SI?

The candela is defined as The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency $540\cdot10^{12}$ hertz and that has a radiant ...
1k views

### Are there reasons for the discrepancies in absolute temp units - Kelvin vs. kelvins vs. degrees Kelvin?

Before 1968, the units for absolute temperature were described as "degrees Kelvin" or "degrees absolute." After that, the SI system got rid of the idea of "degree" for absolute temperature, so the ...
1k views

### Do the trigonometric functions preserve units?

I saw an exercise where you had to calculate the units of $C_i, i=1,2$ from an equation like this: $v^2=2\cdot C_1x$ and $x=C_1\cdot \cos(C_2\cdot t)$ where $x$ means meters, $t$ means seconds ...
479 views

### How large is the information collected from an inverse femtobarn of collisions?

I ran into this while looking at measures of humongous amounts of data. How does the information (data) collected in an inverse femtobarn exposure compare to a gigabyte of data ?
597 views

### Is the second defined arbitrarily? [duplicate]

According to the definition a second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of ...
2k views

### Speed of light definition

I understand the speed of light to be an EXACT number. If permeability of free space has a factor of pi in it, how can this be?
28 views

### Classification of plane angle as fundamental or derived

I recently started with my high school studies and the chapter I am stuck at is units and measurements. I was told about two types of physical quantities and my teacher gave me the following ...
69 views

### Convert cubic feet per sec to feet per sec

Is there a way to convert cubic feet per sec to feet per sec. Or in general volumetric flow to velocity? I want to know the time taken by water to travel from point A to point B. I have the distance ...
1k views

### What is the SI Unit for intercept of a graph

I have a graph that i drew and i got all the point and a question came up that what is SI Unit for intercept of a graph. I have no idea what the answer is. (I'm a total newbie in Physics)
2k views

### Why does $E=mc^2$ give results in Joules?

Why does $E=mc^2$ give results in Joules ? Why didn't Einstein need to create a separate unit and how come using two standard units (speed of light in km.s and mass in grams) give another result in ...