# Questions tagged [metrology]

The study of measurements

313 questions
Filter by
Sorted by
Tagged with
71 views

### Why there is no unit of energy? [duplicate]

In the SI System (https://en.wikipedia.org/wiki/International_System_of_Units) there are 7 units, but no unit of energy, though this is surely the fundamental unit of physics. Yet the Joule is usually ...
• 590
1 vote
57 views

### The definition of Time and second

From the book "University of Physics 15th edition", in chapter 1, they talk about the fundamental units. They stated that the definition of unit of time is based on an atomic clock, where 1s ...
• 11
48 views

### How is the Heisenberg Uncertainty calculated value added to a measurement error bar?

It is not clear to me if the calculated HU for a specific experiment is added to the measured value Gaussian curve together with the statistical errors (i.e. added to the total error bar) of this ...
• 3,011
86 views

• 251
1k views

### Thermometers and the Celsius scale

Why is the Celsius scale not useful for making extremely accurate temperature measurements? We know that $\delta L/L=\alpha \delta T$. By setting $\delta T$ to $1°$C we can find how much mercury or ...
• 304
11k views

### Why should a clock be "accurate"?

Having read that atomic clocks are more accurate than mechanical clocks as they lose a second only in millions of years, I wonder why it is necessary for a reference clock to worry about this, if the ...
• 437
59 views

• 555
302 views

### Fundamental quantities in physics

I observed that the fundamental units like meter, kilogram, ampere, kelvin and candela are all indirectly dependent on one single fundamental value 'second' and each other. For example: Meter:- 1 ...
178 views

### Scientific Notation for Different Units of Charge

Sorry in advance for the basic question, I’m pretty new to physics. I’m doing some electromagnetism homework and so far in class we’ve used only nano and micro coulombs in our force and electric field ...
• 33
86 views

### Are inverses of fundamental SI units also physical units?

I know that Hertz (1/s) is a physical unit, but are there physical units for 1/m or 1/kg too? For example, in the formula for resolvance of a diffraction grating, $\frac{\lambda}{\Delta \lambda} = mN$,...
1 vote
39 views

### Limits of electromagnetic source detection? [closed]

What distance from the observer a lightbulb must have so you can't say the light was from this lightbulb? Is it possible in astrophysics to detect a farest star or galaxy only measuring separate ...
90 views

### How is atomic time defined in Hartree units?

As in Planck units, I believe Hartree units have a fundament unit of time, Atomic Time, something like $2.4188843265857(47)×10^{−17}$ s. My question is what exactly is it that happens in this time? ...
57 views

### How to account for nominal resolution in calibration measurements?

I have a real world question but I think it'll be clearer if I phrase it like a "homework" question so here goes*: A physicist is trying to use a laser measuring device to determine the ...
• 151
498 views

### Is the speed of light exactly 299792458 m/s?

I want to know if there is any uncertainty in the measurement of the speed of light. If yes, please mention the uncertainty.
327 views

### Why does the metric system use the Celsius scale rather than the Fahrenheit scale?

Part of why I ask this is because the Fahrenheit scale has a higher degree of precision than the Celsius scale. And the Kelvin scale could just as easily been adapted to be an extension of the ...
• 391
159 views

### Why are centimetres still in use? [closed]

When SI units were adopted in the UK old, imperial, units were dropped but with a few exceptions for very specific purposes eg pints for beer, pounds and ounces for babies birth weight and feet and ...
320 views

### How are the seven base units related to each other?

How are the seven S.I. fundamental or base units of measurement of physical quantities interrelated or dependent on other base units: Universal constants define these seven SI basic quantities. For ...
32 views

### Subatomic natural units

In High Energy Physics it seems to be common use to measure everything in terms of eV powers, by assuming $\hbar = c = 1$ (dimensionless). Often times this system of units is referred as Planck units, ...
4k views

### Definition of a joule

I'm not getting the definition of a joule. From the definitions I've read if I apply one newton of force to any object, now matter how heavy/ how much mass it has, over one metre in a single direction ...
• 271
142 views

### Is a second now defined using 9,192,631,770 Hz or 9,192,631,770.2 Hz of a caesium atom?

This is obviously splitting hairs, to an extent, but I am genuinely curious... And it does make a difference... In several places, I have seen that .2 added on to the figure... Did the length of a ...
• 3,443
103 views

### When and why are units measured in degree?

I was always under the impression that a unit is measured in degree if it is a relative unit. While this is true for temperatures and angles, I cannot find any reference to this definition or any ...
160 views

### Symbol of quintal

Depending on the source, I have come across the symbol of quintal as 'q', 'qt'and 'qtl'. Is there a standard symbol for quintal. If yes what is it?
• 188
1k views

### Why the need for more sophisticated atomic clocks?

Why is there such a huge effort in building more sophisticated atomic clocks when the current ones achieve $1$ second of error every few hundred million years? Some achieve frequency stability close ...
• 69
3k views

### How are physics theorems so accurate, relative to fixed measurement systems? [duplicate]

I admit this sounds like a dumb question. I know very little about physics, but I've always wondered that given the fact that: A unit of measurement is a very specific constant value. Then how can a ...
• 305
45 views

### Expansion of universe effects on mass

It has been noted that the metal alloy model previously used to characterize the value of 1 kilogram has been steadily increasing in mass due to unknown causes. Could this be a local effect of ...
12k views

### How do you make more precise instruments while only using less precise instruments?

I'm not sure where this question should go, but I think this site is as good as any. When humankind started out, all we had was sticks and stones. Today we have electron microscopes, gigapixel cameras ...
• 2,621
1 vote