# Tag Info

6

This is a relatively tricky one, because it involves the differences between the $\mathbf B$ field and the $\mathbf H$ field in the SI and CGS systems, and those relationships change in the different systems. In short: Oersteds are used to measure the $\mathbf H$ field in CGS units. Teslas are used to measure the $\mathbf B$ field in SI units. In the SI ...

9

They are technically units for incommensurate quantities, but in practice this is often just a technicality. The magnetic field that makes sense ($B$) is measured in teslas (SI) or gauss (CGS), and the magnetic field that people spoke about 100 years ago ($H$) is measured in amps per meter (SI, also equivalent to a number of other things) or oersteds (CGS). ...

4

From a quick google search, it seems that Oersteds are used for defining magnetic field strength and Teslas are used for defining magnetic field strength in terms of flux density. They seem to not really be meant to be converted between, though you technically can (as evidenced by the other answers here). This website and this website might be helpful to ...

0

Quoting from the Wikipedia page on the CGS system: The e.s.u of charge, also called the franklin or statcoulomb, is the charge such that two equal $q=1\:\mathrm{statC}$ charges at a distance of $1\:\mathrm{cm}$ from each other exert an electrostatic force of $1\:\mathrm{dyn}$ on each other. The e.m.u. of current, also called the biot or abampere, is the ...

1

I'm not up to the task of rebuilding geometry from the ground up, but my intuition is that this is sensible and it can be done consistently. It seems to be true that logarithms (or inverse trig functions) take you into a different (transcendental) numerical realm, and exponentials (or trig functions) take you back, and adding quantities from different realms ...

3

It is not incorrect, it is incomplete. A physical value consists of: Magnitude - this is "the number" Unit - "type" of the value Physical values are not only magnitudes, they also have a unit. It is inseparable. Unit defines a physical meaning to a value. You could formally treat it as a pair (magnitude, unit). All calculations in physics are done this ...

2

Why does Light-years or Parsecs seem to be the standard rather than SI? In the solar system astronomy, the astronomical unit is much more widely used rather than meters for distance, days (86400 seconds), Julian years (365.25 days), or Julian centuries (36525 days) are used rather than seconds for time, and the solar mass is used rather than kilograms for ...

3

The basic fact is units are a demarcation of the quantity of something, so they can never be used as a fundamental equation. As you have mentioned F=ma can also be written as N=kg ms^-2 , then how can you say that the equation is of Newton's Law only? The equation can be used to define force dimensionally which is [M L T^-2].

1

SI units are created for everyday life and therefore convenient to use in everyday life. The following sentences makes sense in modern human mind; "I live 300 meters away from here" or "My new boat is 35 meters long" or "I am bust now please call in 5 minutes". Moreover the calculations with these numbers are easy. If you hear someone saying "I drove 50 km ...

7

Light years and parsecs have been used since long before SI existed, so a lot of it is tradition. But using light years also makes it very obvious how long the light has traveled to get here, and thus which era of the universe we are seeing the object in. Something that is 11 billion light years away dates from the era of early galaxies, for example. If you ...

4

If a physics equation is to be valid, it is necessary for the units to work out, but it is not sufficient. The handling of dimensionality only provides part of the information. Units and dimensionality are good checks to make sure you did the equation right, but the mere fact that the units were right does not automatically mean the equation was right. ...

0

A "naked" number such as $3$ does not denote anything in the real world. In order to represent on paper a real world collection of items, we need a numerator and a denominator, e.g. $3$ Apples. But to represent on paper a real world amounts of substance we need in addition a unit to, so to speak, "discretize" the amount into a collection of units, e.g. $3$ ...

5

Disclaimer: I have no idea what question everyone else is answering; none of the other answers seem to address the question as I understand it. We don't use units in formula because not everything we want a variable to represent (like $a$ for acceleration) has its own nice unit. Acceleration is a perfect example: it's just too long to say "meters per ...

7

For one, the laws of physics are the same whether you're working in SI or Imperial. $F = ma$, regardless if your m is in kg, pounds mass, or solar masses. Actually, in college we had a bonus challenge to give the answer to a problem in craziest units of energy that we could come up with that actually worked. "Slug lightyears" was a pretty good one, but the ...

4

The purpose of units is to assign numbers to measurements. They are necessary but of secondary importance to the thing being measured. Scientists want to describe the real world with their equations, not just their measurement tools.

46

A specific parameter might correspond to a specific (SI) unit, but not all units correspond to a specific parameter! Kinetic energy is \begin{align} K&=\frac{1}{2} mv^2 \\ \text{Joules}&=\frac{1}{2} \text{kilograms}\times\text{meters}^2/\text{seconds}^2 \end{align} We also have gravitational potential energy: \begin{align}U&=mgh \\ \... 7 Writing equations using only units would not work at all for dimensionless equations. For example the Snell's lawn_1 \sin\theta_1=n_2 \sin\theta_2.$$You would also lose many of the dimensionless (but usefull) parameters in physics such as the Lorentz factor$$\gamma=\frac{1}{\sqrt{1-v^2/c^2}}. Also consider equations whose all variables have the same ...

7

For simple equations, the two might be equivalent. Certainly, dimensionally an equation must always be correct. But there are plenty of situations where the units may not obviously reflect a particular quantity; and clarity of communication improves understanding. Take electrostatics. If I say "1 Volt" you know what I mean; an electric field is "Volts per ...

0

The stiffness matrix can be multiplied by an displacement vector $\begin{pmatrix}\Delta x\\ \Delta y\\ \Delta \theta\\ \Delta \psi\end{pmatrix}$ and then should give a vector containing forces and torques: $\begin{pmatrix}F_x\\ F_y\\ \tau_\theta\\ \tau_\psi\end{pmatrix}$. Knowing the units of the vector components, you should be able to derive the units of ...

6

W.u stands for Weisskopf unit: [ref 1, ref 2]. Despite being called a 'unit', it does not have a universal value; the value of the Weisskopf unit depends on the mass number of the nucleus in question and which transition the nucleus is undergoing ($E\lambda$ or $M\lambda$). The references contain expressions for the value of a Weisskopf unit in terms of $A$ ...

5

In nuclear Physics estimates can be made using the shell model of the nucleus of the gamma ray transition rates in excited nucleii and such estimates are named after Victor Weisskopf. A measured rate is compared with the Weisskopf estimate and the ratio is said to be in Weisskopf units (W.u.).

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