# Questions tagged [units]

Units are standards of measurement used for different types of quantities.

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### How can the magnetic $B$-field of a photon or light wave be directly compared to the electric $E$-field if the units are different? (Tesla vs. V/m)

I know that the electric field of an electromagnetic wave has the strength (or intensity?) of the magnetic field times the speed if light $c$. (E = B × c)... But, the units are not quite the same (I ...
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### How to conceptualize the unit Temperature?

For a long time, I was under the impression that Temperature is just a shorthand for the "average energy" within a system, but I discovered this is wrong (although sometimes this is a valid ...
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### I am stuck on Precision Applying Problem [closed]

The stability of the Cesium Clock used as an atomic time standard is such that two Cesium clocks would gain or lose $1$ s with respect to each other in about $300,000$ y. If this same precision were ...
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### What is the relationship between magnetic field quantities $B$ and $H$ in cgs units?

In cgs units permeability of free space is dimensionless. If so, then in cgs units won't $B$ and $H$ have same units as they are related by equation: $B=\mu_o H$ But instead why they have different ...
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### What is the difference whether a space is used between number and unit for temperature? [closed]

When talking about temperature, or temperature change, -32.5 ℃ (with space), and -32.5℃ (no space) are two common expressions. Are they the same? If there is no space is used, there is a problem about ...
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### Name for inverse of heating or cooling rate?

I would call the average change in temperature of a body per unit time « cooling rate » or « heating rate » depending on the sign. Is there a name for the inverse of this quantity, ie the time it ...
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### Is the equal sign in "$0 ^\circ \mathrm{C} = 273.15\, \mathrm{K}$" fair?

I'm in doubt whether the equal sign in an expression like "$0 ^\circ \mathrm{C} = 273.15 \,\mathrm{K}$" is fair because, normally, if $A=B$, then, say, $2A=2B$, which is hardly applicable to ...
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### Pankove optical absorption equations: cgs problem?

I'm embarrassed to be asking this question here, but I've been beating my head against an equation for two weeks and can't get the numbers to work. I'm trying to plug in numbers into Jacque Pankove's &...
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### Why gravitational constant $G$ does not have the factor of $4π$?

We know that electrostatic constant $K=\frac{1}{4\pi\epsilon}$. This $4\pi$ came from the surface area of the surface in which charge is enclosed. Then, why don't gravitational constant has the factor ...
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### Unit analysis of classical equations, and why charge and mass may be manifestations of the same thing? [closed]

Consider Newtons force for universal gravitation, $$F_g=G\frac{m_1 m_2}{r^2}.$$ Nothing new here, but bare with me. This is a strange idea, hard to articulate adequately without a greater ...
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### What determines the existence of a physical dimension, as in dimensional analysis?

Dimensional analysis in physics is based on the idea that only commensurable quantities can be added, subtracted, equated or otherwise compared. The five dimensions which appear in dimensional ...
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### Computing the maximum force a rod can bear [closed]

Suppose I had a rod of diameter $d$ composed of some material with tensile strength $T$. If I then exterted a pulling force $F$ on the ends of the bar, how do I compute the force $F$ for which the ...
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The question in the problem sheet reads The de Broglie wavelength of a non-relativistic (nr) electron with kinetic energy E$_{kin}$ can be written as $\lambda_{nr} = \frac{\delta}{\sqrt{E_{kin}}} \;\... • 1 2 votes 2 answers 90 views ### Calling angles dimensionless and simultaneously distinguishing between numbers and angles is inconsistent, isn't it? The SI Brochure says that each angle is the ratio of two lengths - i.e. a number - and therefore a derived and dimensionless quantity. On the other hand, the papers  and  suggest that angles are ... • 1,382 0 votes 1 answer 91 views ### Solving an ODE numerically with different units I am simulating a leaky integrator with$R_m = 10^8 \, \mathrm{\Omega}$and$C_m = 10^{-10} \, \mathrm{F}$with an input current of$I_{in} = 10^{-9} \, \mathrm{A}$for$0.1 \, \mathrm{s}$: $$C_m \... 0 votes 2 answers 42 views ### Fundamental Units and Concentration I am working with the Cahn-Hilliard equation which is typically given in terms of a dimensionless concentration, u. I generally find myself being confused when dealing with dimensional equations ... • 131 0 votes 1 answer 33 views ### Free energy Functional and its Relation to Pressure and Chemical Potential I am reading a paper and trying to follow along some of the mathematical steps and reasoning; however, I can't quite understand one aspect. The authors give an expression for the free energy ... • 131 0 votes 3 answers 115 views ### Unitless variable screws up calculation All, the following equation represents the budget of the specific humidity q (which is the ratio of mass of the water vapor to the mass of the dry air, usually written as gram/kg or kg/kg).$$ \... • 31 1 vote 2 answers 130 views ### Does taking the$\logof a quantity change the unit of the quantity? \begin{align} &= c [P - b \ln(P+b)]_1^{500} \\ &= 0.125 \frac{\text{cm}^3}{\text{g}} \times \left[\begin{aligned}&500 \text{ bar} - 2700 \text{ bar} \times \ln(500 \text{ bar} + 2700 \text{... 0 votes 2 answers 61 views ### The utility of HorsePower vs Torque [closed] All vehicles have only 1 gear Final-drives allow peak RPM (HP) at red line = 35 MPH Wheel-sizes are equal Tires have perfect traction (yet, no rolling resistance, etc) Goal: isolate all variables ... 0 votes 1 answer 51 views ### Why angle and solid angle not included in fundamental physical quantity? [closed] So I recently studied about physical quantities. I doubt what actually Fundamental physical quantities is?? Book simply says it as 'quanties which doesn't depend on other physical quantities" ... 14 votes 3 answers 628 views ### Isn’t natural units prone to mistakes? Suppose I am deriving a length contraction formula using natural units. If I arrive atL = L_0 \sqrt{1 - v^2}$, I know that I should divide$v^2$by$c^2$to get the correct answer in SI units. But ... • 145 -3 votes 2 answers 92 views ### Can you measure the space-time interval using a meter? [closed] The unit of the space-time interval is the meter: $$(\mathrm{d}s)^2 = (c \mathrm{d}t)^2-(\mathrm{d}x)^2-(\mathrm{d}y)^2-(\mathrm{d}z)^2.$$ Can it be measured with a meter? How? • 1,480 2 votes 0 answers 43 views ### Are there Imperial or United States Customary units of electromagnetism, luminous intensity, or moles? In the UK, Boris Johnson's somewhat beleaguered government has recently announced a consultation on using Brexit to ensure that "more choice can be given to businesses and consumers over the ... • 245 0 votes 0 answers 14 views ### Gyromagnetic ratio: units problem I tried to calculate the gyromagnetic ratio of the proton with this formula: $$g_p = \frac{2.m_p.c^2}{e.\hbar.c^2}.\Delta E \approx 5,28$$ with$m_pc^2 \approx 938 \ \mathrm{MeV}$,$e \approx 1,67.10^{... 87 views

### How do I determine what physical constants can be combined to derive a set of natural units?

I am looking into natural units (units of measurement based only on universal physical constants). Different systems of natural units use different physical constants as their defining constants. If I ...
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### Couldn't understand an example used in explaining fundamental and derived quantities

In my book under topic of fundamental and derived quantities, there is an example which is supposed to explain this concept and it goes like this, As a simple example, if a unit of length is defined, ...
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### Magnetic flux from maximum induced emf as a function of angular velocity?

I've done an experiment where I have a small rotating coil in a uniform magnetic field and I've measured the induced emf in the rotating coil as a function of time for several different angular ...
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### Different units of measure for the specific intensity and the components of the Stokes vector

Good morning to all, I'm a bit confused about the fundamental physical quantities adopted in the theory of radiative transfer. In particular, many texts show the scalar integro-differential equation ...
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### Why is $(\text{kg} \cdot \text{m}^2)/\text{s}$ written as "Joule second" when there is no joule present, and time is divided and not multiplied?

The Joule is $(\text{kg} \cdot \text{m}^2)/\text{s}^2$, which is not contained in $(\text{kg} \cdot \text{m}^2)/\text{s}$, the Joule second. So, why does the unit name contain "Joule"? Also, ...
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### CGS of resistance

What's the CGS unit of resistance? Is it the same as that in SI system, i.e. ohm (Ω)? I googled but found no explicit answer.
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### Why are there two different units for charge; the coulomb and elementary charge? [closed]

Why are their two different units for $q$ charges?
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### Imperial units - mass and force

I was rudely informed on an answer here that 1 lb is a force and I 'didn't understand' mass. (I am not interested in relative merits of systems of units - only dimensions..) Now - we know that Force ...
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### Units in vector spaces

I am confusing myself about where physical quantities become mathematical objects. Where does one end and the other begin? E.g. displacement $\vec{s}$. A displacement is a physical quantity that can ...
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### Electromagnetism using differential forms, question about names and units

When formulating classical electromagnetism using differential forms, Maxwell's equations can be written as  \begin{align} dD &= \rho \\ dB &= 0 \\ dE &= -\dot{B} \\ dH &...
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