Questions tagged [order-of-magnitude]

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Order of magnitude radiation measures conversion?

Standard unit of ionizing radiation dose is Sievert. I can get a rough idea of how dangerous absorbing various amounts is from https://xkcd.com/radiation/ and other sources. There are many other ...
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How to compare scales in a thermodynamic system

Suppose we have a generic system with hamiltonian \begin{equation} H= JH_1 +\mu H_2 \end{equation} where $J$ and $\mu$ are couplings and $H_1$ and $H_2$ are just two dimensionless parts of the ...
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1answer
56 views

Why does the universe manifest scale?

I'll try outline my question in clear terms, articulating specific aspects that are its primary motivators. I'm just beginning in my exploration of physics as a student, but a persistent question that ...
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2answers
113 views

Why do people not have gravitational attraction? [closed]

Gravitational theory says every thing that has mass attract each other. So why don't people attract each other and overlap
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2answers
27 views

How do you calculate Reynolds and Mach's numbers before solving the Navier-Stokes equations?

Apologies in advance if the question is trivial. I am accustomed to electromagnetics but an amateur on fluid dynamics. My understanding is that the Navier-Stokes equations are solved to determined ...
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1answer
36 views

Sun's energy and grand unification

According to Wikipedia, The approximate grand unification energy value is equal to $1×10^{25}$ eV or $10^{16}$ GeV This is equal to $1602176$ J. Now, The Earth receives 174 petawatts (PW) ...
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62 views

When is the order of magnitude not equal to the exponent of scientific notation?

Explain why the order of magnitude is sometimes not the same as the exponent in scientific notation. It is because of the units?
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1answer
112 views

When do you use Quantum Mechanics? [duplicate]

Given a problem, how does one know whether to use quantum mechanics or classical mechanics? Take for example electron scattering from a nucleus. The electrons are given a wavefunction in this case. ...
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3answers
157 views

Where is the line between Quantum and Relativity?

Its often said QM is for the very small and GR for the very large. This brings to mind that there should be some limit at which one starts to apply and the other stops. Now I know there are more ...
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1answer
35 views

Different approch on Order of magnitude confusion [closed]

My problem: How many orders of magnitude larger is the sun $(1.99$ x $10^{30} kg)$ than the moon $(7.32$ x $10^{22} kg)$. Justification 1: So according to my teacher, we can do this question by ...
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2answers
61 views

What is the point of generalizing a more specific result to an order of magnitude?

In my textbook, an example wants me to find an estimate of the number of cells in a human brain. It gives the volume of the brain as $8 \times 10^{-3}\ \rm m^3$ which it then estimates further as $...
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4answers
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If an apple is magnified to the size of the earth, then the atoms in the apple are approximately the size of the original apple

Quoting from the Feynman Lectures on Physics - Vol I: The atoms are 1 or $2 \times 10^{−8}\ \rm cm$ in radius. Now $10^{−8}\ \rm cm$ is called an angstrom (just as another name), so we say they are ...
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1answer
197 views

What would happen if gamma-ray burst from a supernova burst hit the sun? [closed]

What would happen if gamma-ray burst from a supernova burst hit the sun? Would the sun explode? Would the sun expand. Have a longer lifetime? Shorter Lifetime? Because people are nit-picky about stuff ...
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60 views

Why are the Planck charge and Planck mass values so much larger than observed particles? [duplicate]

The Planck charge and mass values seem rather unusual. Is this an indicator that there is a lack of understanding in current theory, or is there a logical explanation for the magnitude of these two ...
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1answer
158 views

How much bigger is $a$ than $b$ when we write $a \gg b$?

When a condition of some physical quantity $a$ and $b$ is presented, say $a \gg b$, how much bigger do we say $a$ has to be than $b$? $10$ times? $1000$ times? $10^6$ times? Some context: I am ...
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1answer
45 views

How much faster is the chain reaction in a power plant than that of natural uranium ore? What about a nuclear bomb?

How much faster does the fissile uranium in a power plant 'break down' than an equivalent amount of naturally decaying U-235? How much faster is the chain reaction in a bomb compared to a power plant?...
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1answer
124 views

Theory on how to break the speed of light [closed]

I am 14 years old and I think I have a good grasp on physics but I have a question. Imagine you were to shrink everything in the universe (except for yourself) down to the size of an atom and it was ...
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2answers
125 views

Do you create gravitational waves by clapping your hands?

I was thinking that given that GW's can be created by the merger of both black holes and neutron stars I don't see why any two colliding objects wouldn't also be able to create gravitational waves, ...
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45 views

Physical reason for light incoherence

In my physics notes it is stated that the light phase changes every 10^-8 seconds, hence light from 2 light original light sources. I understand how light from incoherent light doesn't allow for ...
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103 views

Confusion about uncertainty principle

A book is lying on the table. It is at rest. Its velocity is 0 and uncertainty in velocity $\Delta v$ is 0. Does this mean that uncertainty in its position is ∞?
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2answers
500 views

Magnitude of Black hole entropy

I was looking with a friend of mine through time travel related stuff, and for some reason we ended up at the Hawking equation for black hole entropy which is: $$S= \frac{\pi Akc^3}{2hG}$$ where: A:...
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23 views

how long does it take for a photo-conductor to become a conductor when exposed to light?

How fast does a photo-conductor reach full conductivity when exposed to the appropriate wavelength of light? How fast does it cease to conduct when the radiation ceases? It might vary, so I could also ...
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Heating a city with air conditioning units? [closed]

A couple years ago I have estimated that the heat generated by air conditining units is larger than the heat from the sun. So I assumed that there is about 1 kW of solar radiation per m². In the ...
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1answer
95 views

If I have electric current going through a superconductor and cut off the power in what fraction of a second will the electrons stop moving?

If I have electric current going through a superconductor and instantly cut off the power supply(rendering the circuit open, and not at all closed), In what tiny fraction of a second will the flowing ...
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1answer
109 views

Why can't we prepare the moon for colonization by shipping oxygen to its atmosphere?

Why can't we take oxygen to the moon in order to civilize it? We can store oxygen in rockets then take them to moon and release it there. Even if the moon's gravity is weak, it will hold on to the ...
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2answers
64 views

Order of magnitude (serway 9th) [closed]

In my text book I have following example to present computation of order of magnitude: $$0.008 \space 6\space m \sim 10^{-2}m$$ I dont understand whether I am supposed to multiply $0.008$ with $6$, ...
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1answer
63 views

What do we mean by order of a parameter?

This questions looks a bit stupid. But I am confused on a statement which says that the value of some xyz parameter is expected to be of order 1. Does it mean that its value is less than $10^1$ or its ...
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3answers
5k views

How many photons enter our eyes per second when looking at the blue sky on a sunny day?

How many photons enter our eyes per second when looking at the blue sky on a sunny day? Say the sun is directly over head and you are looking at the blue sky on the horizon. Say that the pupil is 2mm ...
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1answer
161 views

Ray Optics: Reflection and Refraction

Suppose you have extremely sensitive photographic film and you can expose it in an optical system for a very long time. Will you be able to record a virtual image on film?
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1answer
675 views

What is a “scale length”, and how do I calculate it for galaxies?

I'm trying to work through, understand, and apply concepts regarding mass models of galaxies. Looking at the Hernquist model, I'm finding the equation $$Φ(r)=−\frac{GM}{r+a},$$ where a is the scale ...
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2answers
678 views

Why is the string length around the Planck length?

In string theory, it is assumed that a string is about the size of a Planck length, $$\ell_{string} \sim \ell_{Pl} \simeq 10^{-35}\,\text m.$$ Why that length? Why not for example a hundred times ...
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716 views

What's the greatest range of orders of magnitude?

There's a famous claim along the lines of "40 dp of PI are sufficient to calculate the circumference of the Observable Universe to the width of a hydrogen atom" I don't know the accuracy and detail ...
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Quantum Gravity In Particle Accelerators

It is my understanding that we have no fully working model for Quantum Gravity. However, I imagine one would need to take quantum gravity into account when making discoveries in, say, particle ...
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1answer
813 views

Can we really neglect gravitational strength?

In an atomic and particle physics model, the gravitational force is ignored or neglected. Physicists might have freedom of ignoring such minute scale value. But can we really avoid it in calculations? ...
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2answers
165 views

Kolmogorov scale why $d\sim \left( \frac{\nu^3}{\epsilon} \right)^{1/4}$ and not $d\propto \left( \frac{\nu^3}{\varepsilon} \right)^{1/4}$?

I am looking at the Kolmogorov scale and in numerous sources (e.g. this one). I have seen the following: $$d\sim \left( \frac{\nu^3}{\epsilon} \right)^{1/4}$$ for the Kolmogorov scale. I can see why ...
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2answers
3k views

What's smaller: a neutrino, or a string from string theory [closed]

I've recently read an article that stated "If an atom were as big as the solar system, a neutrino would be the size of a golf ball". I watch the science channel, and on (I believe) the show How the ...
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2answers
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In geometrical optics, how can we say that rays coming from a distant object are parallel to one another?

If two rays are not parallel in the start, how can they become parallel at the instant when they strike the lens of a telescope? If they don't become parallel, why do we consider them to be, in the ...
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82 views

Ignoring orders of magnitude in a problem

I am so lost in this solution to a physics problem. In the problem I find the gravitational force on the Moon due to Earth and the gravitational force on the Moon due to the Sun They are about $1....
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340 views

Pressure inside a typical white dwarf

Does any one know the order of magnitude of pressure inside a typical white dwarf (better with reference)? Thanks! I think it should be $m_e^4c^5/h^3$ (may be multiplied by $\pi$), which is $10^{22} \...
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Why is 7 TeV considered as a big amount of energy?

Considering that $7$ TeV is more or less the same kinetic energy as a mosquito flying, why is it considered to be a great amount of energy at the LHC? I mean, a giant particle accelerator that can ...
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Is there a rigorous definition of 'much greater than'?

I have encountered $\gg$ in many physics text books where it's used as a relation between constants or functions but in none of the text books I have read is it properly defined anywhere. If $A \gg ...
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Quantity of force between two magnets

I have compiled data detailing a magnetic field. If I assume a duplicate magnet is set up across from it, how can I calculate the repulsive force between them?
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How do you quantify the “much greater” operator “$\gg$”?

I'm specifically asked to compute the charge in the Earth-Moon system knowing that the gravitational force between the two bodies is much greater ($\gg$) than the electrostatic one. However, I don't ...
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1answer
150 views

Mammal Population Estimates as a Fermi problem [closed]

Can We estimate the order of magnitude of number of mammals on earth? Can this be treated as Fermi problem or not?
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3answers
215 views

Will my car consume more if I plug in my laptop?

I drive a Volvo v40 with a 110hp diesel engine. It usually use 4.6L/100km. If I plug my laptop which is 85W, will this change the fuel need? If so, by how much?
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661 views

“Weakness” of gravitational force [duplicate]

I often hear that gravitational force is much "weaker" than electroweak and strong forces. But how can you compare the strength of interactions without the parameters like mass, charge on which it ...
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2answers
2k views

Measuring the nearest order of magnitude [closed]

The world's largest ball of a string is about $R=2 m$ in radius. To find the nearest order of magnitude, what is the total length $L$ of the string in the ball? I have tried this in the following way ...
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2answers
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Temperature on the surface of the sun calculated with the Stefan-Boltzmann-rule

In a German Wikipedia page, the following calculation for the temperature on the surface of the Sun is made: $\sigma=5.67*10^{-8}\frac{W}{m^2K^4}$ (Stefan-Boltzmann constant) $S = 1367\frac{W}{m^2}$ ...
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1answer
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Intuitively, why does removing solutes cost $k_B T$ of free energy per molecule?

I can calculate that if you want to, for example, desalinate water, you will have to pay a free energy cost of $k_B T$ for each ion you remove. In other words, removing an ion from a volume of water ...
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1answer
363 views

Order of magnitude

When finding out the order of magnitude of quantities, as said in my textbook, we compare the numerical part with $3.2$ (approximately $\sqrt{10}$ or rounded off version of $3.162$) Thus, $9.12 \...