The result of sampling the property of a system

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How to (safely) measure the surface area of a human body? [on hold]

Here's a question just for curiosity. You measure mass on scales. You could measure volume by submerging in a bath with a snorkel system. But how could you measure a living human's surface area ...
0
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1answer
16 views

How could the BOE (barrel of oil equivalent) be measured?

The barrel of oil equivalent was given as an example of gigajoule scale energy. "The barrel of oil equivalent (BOE) is a unit of energy based on the approximate energy released by burning one barrel ...
0
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1answer
26 views

Calculate how many photons hitting a sample that are absorbed

In the determination of the quantum yield of a photoisomerization, $\Phi,$ the following is needed $$ \Phi = \frac{\mbox{Number of "reactions"}}{\mbox{Number of absorbed photons}} $$ The photon flux ...
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0answers
22 views

Venturi “Turn Down” [closed]

I'm sorry if this isn't the right forum for this question, but physics is the only stackexchange which had many results for Venturi. I'm trying to measure the flow of air through a 16mm hose/pipe ...
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38 views

Is there any well-defined, measurable atomic size?

Is there any well-defined, measurable size at the atomic scale? I was under the impression that the van der Waals radius was a good way of describing the size of an atom, but it turns out to be a ...
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0answers
52 views

How to measure size of small particles of $mm$ level?

Is there any way to measure dimension of small particles (mm sized) such as rice grains accurately? I know there are techniques such as laser or x-ray diffraction, but the equipment needed are too ...
6
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1answer
52 views

Is there a tool to measure the chemical potential of a system?

Let's suppose I put you inside a room filled with a gas. You can measure its pressure directly with a barometer. You can measure its temperature directly with a thermometer. Can you measure its ...
2
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0answers
26 views

How precisely do we know the van der Waals radius of hydrogen?

I've found several sources that say the van der Waals radius of hydrogen is $120 pm$. How precisely do we know this number? What order of magnitude is the error on this measurement?
3
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48 views

Simultaneous measurement of non-commuting observables without uncertainty

A pair of non-commuting Observables $\hat{X}$ and $\hat{P}$ does not have a common set of eigenfunctions, i.e., it can not be measured simultaneously. Let us for the sake of simplicity assume that ...
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3answers
55 views

Measuring force of a punch

I'm trying to build a device that can mesure the force of a punch. ​ My initial plan was to build a platform with 4 springs (one at each corner) and an accelerometer in the middle. But, if the ...
3
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0answers
21 views

How are melting points of metals measured?

How are melting points of metals measured? What apparatuses are used and how do they "conceive" heat units?
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1answer
59 views

Why do scientists measure fundamental constants so precisely?

According to the last CODATA report the charge of electron is measured very precisely (10 decimal places). And it's not only about the charge of electron. Why do scientists measure fundamental ...
3
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1answer
64 views

Ideal shape of a water clock

The ancients undoubtedly discovered the ideal shape of a water clock by trial and error. In examining some ancient water clocks I notice the shape is different depending on the size. For example, a ...
0
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0answers
33 views

In which book is it written that the only measurable physical quantity is the length?

Task. In any book I've seen thoughts and images that very clearly illustrate the sequence of actions and implicit assumptions when measuring something. The result has been that in any measurement key ...
3
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2answers
97 views

How mass is determined in dynamics?

Mass is one of the most core and complicated concepts in dynamics. I have tried many books but I still don't have a good idea of how the mass of any object is determined relative to another. In The ...
0
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1answer
47 views

Combining errors. Gamma spectrometry, Poisson distribution

I have run an experiment 3 times and measured the results by gamma spectrometry. For example I get values like this (1 $\sigma$): $100 (10)$ $90 (8)$ $110 (12)$ The above 1 sigma error is based ...
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0answers
50 views

The use of real numbers in measurements in physics?

When we measure something in physics we give a value plus or minus some uncertainty and assume that the measured value can take any value with the range with a certain confidence limit. I don't ...
2
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1answer
70 views

How accurate is Newtonian mechanics? [closed]

How accurate is (theoretical, i.e. the mathematical formulation basing on "formalised" physical constants, such as material density) Newtonian mechanics? What does the accuracy depend on (material ...
6
votes
1answer
1k views

The uncertainty of a metre ruler?

I have been taught that the uncertainty in the measurement of a metre ruler is +-1 mm. However , I was also taught that the uncertainty is half of the smallest division in the measuring instrument. ...
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1answer
64 views

Deriving a POVM from a projective measurement

I understand how to show that every POVM is equivalent to a projective measurement on a larger Hilbert space, but I don't understand why the converse is true. The vast majority of explanations of ...
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2answers
64 views

How are the SI units “generalised”?

How exactly are the SI units generalised from their definitions? E.g. the kilogram is a weight of an object of cylindrical form, with diameter and height of about 39 mm, and is made of an alloy of ...
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2answers
44 views

What is the relation between charge and polarization?

On one hand, in this "Measurement of polarisation" lab manual, polarization is surface charge density, $P=\frac{Q}{A}$, in other words, charge an polarization are essentially the same thing. On the ...
11
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3answers
368 views

What does multiplying two real-world values represent? [duplicate]

I totally get what division means in the real world. "dollars / hour", well, that's the number of dollars you will make in one hour. "kilometers / gallon" is the distance you can go with a gallon of ...
3
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2answers
119 views

How is dark matter detected?

What methods do we use to detect Dark Matter? If I understand correctly, due to lack of electromagnetic interaction it should be able to phase through normal matter nearly like through void - since ...
0
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1answer
33 views

Can conductivity (or resistivity) value be assigned to 2D materials?

2D materials, such as graphene monolayer or graphene nanoribbon, don't really have a cross section. By definition, conductivity is $\sigma = \frac{J}{E}$, and $J$, the current density, is calculated ...
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2answers
70 views

How to measure 8 μm surface bumps?

I want to measure surface bumps which are larger in size than $8\ \mathrm{\mu m}$ on a uncoated plastic mirror. So it can be any number of smaller bumps and that would not effect the application. I ...
4
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0answers
62 views

Measuring the Dirac field

If the Dirac field $\psi(x)$ is to the electron as the Electromagnetic field is to the photon, why is it that we can measure the Electromagnetic field, whereas the Dirac field we cannot?
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43 views

Measuring expectation value in quantum field theory and in quantum mechanics

There is a way of calculating the vacuum expectation value $\langle 0|\hat\phi|0\rangle$ theoretically in a quantum field theory like there is a rule to compute expectation value of any operator A ...
0
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1answer
64 views

Calculating flow rate of a syringe

I wanted to know how would I go about calculating the flow rate of a syringe with a metal tip that is dispensing water using a pressurized air. I have had a look on internet about this and on this ...
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3answers
54 views

Why are measurements standardized the way they are?

Using meters as a base length, squaring or cubing lengths smaller than 0.67m makes the square term larger than the cubed term. This fact causes certain properties of physics (how rain needs to form?) ...
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3answers
26 views

Measuring Activity

The formula for Activity of a radioactive substance is $ \frac{dN}{dt}=A=λN $. If we have an initial number $N(0)$ of some Radionuclide, which has a halflife of, say, 12 hours, is there any ...
3
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0answers
77 views

How does one compute the state of a quantum system following imperfect measurement?

Suppose I have a quantum system $S$ ("system") with Hamiltonian $H_S$ and initial density matrix $\rho_S(0)$. I allow $S$ to interact with another system $P$ ("probe"), which has Hamiltonian $H_P$ and ...
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0answers
76 views

Quantum indirect measurement with probes

I am wondering about the following hypothetical example regarding quantum measurements made by probes: An open, two-plate capacitor, in a LC circuit, has charge, q, that we want to measure. Each of ...
0
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1answer
106 views

Wave Function State Reduction As a Result of Quantum Measurement

This is a question about indirect quantum measurement, involving an observable of an object of interest and a probe that is used to measure that observable. In this experiment, an observable of the ...
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22 views

Is the saturation magnetization of a thin film different if you apply the field at different directions?

I'm getting different values of the saturation magnetization when I measure them in-plane versus out of plane. The saturation magnetization for the out of plane measurement is greater than in the in ...
0
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1answer
30 views

Spinor expectation value and measurement

I have a question about the difference between expectation value and probability of measurement. consider the spinor $\zeta = [-3\ \ 4i\ ]^T$ . The expectation value of $S_x$ is zero because : ...
0
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2answers
45 views

Probability that a measurement will be in some set

Let $\mathcal{H}$ be the Hilbert space of a quantum system and $A$ one observable in $\mathcal{H}$. If $A$ has discrete spectrum $\{a_n : n \in \mathbb{N}\}$ for simplicity, then by the postulates of ...
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0answers
31 views

Position from acceleration, or acceleration from position, to track an object moving the speed of sound

I'm working on a project that involves tracing the motion of a fast-moving object (approx the speed of sound) through a fairly defined trajectory. Would it be more accurate to measure the position of ...
0
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2answers
50 views

Photo Multiplier Tube Usage

How is it determined that a PMT measures only one photon ? By the energy that one photon is supposed to have ?
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1answer
24 views

Is it possible to determine when an accelerometer is in a vibrating state compared to a non-vibrating state?

I would like to know if so, how raw 3-axis accelerometer data could be analyzed and manipulated real-time to register periods of vibration. The device being used has a max sample rate of 62Hz (I ...
0
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1answer
52 views

measuring average directional intensity of LED?

Although I am poor at physics and math I want to compare two light sources of which I haven’t decided the specific brand yet. I was thinking of florescence bulbs versus LED lamps. So something like ...
0
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2answers
64 views

Does measurement really affect polarization?

The answer must be yes but I have trouble dealing with the explanation given here. As you can see the light is polarized at an angle theta to x axis. After it passes the polarizer A which is along x ...
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0answers
51 views

How to practically measure the heat capacity of a large object besides bomb calorimetry?

I need a practical solution to non-destructively measure the heat capacity of a variety of large objects containing many materials and having complex geometry which includes internal hollow areas. ...
0
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0answers
37 views

Given a dimension of a known object in the image, how can we calculate the dimension of other objects in the same image ?

The question considers a very specific scenario in which we have an image with let us say, two rectangle objects. We know width and height of one object. How can we calculate the dimensions of the ...
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40 views

Conductance measurement of InAs/GaSb Quantum Spin Hall Edges

My questions are related to recent article: http://arxiv.org/ftp/arxiv/papers/1507/1507.08362.pdf I can't figure out how their sample (wafers) actually looked like. In particular I can't understand ...
2
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1answer
91 views

Understanding measurement based quantum computing

I am currently reading M. A. Nielsen's review on Cluster-state Quantum Computation (Nielsen, Michael A. "Cluster-state quantum computation." Reports on Mathematical Physics 57.1 (2006): 147-161.). ...
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1answer
62 views

Uncertainty in the average of a series of photon counting measurements

I'm a bit stuck on a problem relating to statistics in photon counting. I'm measuring a spectra with a spectrometer and can set a measurement time and number of times to repeat the measurement in ...
2
votes
3answers
119 views

How is complex permittivity measured?

Complex relative permittivity is defined as $$\epsilon_r = \frac{\epsilon(\omega)}{\epsilon_0}=\epsilon_r^{\prime}(\omega) + i\epsilon_r^{\prime\prime}(\omega) = \epsilon_r^{\prime}(\omega) + ...
3
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1answer
103 views

Inaccuracy at measuring gravity constant with Cavendish experiment

For a scientific work for school I decided to measure the gravity constant with the Cavendish experiment. I set up a structure like the one suggested on this website: ...
3
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3answers
84 views

Calibration of a clock [duplicate]

I was inspired by this interesting question on this forum: How do I measure an earth year without a clock? Say you're stranded on an alien planet without any significant tools. How would you ...