Questions tagged [power]

The time rate of change of energy

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How efficient is the Crookes radiometer?

I remember many years ago, I think at 8th grade, seeing the teacher show us a Crookes radiometer. I remember it being very fascinating. Today I read the wiki article on it, after looking up what it ...
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What is the physics behind the vibrating feeling you get when you touched an inducted metal surface?

When I was last working on the highway, I stopped to help a motorist parked under high tension power lines. I'm driving a large Ford Transit van, fairly box shaped. My van was parked 90 degrees (...
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4 votes
4 answers
639 views

Is brightness proportional to power?

In an exam, I am given a situation where a student investigates how the current varies with potential difference for two bulbs of the same type. For the same voltage, the current of bulb A is twice ...
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How exactly can we shake an object to determine its mass?

It's often said that inertial mass is a measure of how hard it is to shake an object (as distinct from gravitational mass, which is how hard it is to lift an object in a gravitational field). Because ...
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How do we know sound pressure squared is proportional to the power in the sound wave?

I am trying to find the equation that explains why we can say the pressure exerted by a wave (squared) is proportional to the power. I see this claim made a lot but I can't tell where it comes from.
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What do I measure with power meter?

I modulate directly a laser diode by setting the pump current periodically from 0 A to $I_{on}$ A. I do it for different duty cycles and different frequencies. But I need to keep my average power of ...
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3 votes
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Geometric frustration in power transmission networks

I read this article concerning the detection of Braess Paradox on power transmission networks: http://phys.org/news/2012-10-power-grid-blackouts-braess-paradox.html Here is a quote from the article: ...
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Why did a lack of electrical power generated in Texas recently almost cause fires?

This may be more of an engineering problem... A recent newspaper article stated that the rolling blackouts instituted during the recent cold snap in Texas narrowly averted parts of their grid catching ...
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2 votes
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Does a power station directly control the supplied power or voltage?

The electric grid's "job" is to provide to the end user a steady sinusoidal A/C voltage with an RMS value of 120 V and a frequency of 60 Hz. (That's in the United States - other countries ...
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PMDC Motor Characteristic Graph for Given Parameters

TL;DR Need to make a graph which showcases all parameters of a PMDC motor against the torque input. I am trying create a graph similar to as shown below. However, I am confused as to how the torque is ...
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641 views

Complex Poynting vector and energy loss

Consider continuum medium (no boundaries) and the electromagnetic wave Poynting vector $$ \mathbf{S} = \mathbf{E}\times \mathbf{H}^*$$ with $^*$ the complex conjugate. When there is a phase difference ...
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Analytically understanding wireless power transfer

https://youtu.be/M8aZoM9p43k This YouTube link shows a experiment I quickly made. Two resonant lc circuits that use magnetic fields to wirelessly light up a led. I manually found the best frequency ...
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Power in a wind turbine

An wind turbine starts turning by a wind speed of $v_b$ and it stops turning (rated wind speed, to prevent overloading) by a wind speed of $v_g$. By wind speeds between $v_b<v<v_g$, the power ...
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Why does an alternator slow down when the 3 phases are shorted?

I'm designing a small wind turbine and noticed that when i short the three wires coming out of the alternator, the alternator is breaking and the blade RPM is dropping. What is the explanation for ...
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2 votes
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221 views

How to apply conservation of energy to active noise-cancelling headphones?

Active noise-cancelling headphones add the opposite waveforms of the ambient sound in order to cancel out this ambient sound by destructive interference. I do not understand why adding more power in ...
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Estimating the battery capacity using current power consumption and battery percentage

I want to estimate the current maximum capacity (in kWh) having the current power consumption (in kWh) and the state of charge of the battery (in %) available in a time series. I do not have a full ...
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Energy stored in a battery?

I want to calculate the energy lost from a battery to heat, and the energy stored in a battery. Are these formula's correct? $ E_b$ = $I$$V$ x $t$ $ E_H$ = $I$$R$$^2$ x $t$ $E_b$ = energy stored in ...
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Total power consumption of electric device

For some electric devices, like a fan or air conditioner, I read about their power consumption in watts on their specification guide. Does it tell about the power at normal or full speed? or Does the ...
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1 vote
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What is the maximum possible averge power of the geomagnetic electric generator?

Earth's magnetic field strength varies from 25 to 65 μT on the surface. Therefore, magnetic energy is approximately $V\frac{1}{2} \frac{B^2}{\mu\mu_0} \approx 10^{\left (17,5\cdots 18,5 \right )} \...
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What was the first use/cite of "Watt's law"?

Yesterday I saw a reference to Watt's law being used for the power formula P = V I. It's been over 40 years since I learned my basic electrical engineering fundamentals and this is the first time that ...
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Square coil circuit and variable magnetic field problem

I am attempting to solve the following problem. A square coil of side $l = 40.6 \ cm$ and resistor $R = 95.5 \ \Omega$ is contained within the $xy$ plane. Within the same region a magnetic field, ...
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Relation between Radiance and Surface power density (i.e poynting vector intensity)

I am attending a microwave remote sensing course and I have same problem to understand the relation between the radiance and the intensity of poynting's vector. The radiance is defined as: $L(\theta,\...
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1 vote
2 answers
65 views

What is the difference between antenna input impedance and its radiation resistance?

I got to know that the input impedance of the halfwave dipole antenna is given as: $Z_{in}$=1/$I_m$* sin($\beta$(H-|Z|)) where $I_m$ is the maximum current on the antenna( when excited with a 1 volt ...
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1 answer
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Intuitive explanation for coefficient in the Larmor formula

So the Larmor formula tells us the total power radiated by an accelerating point charge that doesn't go too fast with respect to the speed of light is $P=\frac{2}{3}\frac{q^2 a^2}{c^3}$ (written in ...
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1 vote
2 answers
229 views

Why is the half of the total height taken as effective height in case of certain mathematics regarding gravitational potential energy?

A problem gives the depth and diameter of a well full of water, states that a pump can empty the well within a certain given time, and asks to find out the power of that pump. In the solution, while ...
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107 views

Work done on a moving cart in wind

Let us suppose a cart is moving in a rough ground with a propeller attached to the roof of the cart. Let the velocity of the cart at the given point of analysis be V and velocity of wind is -W. The ...
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1 vote
2 answers
193 views

Definition of power spectral density

I was reading here in equation (12) that the power spectral density (PSD) for a signal $f(t)$ and its corresponding Fourier Transform $f(\omega)$ is defined as $$\langle{f(\omega)f^*(\omega)}\rangle=...
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1 vote
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Normalization of Phase Noise / RF Power Spectrum to 1 Hz RBW

I used a Rohde & Schwarz Phase Noise Analyzer to take some phase noise and RF spectrum measurements. All in all, I'm trying to compute the linewidth of a laser from a measurement. I cannot detail ...
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Ray Power Equation for Bent Fiber Optics

I came across this equation while reading the book, Optical Waveguide Theory (by Snyder and Love) on page 182: where P(0) is the initial power from the straight fiber, R is the curvature radius, ρ is ...
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Power in uniform circular motion

Say you have a rocket ship in a vacuum away from all gravitational fields firing its engines to maintain uniform circular motion. Since the engines are burning fuel, it seems to make sense to ...
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Model radiated power from an LED

For a range of LEDs I'd like to model their performance and predict the flux density across an area using manufacturers datasheets. Eventually I'd like to take various power measurements and calculate ...
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1 vote
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20 views

Testing the strength of a $B$ field from loop of wire and limitations of a function generator

I built a coil that receives magnetic fields (Rx) from a sending coil (Tx) a few centimeters away. (wireless power transfer) I send waves to the Tx at the Rx's resonant frequency since its a loop with ...
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0 answers
11 views

A constant number of calories burnt for an exercise lasting different durations

So in the gym, me and a friend doing 2000 m on the rowing machine burnt the same amount of calories, but I did the exercise in 14 mins and he did it in 10 mins. Given that both of us have similar body ...
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1 vote
0 answers
72 views

Noise power spectral density and $|X(\omega)|^2$. Why are those quantities associated usually ? Their definitions are different

I consider a generic signal $X(t)$. We could imagine to fix ideas $X(t)=\sqrt{R}I(t)$ ($R$ being a resistor and $I$ current flowing through it) The energy of this signal is defined as $E=\int_{-\infty}...
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1 vote
2 answers
356 views

Rate of charging the capacitor

Why does the rate at which a capacitor in an RC circuit charges, only depend on $R$ and $C$? We know that for power $$P = \frac{V_{\text{emf}}^2}{R} e^{-t/RC}=\frac{QI}{C} $$ so why are we ignoring ...
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Is there a graphical summary of which modulation technique to use in communication systems using Visible Light Communication techniques?

I am currently looking at various papers written (some listed at the bottom) that discuss how laser pulses can be modulated to send encoded messages to a receiver for communication systems. The more I ...
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1 vote
0 answers
32 views

Power meter zero-offset calibration

I have to measure the output power of a laser diode. To this aim, I am going to use an integrating sphere connected to a photodetector and a power meter. Concerning the photodetector, electrical ...
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1 vote
2 answers
77 views

Confusion in usage of formulae for power dissipated in a circuit

When should we use $\frac {V^2}{R}$ and when to use $I^2R$. Because many times they ask about the power without giving details about series and parallel. I am really confused
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1 answer
225 views

Calculate Retinal irradiance (W/cm² to log photons/cm²/s)

I have been trying to calculate what retinal irradiance value I get with a 1 blue LED system. Since the manufacturer didn´t give the spectral distribution information, I will approximate LED as a ...
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1 vote
0 answers
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Can neutronic emissions be used as a power source?

The free neutron will degrade to a proton electron and anti neutrino in about 15 minutes. Assuming you could slow a beam of free neutrons sufficiently to keep the size of the reactor manageable: Is ...
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1 vote
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How to calculate energy (KWh) from Temperature (T),Air quality (Co2), and illumination (lux)?

I have data of temperature, air quality, and illumination.I want to calculate energy consumption (KWh) from these parameters.My data is shown in the image . Also, examples are also shown in image 2 .
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1 vote
2 answers
343 views

Is the force from an engine constant as a vehicle reaches top speed?

I've come across a textbook question and I've got myself confused by it although it seems like it should be simple. The van accelerates with a (supposedly) constant resistance force up to its top ...
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1 vote
2 answers
49 views

Power factor in Transamision

What is normally the power factor in the transmssion.? Is there some standard or guidelines for that.? Also, the low factor is not desirebale as it leads to more power loss. That makes sense. They ...
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How to calculate theorectical engine max. torque from max. power and RPM?

I'm trying to come up with a formula to estimate an engine's maximum torque (and at which RPM) given other values and some assumptions. I'm approximating an engine's torque curve as 2 linear ...
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1 vote
3 answers
64 views

Would we require energy to walk in a gravity free space?

If there was no gravity would our leg muscles lose energy to make us walk. When we walk we cover strides . when we lift our one leg from rest on ground accelerate it until it reaches top say to speed ...
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0 answers
175 views

How to Calculate the total scattered radiation power and total radiation power.?

Let's suppose I have incident field $E^i_z$. The components of Scattered electric and magnetic field are $E^s_z$, $E^s_\phi$, $H^s_z$ and $H^s_\phi$. The electric and magnetic field components ...
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1 vote
0 answers
173 views

How can the amplitude of the same acoustic wave be different in a interferometer?

I came up with an exercise on Halliday Resnick Krane that asked a question which consfuses me. I premit that I do not look for a solution of the exercise but for suggestions only regarding the ...
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1 vote
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747 views

Sound frequency and power

Although I have studied physics I am not clear on this question: Given that two (or more) sound waves have the same amplitude does the power they carry depend on frequency? What about electrical AC ...
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87 views

Power of solar panel and dc generator

I have made a solar stirling engine and I need to measure its power and compare it to the output power of a solar panel. I need to see which one is more efficient. I am using the same area in both ...
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Luminous intensity: from Volt to Candela/m2

I use a probe for converting optical power to an electrical current. I need to convert Volt to candela/m2. The sensor, a photodiode, measure the brightness of the monitor leaning the probe on the ...
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