Questions tagged [power]

The time rate of change of energy

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28 views

Power as force times velocity and change in KE

I was noodling over problems and coming up with one for my students. The concept of power as force times velocity shows up on the Regents exam and I was thinking of a baseball. Specifically, a ...
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Do gas-powered plants use Higher Heating Value(s) (HHV) or Lower Heating Value(s) (LHV)?

Does anyone know if natural gas power plants, and other utilities for that matter, obtain as much energy as (reasonably) possible from burning methane (or coal, or whatever)? Where would one find that ...
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One piston power and force [closed]

I am trying to figure out, what does the engine power rating mean regarding to single-piston performance. A gasoline or diesel engine is rated for let's say 100kW of power at 4000 RPM. Let's assume ...
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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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26 views

The equation of raising the temperature of the water using the electric heater

I've been confused of the following equation The electric heater exists which can generate $P[cal/s]$. $t:=$time which is required to raise the temperature of the water of $m[kg]$ from $a[°C]$ to $b[°...
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Interpretation of wind power spectral density

I'm reading this article on Dryden Wind Turbulence Model, and also similar Von Karman wind model. The Dryden model gives power spectral densities of turbulent wind. But why is power spectral density a ...
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25 views

Amplitude spectral density vs power spectral density

I'm reading the Wikipedia article on spectral density. It is said: Sometimes one encounters an amplitude spectral density (ASD), which is the square root of the PSD; the ASD of a voltage signal has ...
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1answer
30 views

Why is intensity required here?

Q)A point source of light is placed at the centre of curvature of a hemispherical surface. The radius of curvature is r and the inner surface is completely reflecting. Find the force on the hemisphere ...
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What is the gain of an isotropic antenna?

My question is simple: What is the gain of an isotropic antenna in dBi? I cannot find a consistent answer on the internet. Some sources define it to be 0dBi and others define it to be 1dBi. Thanks.
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Minimum detectable signal of base station in free space

I am studying for tomorrow's exam (Communication Systems, part of Computer Science course), and I've run into this question which involves some radio propagation physics. That's why I decided to post ...
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1answer
96 views

How much energy per area or volume is the solar wind producing?

Would it be possible to erect essentially a giant loop of wire in interplanetary space to create an electric generator? If so, how much power can we get?
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1answer
67 views

Why are there no brushless motors with more than 3 phases (6, 9, etc.)?

My understanding is that adding phases will (up to a point) add the ability to push more power through a motor (more Amps total, same per individual winding wire), generating more torque for similar ...
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How can two equal forces applied on the same mass $m$ cause the same displacement in different amounts of time? [closed]

in my textbook the concept of power is explained through this example: a man lifts a bucket containing cement to a certain height; a freight elevator can lift the same bucket in less time. It then ...
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Finding power stored/given by each element in a circuit [closed]

I will start with my attempt. I recall my professor saying that $i_1 = 0$ due to the fact that current cannot flow through an open circuit. That makes the current-dependent-voltage-source on the right ...
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Is there a way manipulate the shaft power equation to show BHP as the area under the curve of Flow v. Pressure

Pump curves are frequently presented in terms of Flow rate Q (typically x) vs Pressure Head H (typically y) and can have different shapes depending n the technology (centrifugal, turbine etc..) Given ...
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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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How to simply explain correlation between Range, Power and Bandwidth in RF Technologies?

First off, I don't have a great understanding about the physics behind radio frequency technologies. I'm working on a paper evaluating if LoRa infrastructure can help developing communities for a ...
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How can more voltage give more power (do more work)?

I * V = W 1A of current @ 1V is: 1W 1A of current @ 10V is: 10W I understand that 1A is 1C-per-second, which is some amount of electrons. Intuitive so far. How can I get more power if I use the same ...
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74 views

Does the sum of power supply in circuit equal zero? [closed]

Just like in the question, I want to know if the sum of power supply in the circuit is really equal zero and if it is true what is the reason behind it. For what I think, power supply shouldn't be ...
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48 views

Work of a force applied to a point with no velocity [closed]

What can we say about the work of a force applied to a point with a velocity $v=0$? My guess is $\delta W = P(t) dt \implies W = \int F\cdot v \ dt$ and then since $v=0$ we have that $W=0$. I am not ...
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Velocity reference point when calculating power [closed]

As shown in the picture, a person (80 kg) is on a platform (20 kg) that's suspended by a rope which is on a pulley. As they pull on the other side of the rope, they reach a constant velocity of 0.4 m/...
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Is the brick-concrete connection the 'weakest link' in brick made buildings?

When an earthquake strikes,do buildings made of bricks and concrete break mostly on concrete-brick connection spots?If Yes does the brick on the picture prevents this from occuring?
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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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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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Specific Power (gravitometric power density) of gasoline?

This may be off the mark, however I am having a difficulty in finding this piece of information as every search on Google provides results for the (gravitometric) energy density, the mJ/kg, or Wh/kg ...
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Gain saturation and power saturation in lasers

I have difficulty understanding the concept between gain saturation and power saturation in lasers. How are they different?
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Jackson EM Conundrum: Power Spectrum Polarization dependence for Thomson Scattering

I'm having some issues wrapping my head around what Jackson did for his derivation of nonrelativistic Thomson scattering. I'm using Gaussian units although I don't think there's a conflict here ...
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Output power of a QCW pumped Laser

Imagine we have a Nd:YAG Laser which is qcw pumped by a laser diode. First we choose for the pump rep. Rate 10Hz and a pump puls width of 250us. In the second approach we choose 50Hz and 100us. My ...
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Mathematical Definition of Power [duplicate]

I am a high school student who was playing around with some equations, and I derived a formula for which cannot physically imagine. \begin{align} W & = \vec F \cdot \vec r \\ \frac{dW}{dt} & = ...
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How to change power without changing line Resistance and transmission voltage?

I saw highschool physics problem. Problem is this. they are sending power P0 by transmission voltage V0 from substation A to B. In this, loss power in line is P If send power 2P0 by V0,what will be ...
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Why current drops when voltage increases?

https://www.youtube.com/watch?v=ubZuSZYVBng&t=329s In the video above the man put a 200 thousands volt ball near a non-charged one and a very tiny current appear between them. But I think with ...
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Is the change in Kinetic Energy not equal to the work done when mass changes at some rate?

Here the Rate of change in Kinetic energy is not equal to the the power. Please explain
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44 views

Conductivity and loss in electromagnetic waves

I have learned electromagnetic waves in a free space (conductiviy=0) propogates without loss. But if medium have conductivity there are 2 cases good insilator case and good conductor case. In a good ...
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48 views

Electrical energy (J) from voltage (V), amperage (A) and exposure duration (s)

I have that the electrical energy $E$ in Joules (J) is equal to $E = time \times current^2 \times resistance$. Reasoning with SI units, we can verify that this equation does return energy values in ...
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Questions About Highly Coupled Magnetic Resonance

This is one of very few in-depth sources of information I can find online about Highly Coupled Magnetic Resonance: The last sentence of the third-to-last paragraph says that research is underway that ...
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Why is the relationship between power and amplitude squared? [closed]

I read on Wikipedia that "An example scale showing power ratios x, amplitude ratios √x" [1], i.e. that the relationship between power and amplitude is squared. How come? Have this something ...
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Can gravity and hydro power be utilized in a closed system to generate efficient electricity? [closed]

This is my second post on this topic. The first one had some flaws but I resolved them with this new design. This is more of a thought experiment - which I don't think is that outlandish given ...
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How is the fluorescence of a single molecule depending on excitation power?

In my project, I recorded the fluorescence of laser-excited tetracene molecules attached to the surface of solid rare gas clusters with a photomultiplier tube (not sensitive to emission wavelength) ...
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1answer
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Power and power loss during transmission of power

So let's say power to be delivered to homes is 80kW($P_3$=80kW) and the houses have to receive it at 220V($V_3$=220V). The substation is a 4000V to 220V step down transformer and $R_2=15 \Omega$ It ...
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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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Can gravity be utilized to generate hydropower in a clean, repeatable fashion without rivers or lakes? [closed]

I came up with this idea today and am just wondering if it is feasible. It is much easier to communicate via images so I drew the attached image to convey it. The key concept is rolling a ball in ...
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Magnetic flux as a function of distance experiment

For my electrical engineering senior design I created an experiment to test magnetic flux (more voltage) increase from a material in a coil of wire. There is a large sending coil with A.C. voltage and ...
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Understanding the big difference between input power and output power of system [closed]

Screw jack. ( Please see the above picture ) According to screw jack formula F=w×p/2πr ( without friction ) Here F= force , w= weight , p= pitch of the screw , r=length of the lever . Now i want to ...
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Does fuel consumption depend completely on work done?

I recently encountered a question in a physics workbook, a car of mass m kg is lifted by two cranes to a height of h metres, the cranes lift the car in t1 and t2 seconds respectively (t1<t2), then ...
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Power density and dBm units

A document says the spectral level (in Watts/Hz) of a pulse with pulse repetition frequency f (in Hz) and pulse duration t (in seconds) and pulse peak power P (in Watts) is given as below. ...
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71 views

Seemingly contradictory situation in electrical system loss

In a power supply system, we know that we decrease the current and increase the potential difference. If we decrease the current by a factor of 10 and increase potential difference by a factor of 10, ...
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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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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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105 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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