Questions tagged [power]

The time rate of change of energy

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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

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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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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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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What causes the push you back suddenly into your seat feeling when you accelerate cars?

I'm trying to buy a car that really feels fun to drive, and the one I test drove doesn't feel all that fun despite having high horsepower, torque, and acceleration. I'm trying to find a mathematical ...
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Why is it not possible to run electrical plants on oxygen (rocket fuel) or hydrogen rather than coal?

Title basically says it all. I'm not a physicist by any stretch. I'm an IT professional with slightly more than a passing interest in mechanics. I got to thinking the other day how much power is ...
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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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Ohm's law and Joules heating

Ohm's law state that the ratio of V and I gives us a constant value of R provided that the temperature is kept constant throughout. However, in accordance with the joule's heating it would get heated ...
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Is resistance or voltage more effective in reduction of power loss?

It is commonly stated that increasing voltage of the source reduces power loss. If voltage of source is increased current through the circuit increases, resulting in more power loss (than with less ...
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How long could the Sun power the Earth?

With the energy emitted by the sun in 1 second, how long could everyone household on earth be powered? Currently we obviously can't harvest 100% of the sun's energy emissions.
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Is it right that in LCR cicuit Power loss $=VI$ not equal to Power loss $= V²/R$?

I had a doubt today (this is not my homework question, I made it up). Suppose in a LCR circuit at resonance we take out power loss as $P=VI \cos\phi$ (power factor and $\cos\phi$ becomes 1) and in ...
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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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Trouble in understanding question related to the work, energy and power chapter about pendulum

Two pendulums of masses $m_1$ and $m_2$ ($m_1>m_2$) and strings of same length are taken. The densities of the both the bobs are the same ($d_1=d_2$), if we compare the time period of the two ...
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Dimensional analysis of power

I am trying to do the dimensional analysis of power using two different ways Using $P = \frac{dW}{dt}$ $\frac{[W]}{[t]} = L^2\cdot T^{-3}\cdot M$ since $W$ is in joule Using the power formula $P = ...
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Is signal strength in $\rm dBm$ a measure of power or intensity?

$\rm dBm$ is decibel-milliwatts, which should be a measure of power. However, my phone's reading of WiFi signal strength dBm decreases as I walk away from my router. Which implies dBm measures signal ...
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Energy transfer between oscillators [closed]

Suppose I have two mechanical oscillators $a(t), b(t)$, coupled through the interaction $V_\text{int} = \mu^2 a(t) b(t)$. Is there a simple way to express the rate of energy transfer from $a$ to $b$ ...
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Are there thermodynamic obstacles to thermoelectric generation on the surface of Venus?

There has recently been a NASA design challenge for a clockwork Venusian terrain navigation system which uses no sensitive electronics or hydraulics. I was thinking that rovers and probes on Venus ...
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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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Why is Power caused by the driving force

Say we have a car on a rough inclined plane, the maximum constant velocity at which the car can travel is $ V $, the driving force is $ F_d $ and the frictional force is $F$. Shouldn't the power ...
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Determining rate of heat production across two wires of different materials

This a school-level based Physics question, but I'm not able to explain myself the answer. Question : Nickel and Copper wires having same shape, size are connected across a battery in turn. The rate ...
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Why does current flow through the ground rod if lightning strikes the breaker box?

In normal functioning of household power supply, the current flows through the hot and the neutral wire. Why does it not take the path of the ground rod, even when the ground rod, neutral wire and the ...
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When to use Ohm's law as opposed to using power formula?

I am a mechanic trying to gain a better understanding of electrical theory on vehicles. I have a sound understanding of Ohm's law and also the power formula (Power = Voltage x Current) However, I am ...
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What is the power while accelerating a bicycle?

With respect to this question, the power on a accelerated body, that moves in the same direction of the acceleration, can be calculated by $W = Fv$, where $F$ is the external force and $v$ the ...
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Why Doesn't Thermal Radiation Render Thermionic Conversion Unable To Approach Carnot Efficiency?

Thermionic Conversion follows the classic Richardson-Dushmann Equation for thermionic current as a function of temperature squared: $$J_{RD} = A_0 T^2 \exp\left(-\frac{\phi}{k_B T}\right)$$ where $J_{...
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Single-mode laser oscillation in a resonator: time-dependent power

I have some issues with the time-dependent power of a single-mode laser oscillator. We are given the frequency $\nu$ of the resonator mode, the amplification of one period in the resonator is given by ...
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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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Work Equals Torque? Horsepower, Pulleys

While reading one definition of torque, I saw its units are Newton-meter, which is the same as work. But sources usually make it a point to emphasize "even though both work and torque units are ...
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Instantaneous power for a variable force

I know that instantaneous power is defined as the time derivative of work done. For a constant, it is easy to prove that this is just the dot product of force and velocity. However, is Instantaneous ...
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Calculating the time to perform work on rolling shutters

I have rolling shutters, e.g.: I am trying to determine how much time it takes to raise/lower the shutters a specific amount. (Because of the physics involved, the time is non-linear relative to the ...
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2answers
81 views

Why does ground reaction force decrease at the start of a vertical jump?

I don't understand why reaction force would decrease at the start of a countermovement jump. I've tried looking at this diagram and considering moments and how lowering the position of the center of ...
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Power consumption of vacuum tube [closed]

What is the power consumption of a vacuum diode? (or vacuum tube). I'm interested only in the "Anode supply voltage", this source typically has values of around $100\ \text{V}$ but I don't ...
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Why does the power remain constant during the stepping up and stepping down of current in transformers?

I understand quite a bit of transformers, their structure and other concepts and formulas related to mutual induction. However I'm not able to explain why the electrical power has to be constant ...
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What's the point of an RMS value? [duplicate]

the RMS (root mean square) value of $f(x)$ is defined as: $$f(x)_{rms}=\sqrt{\frac{\int^b_a (f(x))^2dx}{b-a}}$$ Why do we do this very specific thing of taking the square, the mean, and then the ...
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215 views

Power of water pump

Consider a water pump pushing water through a pipe: We wish to find the energy which the water pump has to expend to keep the water flowing through the pipe. Writing Newton's second law for water ...
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Power Transmitted along the string by a sine wave [closed]

I have referred several sources which derived the equation for the power transmitted along a string by a sine wave. However, I could not find anywhere what exactly is it? "Along a string" ...
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Why can't we jump very high?

If somebody is able to output $3750$ $\text{N}$ lifting weight on the back doing squats, why can't they jump at $150$ $\text{km}/\text{h}$ without weight? With weight: $$(300 \text{ kg of load} + ...
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Motor's work and power against the gravity

Ok, I've read this answer: Why does holding something up cost energy while no work is being done? But more questions pop up. If there is no work being done does that mean that there is also no power ...
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Why does increasing voltage result in a decrease in current and not rather a decrease in power? [duplicate]

I recently read that in order to conduct electricity over vast distances via power lines they had to step up voltage which results in a decrease in current through the medium. This is reflected in the ...
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Why do current carrying wires with smaller area heat up more than those with larger area?

Consider two wires of length $L$ and resistivity $\rho$, and consider a fixed voltage source with voltage $V$. The first wire has cross sectional area $A_1$ and the second wire has cross sectional ...
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1answer
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What is the meaning of power delivered by the battery after $t$ seconds?

I don't get it when a question asks power delivered by a battery after t seconds, are they talking about the total energy delivered by the battery in time t seconds divided by the time, or are they ...
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Derivation of the Gaussian Beam Pattern

I am currently reading this book by Mireille Levy. It says (on page 40) that the Gaussian beam pattern with half-power beamwidth $\beta$ is gien by: $$B(\theta)=A\text{exp}\left(-2\text{log}2\frac{\...
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Power required to reach X velocity in X time with drag

I wrote a program that adds force to a car like so: Engine Force = Power / Velocity​ Drag Force = -Velocity²​ Net Force = Engine Force - Drag Force = Power / Velocity - Velocity²​ I'd like to ...
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Is combustion engine power defined by speed and torque?

Of a combustion engine, the speed, torque, and power is measured at the crankshaft. For a measured pair of values for speed $N$ and torque $M$, can there be more than one value for power $P$? In other ...
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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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If a light bulb is connected to different voltages, is there a single method to calculate the utilized power?

If a bulb is marked at 100W and 200V, we know that the bulb utilises a power of 100W when connected to a 200V potential difference. So, can we say that the bulb, when connected to twice the potential ...
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Linear relationship between air resistance and power consumption?

Suppose that the air resistance a car encounters is independent of its speed. When the car travels at $15\mathrm{~m/s}$, its engine delivers $20\mathrm{~hp}$ to its wheels. (a) What is the power ...

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