# 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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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 $P = ... 0answers 15 views ### Is signal strength in$\rm dBm$a measure of power or intensity?$\rm dBm$is decibel-milliwatts, which should be a measure of power. However, my phone's reading of WiFi signal strength dBm decreases as I walk away from my router. Which implies dBm measures signal ... 1answer 33 views ### Energy transfer between oscillators [closed] Suppose I have two mechanical oscillators$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$... 3answers 121 views ### Are there thermodynamic obstacles to thermoelectric generation on the surface of Venus? There has recently been a NASA design challenge for a clockwork Venusian terrain navigation system which uses no sensitive electronics or hydraulics. I was thinking that rovers and probes on Venus ... 0answers 11 views ### A constant number of calories burnt for an exercise lasting different durations So in the gym, me and a friend doing 2000 m on the rowing machine burnt the same amount of calories, but I did the exercise in 14 mins and he did it in 10 mins. Given that both of us have similar body ... 1answer 37 views ### Why is Power caused by the driving force Say we have a car on a rough inclined plane, the maximum constant velocity at which the car can travel is$ V $, the driving force is$ F_d $and the frictional force is$F$. Shouldn't the power ... 1answer 26 views ### Determining rate of heat production across two wires of different materials This a school-level based Physics question, but I'm not able to explain myself the answer. Question : Nickel and Copper wires having same shape, size are connected across a battery in turn. The rate ... 3answers 49 views ### Why does current flow through the ground rod if lightning strikes the breaker box? In normal functioning of household power supply, the current flows through the hot and the neutral wire. Why does it not take the path of the ground rod, even when the ground rod, neutral wire and the ... 2answers 52 views ### When to use Ohm's law as opposed to using power formula? I am a mechanic trying to gain a better understanding of electrical theory on vehicles. I have a sound understanding of Ohm's law and also the power formula (Power = Voltage x Current) However, I am ... 5answers 92 views ### What is the power while accelerating a bicycle? With respect to this question, the power on a accelerated body, that moves in the same direction of the acceleration, can be calculated by$W = Fv$, where$F$is the external force and$v$the ... 1answer 29 views ### 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_{...
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 ...