Questions tagged [power]

The time rate of change of energy

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24 views

How do cyceling moltant salt reactors do nuclear fission?

I only have an understand of how PWR's do nuclear fission, looking at the diagram for the salt reactors it's a bit confusing. Where is the fission taking place if the salt is cycling throughout the ...
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Work, power and energy in the case of chain [closed]

Please read my question carefully it is not homework question.i did my own work here I am jee advanced aspirant and i have no teacher only Google is my teacher so please don't flag my question as ...
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Work power and energy in the case of chain [closed]

Please check my work. And my solution here. But my answer does not match with the actual answer. Where I am wrong here?
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56 views

Wave equation and skin-depth

I initially posted this question to the math stack overflow, but realized that it is really a physics question This is a follow-up question to one of my unanswered questions https://math....
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What does an $x$ Watt bulb actually means?

Let's say I have a 11W bulb in my home, connected to a 220V power supply. What exactly does it mean that this bulb is 11W? As far as I know, the Wattage is determined by the formula $W = V * I$, so it ...
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Is any work done when we force is being applied on any point but only the point of application moves but body does not

In most of the cases when we apply force to any rigid body on any point the body itself starts moving but what about the elastic sort of body ,if I apply any force and only that particular point moves ...
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65 views

Rollercoaster physics! [closed]

I'm completing an assignment about roller coaster physics, however, I'm having difficulty understanding a concept. The image depicts a rollercoaster with a motor, however, I am not sure what it means ...
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48 views

Is power delivered( or work done) by battery completely converted into heat?

I am currently a $9^{th}$ grade student. I recently learned about power delivered by batteries and also the heating effect of current.Consider a circuit consisting of a battery of e.m.f $E$ and a ...
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115 views

Find relation between displacement and time

“A body is moving unidirectionally under the influence of a source of constant power. Find the relationship between displacement $s$ and time $t$.” I proved it using dimensional homogeneity. Power ...
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Resistive heating - calculating how much a wire will heat up before it melts

I wanted to offer a simple model for students to work with about resistive heating. Since $P=\frac{V^2}{R}$ we can calculate how much energy will be put into a wire over time, if we know the voltage ...
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What’s the difference between energy in an electromagnetic wave and transmit power?

As I understand, Energy of an electromagnetic wave has a direct relationship to frequency. A higher Energy wave is smaller in length at greater frequency. But Transmit Power can be increased without ...
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Why are PWR's being build instead of FBR's or LMFBR's?

Why are PWR's still being built in modern times when we have the option to use designs that allow intake of raw uranium without the need for enrichment and allow for recycling of waste material. I don'...
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Finding power using work as $Fd$ or as change in energy [closed]

This is a question I had been given a few years ago and I have found two different answers depending on how I find work. If I use $W=Fd$ vs $W=E_f-E_i$, I get two separate answers. I must be missing ...
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33 views

Switching light off or not?

Is it better to switch off a tube light and turn it on again or not switching it off in the first place for a small amount of time? Google searches say "Fluorescent lights do take a small surge ...
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45 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 $$<{f(\omega)f^*(\omega)}>=S_f(\...
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What is the braking power generated by an air brake system on a heavy truck? [closed]

Can't really find good resources on the Web. So what is the braking power generated by an air brake system on an average heavy truck/lorry ? Assuming let's say it's going from 100 km/h to a full stop. ...
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How to calculate energy from power I receive every 5 minutes [closed]

I have to calculate energy from power measurements, which I receive every 5 minutes. The values are summed and assigned to measurements, then monthly measurements and finally yearly measurements. ...
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A formula for power [duplicate]

My question is this, $P= dW/dt.$ dW=F.dr That leads to P= F.v But how?! The book (Schaum's theoretical mechanics) stated that this formula is true. But I think it is true just in case that F is ...
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Why is the power of a car limited?

As an introduction this is what's written in my textbook: $Power=\frac{work\ done\ by\ driving\ force}{time}$ $=\frac{Driving\ force×distance}{time}$ (as distance over time = speed) $= Driving\ force×...
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Opposite currents in adjacent superconductors

I understand that the maximum current that may be carried in a superconductor is limited by the magnetic field the superconductor is in, which is very often almost exactly the field generated by the ...
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How to inject the maximum acoustic power into a block of granite?

I know that we can use transducers that are glued to a surface to achieve this. If I want, for example, to have 200 watts of actual acoustic power in the audible range in the granite, is a transducer ...
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37 views

Electrostatic potential…in electrodynamic regime in the expression $P=U.I$

Let us consider a two terminal electrical component. It is typical to calculate the power it consumes by considering the voltage drop $U$ and the current passing through it $I$ and to say it absorbs a ...
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Power formula at transformer derivation

For the power of an ideal transformer, there is the formula $U_1\cdot I_1 =U_2\cdot I_2$ which can be derived from energy conservation by saying that there are no energy losses and therefore the ...
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Power transferred in a transformer

Power absorbed from the voltage source in a circuit is $=V_{rms} \ i_{rms} \ \cos(\phi)$ where $\phi$ is the phase factor. So in the primary circuit power absorbed from the voltage source is$=V_{rms} \...
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Using light as an energy condenser

If I created a sphere with the internal surface perfectly mirrored and shone an intense light into it from a small aperture, what would happen to all the photons being accumulated? If I then at some ...
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Why is max power below natural frequency

I can't understand why intuitively the max power is at the natural frequency. We were told in our notes that the power absorbed is exactly equal to the rate at which energy is dissipated. Yet, if we ...
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Why is the total energy of system considered to be a state function? [closed]

Mechanical power is not a state function because it changes with the change of the path taken from a specific initial point to a specific final point. The same for thermal power. It is not a state ...
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Why doesn't the Stress Power in the theorem of the expended mechanical power contribute to the kinetic energy?

The following is the formula of the expended mechanical power: $P(t)$ $=$ $\frac{d}{dt}$ $\int \frac{1}{2} \rho\ v^2 dV$ $+$ $\int \sigma \colon d\ dV$ where $P(t)$ is the mechanical power entering ...
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Interpreting a normalized Power Spectral Density (PSD)

I am using software to produce power spectral density (PSD) plots of time-series (voltage versus time). Unfortunately, the units of the produced plots are alien to me. I'm used to reading and ...
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Can an open core transformer with primary and secondary wrapped on top of each other also act as an electromagnet?

If we take a rod and wrap it with a coil with N turns and then wrap another secondary coil on top of the primary with same N turns, the core has open ends, will this type of transformer act as an ...
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Near and far field EM radiation

When we talk about radiation, we often talk about the near and far field radiation. For the near field, the Poynting vector falls off like $1/r^3$ and the far field Poynting vector falls off like $1/r^...
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Heat Energy $\propto 1/R$ or $\propto R$?

I know that heat energy can be calculated by $I^2 R t$. Therefore, it's directly proportional to resistance. However, $$I^2 R t = \frac{V^2}{R}t$$ In this case, won't the heat energy be inversely ...
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Generating electrical current using sea or ocean currents

I'm curious to know is this concept can work. Place two metallic plates in a body of salt water. Connect a power supply in order to attract ions close to each plate. Place two coils as illustrated ...
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How does Ohm's law work? Can a resistance change depending on the applied voltage?

I'm refreshing my memory on electromagnetism, and even though I thought I completely understood Ohm's law, today, while reading my textbook I came across the following. My question is, what does '...
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Deriving the $E=g\cdot Q\cdot h\cdot t$ formula for hydraulic energy

Goal. Derive $E=g\cdot Q\cdot h\cdot t$ where $g, Q, h$ and $t$ are the gravitational acceleration $9.8m/s^2$, the volumetric flow rate in $m^3/s$, the height in meters from the reservoir to the ...
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Why is not used only watts or vars?

The watts are for real power and the VA for reactive power, but isn´t the same thing at the end? Why does not use only watts or only vars?
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Power of Bulbs Connected in Series and Parallel

Let's say that I have a circuit consisting of a power supply of $220V$ and $2$ bulbs of power-voltage rating $40W-220V$ and $60W-220V$ in series. How would I calculate the current flowing across the ...
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Is it possible to lift an object from rest with constant power?

This is inspired by the following question. Consider some object which I want to lift from rest with a constant power throughout the whole process; the power I apply when lifting the object from rest ...
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Which way might commercial fusion power affect the earth's energy balance?

The ITER fusion-energy project might prove the feasibility of commercial fusion-energy generation by mid-century. This would be a comparatively clean method of energy generation, and subject to ...
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How to find how many atoms have to decay to get some amount of power? [closed]

How do I get the power when there is no charge, or anything else given in the question? Is there a power formula that I'm not aware of? Please help! A nucleus of plutonium-238 ($^{238}_{\ \ 94}$Pu) ...
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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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Mechanics: if there's a change in power doesn't that mean there is a change in driving force, and thus velocity? [closed]

here is the question: a car of mass 1100 kg is moving on a road against a constant force of 1550N resisting the motion. the car moves along a straight horizontal road at a constant speed of 40m/s. (a) ...
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132 views

$P = IV$ and $V = IR$

$P = IV$ and $V = IR$ If $v$ is proportional to $i$, then, for transmission cables, how can one increase the voltage but somehow decrease current? Also, if you decreased current but increase voltage, ...
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103 views

Why can't we use $P=VI$ in some cases?

Here is one example of a situation wherein one can't use the relation $P=VI$: The supply voltage to a room is 120V. The resistance of the lead wires is 6 Ohms. A 60W bulb is already switched on. What ...
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130 views

Power and work contradiction

A body is starting from rest. A force is acting on it for a short period of time. In that given time, power delivered to it at any instance $t$ is given $$P = F \cdot v_1 = ma \cdot v_1 = mv_1^2/t,$$ ...
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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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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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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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