Questions tagged [power]

The time rate of change of energy

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Power of a Hydraulic Pump and Rate at which Kinetic Energy is imparted to the water

Suppose there is a Horizontal pipe of Length $L$ which pumps water through an end with a speed $v$, there is no friction. A pump is on the opposite side. So what will the rate at which of kinetic ...
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461 views

Maximizing power, current, voltage

We're asked to maximize voltage across the resistor RL, then do the same for current and power. But to find a value of R that maximizes VL, I think we'd just say infinity. And to maximize current ...
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50 views

Why is the brightness of a light bulb dependant on power?

If: Temperature is a measure of the average (kinetic) energy of the particles which make up some material Power is measure of energy transferred per second (J/s) The temperature of the filament in a ...
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3answers
141 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 ...
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Modified affinity laws - effect of motor efficiency

I have a problem regarding the affinity law that describes the relation between power and fan speed. If I am not mistaken, this is applicable if the motor efficiency is constant. In my case it is ...
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2answers
39 views

Power Demand for Motion of Vehicle on a Road and Plain in Air with Air Resistance

Let's say we have an ideal motorcycle without frictional losses (no tire slip, ideal machine without losses). But we want to consider wind force which is proportional to the square of speed $F = k \...
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2answers
230 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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5answers
83 views

Inclined Stationary Bike

Why would it make you work harder if you incline a stationary exercise bike? What's the physics involved? Assuming all other factors remain constant and only the incline changes, why would you burn ...
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1answer
236 views

What exactly is average power?

This question arises due to a problem I'd been solving recently: The question was to find the average power supplied by the water over the six hour period. Now I have read this post which ...
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2answers
73 views

Example for velocity being inversely proportional to force

We know that force is inversely proportional to the velocity when power is constant. Can someone provide me with a real-life example where an increase in the net force acting on a body actually ...
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1answer
56 views

Can average power be non zero, but instantaneous power be zero

Q. A wind-powered generator converts wind energy into electric energy, Assume that the generator converts a fixed fraction of wind energy intercepted by its blades into electrical energy. For wind ...
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2answers
137 views

Why can't we use $P=VI$ here? [closed]

I have this question: An electric bulb is rated 220 V and 100 W. When it is operated on 110 V, the power consumed will be: (a) 100 W (b) 75 W (c) 50 W (d) 25 W I use P=VI to find Current when Power ...
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Why do high current conductors heat up a lot more than high voltage conductors?

120 volts x 20 amps = 2,400 Watts However, if I increased the voltage and lowered the current, you can also use a smaller wire size (more inexpensive), also have less heat and achieve the same watt ...
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2answers
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Work done by a pump to empty a well [closed]

I've come across problems that ask to calculate work done by a pump to empty a full cylindrical well. The result obtained through integration is $mgh/2$, where $m$ is the mass of water in the well and ...
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Question about the work done by a frictionless rod on a ball on it (Mechanics) [closed]

Please refer to the question, everything is mentioned there. The answer is option (b) which is $mv^2$ I tried solving this, but could not do it. The problem here is the plane we are using, if it was ...
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1answer
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Why don't we take kinetic energy into account when finding the change in potential energy?

An engine draws water from depth of $10 \;\text{m}$ with constant speed $2 \;\text{m/s}$ at rate of $1 \;\text{kg/s}$. What is the power of the engine? While solving the question, I found that we ...
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2answers
206 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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256 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
43 views

What exactly carries power in a transformer?

Suppose we have applied a voltage to a transformer with an open secondary, we will get a certain flux in the core and a certain voltage at the secondary depending upon the turns ratio. Now, suppose we ...
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0answers
102 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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32 views

Power of a Sr-90 RTG

I am trying to make sense of the numbers that I have found in a scientific paper (DOI:10.4236/ojbiphy.2013.31A011). As can be seen on the photo, it is written that the mass, which I am pretty sure is ...
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4answers
57 views

Definition of Power

I wanted to clear my doubts regarding the true definition of power. Imagine a mass falling from a height and reaching the ground thanks to gravity. The power of this event would be the work done by ...
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6k views

Can power (in the context of work and energy) ever be negative?

Consider this question which HAS definitely been asked but i ask you now to question your first inclination to just 'do the question' and explain the thought process. Restrictions: assume you only ...
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1answer
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How do cycling 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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Linearity from fractional equation? (solar cell)

I am doing a little research into solar cells, namely the fill factor. It is a ratio, defined as maximum power divided by theoretical power [of the solar cell]. The maximum power and theoretical power ...
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2answers
126 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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1answer
62 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 ...
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314 views

How much heat can be adsorbed in a solar parabolic trough?

How do I know the produced heat on the focal point of a solar parabolic trough per different radius if made from steel and also from glass?
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58 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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672 views

How to calculate total power generated?

I have a circuit with a piezoelectric transducer which converts mechanical energy to electrical. The ceramic is connected to a 1 Megaohm resistor and a voltmeter which reads voltage. If I want to ...
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3answers
54 views

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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2answers
61 views

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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1answer
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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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606 views

Measuring output optical power of an LED

How do I measure the optical power output of a LED? I have a detector that measures the output power of a laser incident on a small opening. The problem with using the LED with the same detector is ...
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4answers
68 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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1answer
57 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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3answers
143 views

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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1answer
28 views

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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1answer
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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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1answer
68 views

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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1answer
34 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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2answers
329 views

Is it resistance of filament or current thorugh it which determines brightness of bulb?

SO brightness of bulb. Does it depend on the resistance or current in it? I have two bulbs keeping everything same except resistance. So Bulb A - high resistance Bulb B - Low resistance which bulb ...
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2answers
57 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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Why aren't (domestic) kettles insulated?

In my experience of buying and using kettles, I have come across none which are insulated. The obvious reasons as to why it would be beneficial being that heating time would be reduced, similarly, ...
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1answer
88 views

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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1answer
41 views

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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1answer
60 views

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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115 views

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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274 views

Why does potential energy time-derivative depend only on the mass flow?

This is a homework exercise, but I don't want specifically to tell me how to resolve the exercise. I want you to help me what's wrong here, and why I can't do it. Water is pumped from a lake to a ...
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24 views

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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