Questions tagged [power]

The time rate of change of energy

581 questions
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For a car engine, why does velocity increase as force decreases?

Take the equation P = FV For a fixed power of an engine, applying a greater force gives a smaller velocity while a smaller force gives a greater velocity. To me this sounds counter-intuitive. I ...
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Conducting ring free-falling in a variable magnetic field

Consider a free-falling conducting ring oriented horizontally to Earth's surface. It then enters a region with magnetic field $B = B_o(1+\lambda z)$ where $+z$ is the vertical direction upwards. By ...
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How is radiance conserved? - Power/intensity misconception

Before getting to the main question I'd like to clear some pretty basic concepts regarding energy/power/intensity as my physics is pretty rusty. According to wiki In radiometry, radiant flux or ...
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mechanical power in moving coordinate system

Suppose I have an object resting on top of a table. There are two forces acting on the object: gravitational pull and the supporting force from the table. It is pretty obvious, that the table is doing ...
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How to increase power in a geared system using P = (omega)(torque)

Could someone give me a basic, intuitive explanation on how to increase power in a geared system using the P = omega*torque formula? By geared system I mean two connected gears, where the first one ...
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Is there a way to express stream power for an entire watershed?

Fluvial geomorphologists describe stream power, $\Omega$, as the 'rate of energy dissipation against the bed and banks of a river per unit downstream length'. It is expressed as a function of water ...
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Whose electric energy consumption 'copper' or 'aluminium' element in Water heater will be greater?

Considering water of 20 litres 25 degree celsius of temperature needs to be raised. Using the heat equation q=mcpdt considering cp of water to be 4.2 kJ/kgk we get q=2100 kJ which equals 2100000 J. It ...
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what does the “T” really represent in the signal power equation

In the signal power equation $$P(g(t)) = \lim _{T\to \infty }{\frac{\left(\int_{-\frac{T}{2}}^{\frac{T}{2}}g\left(t \right)^2dt\ \right)}{T}}$$ What does the $T$ really represent ? From my ...
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What is the sound power produced at a defined frequency by a woofer of given diameter and excursion?

Assuming an infinitely rigid woofer of a given diameter and oscillating with a given excursion at a given frequency, what is the SPL (sound pressure level) at 1 m distance? I found https://en....
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What is the physics behind the vibrating feeling you get when you touched an inducted metal surface?

When I was last working on the highway, I stopped to help a motorist parked under high tension power lines. I'm driving a large Ford Transit van, fairly box shaped. My van was parked 90 degrees (...
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How to determine the best fit battery for a battery powered application? [closed]

This is an attempt to get some scientific information about how to best use batteries (I'm going to refer to both single cells or assemblies of cells as batteries for the sake of this question) in ...
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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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How do microwave transformers avoid power loss through the skin effect?

I was reading about the skin effect and how the effect increases for higher frequencies. I also read that Litz wires can be used to counteract the effect, but usually only up 1 Mhz frequency. Is ...
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Friction energy dissipation calculation

In a moving block, a variable force pushes it. Friction is acting against the movement. Below equation specify the amount of energy could be used to give motion: $$E_1 = \int F_T(t) .v(t) .dt$$ ...
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Transmission line loss [closed]

Let us suppose that a transmission line is carrying power to a resistor $R$ The resistor $R$ requires a power of $P=IV$. Applying the most simple case where a transmission line is supplying a current ...
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Why we say that electrical energy is power over time p x t? [closed]

How electrical energy can be power over time when electrical energy is actually potential energy? from a lot of sources (wiki ,study.com..) comes the information that electric energy is potential or ...
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Why we need Watts in electricity if power is done by the Amps?

If: I = V / R And a light bulb needs X amount of electrons flowing per second (I=amps) to create light. Why do we use watts on the first place if all the light bulb cares is the number of ...
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Power and work of tension force of a rotating body [closed]

Suppose that we have a body rotating on a massless string with a uniform velocity. What can we say about the work done by this tension. Moreover, what is the power related to this work. I do not have ...
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Why do power stations use steam to drive turbines?

Wouldn't it make more sense to use a liquid with a lower specific heat capacity/latent heat of vaporisation so that less energy was used up in converting the water to steam?
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What is the relationship between power and energy? [duplicate]

What is the relationship between power and energy? Of course, this is a very basic and fundamental question, but it's one that many people learning physics struggle with. I'm asking this to have a ...
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Work, Energy, and Power- Hooke's Law & graphing

The purpose of the work, energy, and power lab is to demonstrate the validity of Hookes Law over a limited range of displacements, measure the spring constant of several springs, and determine the ...
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How to devise a microwave power sensor?

I am fairly aware of the general thermistor/ diode/ thermocouple methods of sensing microwave power. Currently my requirements are simpler: A device that senses only microwave power incident on it - ...
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Say we have a waterfall and we know that 10kg of water falls over the ledge per second. We are also given that the acceleration due to gravity is 10m/s^2, and the distance to the ground is 5m. We are ...
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Energy Creation Using Water, Generators, and a Pump

So a friend and I are arguing over an infinite energy idea that he had about using gravity, that is an almost infinite source of energy. Water to use to push a generator by dropping and hitting the ...
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How to convert RF absorption rate (SAR) values to electric field values?

I'm currently reading a bunch of journal articles that describe biological effects of electromagnetic fields. They often refer to the field in terms of specific absorption rate (SAR) values (W/kg). ...
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Why power is proportional to resistance in $I^2R$ and inversely proportional in $V^2/R$?

In one relation electrical power is directly proportional to resistance while in other it is inversely promotional.
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What are the natural forces that slow the neutron down?

In a nuclear bomb, I've noticed that slow neutrons don't affect plutonium 239, if we're using this kind of atoms (plutonium 239), we need fast neutrons to make a sustainable chain reaction. Also I've ...