Questions tagged [power]

The time rate of change of energy

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127 views

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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2answers
216 views

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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0answers
105 views

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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1answer
40 views

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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0answers
32 views

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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1answer
60 views

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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1answer
100 views

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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1answer
28 views

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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0answers
342 views

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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0answers
45 views

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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1answer
83 views

Question about Power lost in a wire given voltage and current

The question is as follows: A high-voltage transmission line carries $1000 \mathrm{\,A}$ at $700 \mathrm{\,kV}$ for a distance of $100 \mathrm{\,miles}$. If the resistance per length of the wire is $0....
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1answer
33 views

How is it that a single transmission line can be modeled by a two port network?

I am studying electric power distribution. I've read in Kirtley's Electric Power Principles (2010) that a transmission line can be modeled by the 'pi' model. Which is a two port network with series ...
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1answer
655 views

What does the Force in the equation P=FV represent?

So this is the question in context. A car of mass 1200 kg tows a trailer of mass 300 kg along a straight horizontal road. The resistance to motion is modeled as being a constant force of magnitude ...
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1answer
954 views

How to calculate the power produced by a single cylinder petrol (car) engine at a certain rpm? [closed]

I want to know: What are the main parameters that make power/torque generated by engines different and what makes some engines more efficient? (except friction and things like that!) How do I ...
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1answer
478 views

Measuring voltage output of a Wimshurst machine

I see that there are a few variables that effect the voltage output of the Wimshurst machine. For one the size of the disk, the amount of conductive strips on the disk, and obviously the spark gap ...
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1answer
119 views

What are some good analogies about electrical power? [closed]

What are some good analogies about electrical power? I understand the mathematical part of $P=VI$, but I would like to somehow visualize it in a practical electric circuit.
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0answers
82 views

Building a CO2 laser power supply [closed]

So i bought a 15,000 volt franceformer for $160. This is used to power my CO2 laser that is also homemade. The power supply is the most expensive part by a landslide so I decided to ask around. I ...
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2answers
1k 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
23 views

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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1answer
441 views

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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2answers
63 views

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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2answers
134 views

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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4answers
114 views

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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1answer
117 views

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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2answers
100 views

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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2answers
7k views

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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1answer
226 views

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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1answer
29 views

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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1answer
66 views

Question about power

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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2answers
47 views

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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1answer
258 views

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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1answer
484 views

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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1answer
98 views

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 ...
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1answer
385 views

Power consumption of a qubit?

I keep hearing “A quantum computer will consume so much power that it'll need a dedicated power plant.” I'm trying to find literature to back this up. I would love to get an estimate for the ...
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1answer
100 views

What assumptions would I have to make to rigorously prove power equals force dotted with velocity, $P=\mathbf{F}\cdot\mathbf{v}$?

$\newcommand{\F}{\mathbf{F}}$$\newcommand{\r}{\mathbf{r}}$$\newcommand{\v}{\mathbf{v}}$$\newcommand{\d}[2]{ \frac{ d #1 }{ d #2 }}$ There’s a well-known known formula for the power $P$ exerted on a ...
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0answers
2k views

How to calculate theorectical engine max. torque from max. power and RPM?

I'm trying to come up with a formula to estimate an engine's maximum torque (and at which RPM) given other values and some assumptions. I'm approximating an engine's torque curve as 2 linear ...
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1answer
102 views

Can a fusion bomb make electricity? [closed]

Would blast energy remove a single piece of pre-cut earth above an underground explosive? Could you then break apart the piece of earth and lower it back into the cavity as a way of generating power? ...
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2answers
73 views

Electromagnetic plane wave interaction with metal plate

Given a plane wave with $\overrightarrow E$ field parallel to $z$ axis and a square metal plate in $xy$ plane. Suppose we know the power of the incident wave and the size of the plate, how can we ...
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1answer
285 views

The physics of a dog sled — how much power or energy over time?

I'm looking for some equations that govern the physics of a dog sled. Let's say we have 8 dogs pulling a sled. Each dog provides a force of F, the tractive effort. The sled weighs W and there's a ...
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2answers
94 views

Electric power and resistance dependance

According to the equations, $$P=VI =I^2R\,\text{ and voltage } V=IR$$ it seems clear that when the resistance is lower by fixing the voltage at constant, the current is therefore, higher, generating ...
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1answer
45 views

“Reference dependence of fuel gauges” in classical mechanics

The power exercised by a force on an object is dependent on the velocity of the object and hence the frame of reference. I am wondering wherein the error lies with arguments such as the ones presented ...
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1answer
50 views

Watts and Newtons revisited

At the junkyard, an electromagnet holds a 1 ton car freely in the air (no acceleration exc. gravity). Assuming 100% power efficiency, how many Watt electrical power are needed? I know that the magnet ...
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2answers
163 views

Why isn't work a logical quantity? [duplicate]

Please read till the end, before you answer. I'll start off with the examples to get to the point. Drop a block from 1 meter and it will have speed v when it reaches the ground, with total work ...
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2answers
80 views

Why does having a high input impedance and low output impedance cause minimal voltage drop when you connect two circuits?

Say you connect Circuit A with low output impedance to circuit B with high input impedance. Why does this cause minimal voltage drop compared to connecting them the other way around (high output to ...
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0answers
89 views

Wind Power in a Wind Tunnel

Two questions in one day! What? Okay, so the power equation for air flow is as follows: $$ P = 1/2 \cdot \rho \cdot A \cdot V^3 $$ In a wind tunnel due to the Bernoulli principle as the area ($A$) ...
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3answers
698 views

Question about the relationship between resistive drag, dissipated power, and velocity

This is from a high school physics exam for 16 year olds that I did two years ago. I still don't understand it. The power $P$ required to move an object through a medium at constant speed depends ...
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4answers
839 views

Units in the power equation

I have a question with the formula power = energy / time. I know it's a simple formula, but when substituting into this formula, Does the energy have to be in joules or watts (no kJ or something ...
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2answers
50 views

Power of an engine

Suppose a car of mass $m$ starts out with an acceleration $a$ in the positive direction (say +ve x-axis), against a constant Resistive force $R$. At the instant it's velocity is $v$, what is power of ...
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1answer
655 views

Converting lux to watts with known wavelength

I was wondering if there was a possible way to convert lux to watts accurately if the wavelength is known. I know that for the sun there is an approximate conversion of 0.0079W/m$^2$ per Lux using ...